4. Electricity and magnetism Flashcards
Describe the forces between magnets
Magnets attract or repel other magnets
Describe the forces between magnets and magnetic materials
Magnets will always attract other magnetic materials
What causes magnetic forces?
Interactions between magnetic fields
What are 3 magnetic materials (ferrous)?
Iron, cobalt, nickel
What are 3 non-magnetic materials (non-ferrous)?
Plastic, aluminium, copper
What are 3 methods of magnetisation?
Stroking with a magnet
Use of direct current (d.c.) in a coil
Hammering in a magnetic field
What are 3 methods of demagnetisation?
Hammering
Heating
Use of alternating current (a.c.) in a coil and slowly remove the material
What are magnetic field lines?
- A way of picturing the space around a magnet
- They show its strength and direction
Describe an experiment to identify the pattern of magnetic field lines, including the direction
Place the magnetic on plain paper
Draw around the magnet using a pencil
Make a dot one end
Place a compass needle in line with the dot
Make a dot where the compass needle points to
Point the compass needle to the next point
Make 4 more dots and connect them
Repeat with the compass needle on other points along the magnet
What are 2 rules to follow when drawing magnetic field lines?
- They always go from north to south
- They never touch or cross other field lines
What are 2 magnetic properties of soft iron?
- Easy to magnetise
- Easy to demagnetise
What are 2 magnetic properties of steel?
- Hard to magnetise
- Hard to demagnetise
What are permanent magnets made out from?
Hard magnetic materials
What are electromagnets made out from?
Soft magnetic materials
What is meant by the term hard magnetic material?
Magnetic materials that can be permanently magnetised
What is meant by the term soft magnetic material?
Magnetic materials that can only be temporarily magnetised
What is the design and use of a permanent magnet?
- Hard magnetic material
- Fridge doors
What is the design and use of an electromagnet?
- Solenoid
- Scrap metal moving
What are the 2 types of charges?
Positive and negative
What happens between unlike charges?
Attraction
What happens between like charges?
Repulsion
What is the SI unit for charge?
Coulombs (C)
Describe simple experiments to show the production and detection of electrostatic charges
Suspend one of the materials using a cradle and a length of spring
Rub on end of the material using a cloth
Rub the second piece of insulating material using a cloth
Hold the charged end of the second piece close to the charged end of the suspended piece
If the first piece is repelled, the materials have the same charge
What does charging a body involve?
Addition or removal of electrons
What is the direction of an electric field at a point?
The direction of the force on a positive charge at that point
Describe an electric field?
A region in which an electric charge experiences a force
Describe charging by induction
- Charged object is held close to the conductor
- Electrons in the conductor are able to move towards or away from the object
What is an electrical conductor?
A material that allows charge to flow through it
What is an electrical insulator?
A material that does not allow charge to flow through it
What are 2 examples of electrical conductors?
Metals and graphite
What are 2 examples of electrical insulators?
Non-metals and plastics
Non-metals and plastics
A negatively charged particle
What is current?
Rate of flow of charge
What is used to measure current?
Ammeter
What is the equation for current?
I = Q ÷ t
What causes current in metals?
Flow of electrons
What is the SI unit of current?
Amperes (A)
How is an ammeter connected to a circuit?
Always in series
What is conventional current?
Positive to negative
What is the electromotive force (e.m.f.) of an electrical source of energy measured in?
Volts (V)
What is the definition of e.m.f.
Energy supplied by a source in driving 1C of charge round a complete circuit
What is potential difference (p.d.) measured in?
Volts (V)
What is 1 V equivalent to?
1 J/C
What is used to measure potential difference?
Voltmeter
What is potential difference?
Amount of energy transferred by each unit of charge passing between those two points
How is a voltmeter connected to a circuit?
In parallel with the part of the circuit you want to measure the potential difference of
What is resistance?
Opposition to current
What is the formula for resistance?
R = V ÷ I
Describe an experiment to determine resistance using a voltmeter and an ammeter
Set the power supply to a low voltage (1 or 2 V)
Use a voltmeter and ammeter to obtain voltage and current readings
Substitute readings into the equation R = V ÷ I
What is the relationship between length and resistance?
Length is proportional to resistance
What is the relationship between cross-sectional area of a wire and resistance?
Cross-sectional area of a wire is inversely proportional to resistance
Cross-sectional area of a wire is inversely proportional to resistance
Battery/power source → circuit components → surroundings
What is the formula for power?
P = VI
What is the formula for energy transferred?
E = Pt / E = VIt
Describe the current in a series circuit?
The same at every point
What is the sum of combined resistances in a series circuit equal to?
The sum on individual resistances
What is the sum of the p.d.s across the components in a series circuit equal to?
Total p.d. across the supply
Describe current in a parallel circuit?
Current from the source is larger than the current in each branch
Describe current in a parallel circuit?
Current from the source is larger than the current in each branch
Describe combined resistance of two resistors in parallel?
Less than that of either resistor by itself
What is current from the source in a parallel circuit equal to?
Currents in separate branches
What are the advantages of connecting lamps in parallel in a lighting circuit?
Lamps can be individually controlled, using their own switches
If one lamp stops to function the other will continue to work
Each lamp gets maximum p.d.
What is a variable potential divider also known as?
Potentiometer
What is a variable potential divider also known as?
Potentiometer
What is the relationship between p.d. and resistance across 2 resistors?
The resistor with a largest resistance will have greater p.d. than the other
What does a potentiometer consist of?
A coil of wire with a sliding contact along it
What does moving the sliding contact do in a potentiometer?
- Increases or decreases the length of wire
- Increases or decreases resistance (and hence p.d.)
What is a thermistor?
A resistor that changes its resistance when it is heated
What is the relationship between temperature and resistance in thermistors?
Temperature is inversely proportional to resistance
What is a light-dependent resistor?
A resistor that changes its resistance when light shines upon it
What is the relationship between light and resistance in an LDR?
Light is inversely proportional to resistance
What is the formula to find V.out from a potentiometer?
V.out = V.in × (R2 ÷ R.Total)
What is the function of a transducer?
Takes in physical energy and converts it into an electrical signal that can be read
What does a relay consist of?
A coil and magnetic switch
Describe the action of a relay
When there is a current in the coil it creates a magnetic effect
This attracts the switch, closing it
This can be used to control a switch in a separate circuit
What is a diode?
A component which only allows charge to flow in one direction through it
In what direction must the diode point to to have a current?
Positive to negative
What does a diode do to an a.c. power supply?
Rectification
What is rectification?
Conversion of a.c. to d.c.
How is an LDR used, along with a relay, to control a bulb?
*Light sensitive switch
When it gets darker, resistance increases
This causes an increase in p.d.
Increase in p.d. causes an increase in current in the coil
A large enough current in the coil will attract the switch, closing it
This allows a current to pass through
Exam tip ; )
Temperature-controlled (temperature-operated alarm) circuits work the same way as LDR-controlled circuits
What are the 2 types of signal?
Analogue and digital
What is an analogue signal?
One that is continuously varying and can take any value
What is a digital signal?
One that can only take one of two (discrete) states
What is a logic gate?
An electronic component that gives a particular output, depending on the input(s) to it
What is the hazard of damaged insulation?
Electric shock
What is the hazard of overheating of cables?
Fire
What is the hazard of damp conditions?
Electrocution
What does a fuse do?
Protects a circuit
Explain the use of fuses
If the current in the wire becomes too large
The wire heats up and melts
The wire breaks, stopping the current and breaking the circuit
Explain choosing appropriate fuse ratings
The fuse should always have a higher current rating than that needed by the appliance - without being too high
Explain the use of circuit breakers
An automatic switch which
Current rises over a specified value
Electromagnet pulls the contacts apart
Breaking the circuit
Explain the action of earth wire
Earth wire provides a low resistance path to the earth
This causes a surge of current in the earth wire and also in the live wire
High current in the fuse causes it to melt and break
This cuts off the supply of electricity to the appliance
How is an e.m.f. induced in a conductor?
When the conductor is moved across the magnetic field, it cuts through the field lines
This induces an e.m.f. in the conductor
What does the direction of the induced e.m.f. always do?
Opposes the change that produces it
Describe an experiment to demonstrate electromagnetic induction
If a wire is passes across a magnetic field/changing magnetic field
A small e.m.f. is induced in the wire
This can be detected by a galvanometer
What are 3 ways to increase the magnitude of an induced e.m.f.?
Moving the wire faster
Using a stronger magnet
Adding more turns to the coil
What is alternating current (a.c.)?
One that continuously changes its direction
What is direct current (d.c.)?
One that is steady and only flows in one direction
What is direct current (d.c.)?
One that is steady and only flows in one direction
What is direct current (d.c.)?
One that is steady and only flows in one direction
Which electrical components produce alternating current?
Mains electricity
Which electrical components produce direct current?
Electric cells (batteries)
Describe and explain a rotating-coil generator and the use of slip rings
As the coil rotates, it cuts through field lines
This induces an e.m.f. and hence a current
The size of the e.m.f. could be increased by: (List)
Slip rings transfer current to the metal brushes whilst allowing the coil to rotate freely
What are slip rings in a rotating-coil generator attached to?
The ends of the coil
When is induced e.m.f. greatest in a rotating-coil generator?
When the coil is horizontal
Why is induced e.m.f. greatest when the coil is horizontal in a rotating-coil generator?
The coil cuts through magnetic field lines at the fastest rate
Exam tip ; )
Remember to say ‘add more turns to the coil’ and not ‘add more coils’ – if you give the second one it will be marked wrong.
What is a transformer?
An electrical device that can be used to increase or decrease the voltage of an a.c.
What can a transformer not work with?
Direct current
What does a step-up transformer do?
Increases the voltage of a power source
What does a step-down transformer do?
Decreases the voltage of a power source
How does a transformer work?
A.c. is supplied to the primary coil
A changing magnetic field is produced by the coil
The field passes through the soft iron core and secondary coil
Changing field in the secondary coil induces an e.m.f.
When is a transformer used?
High-voltage transmission of electricity
What is are 2 advantages of high-voltage transmission?
- Reduces power loss
- Thinner, cheaper cables can be used
Explain why power losses in cables are lower when the voltage is high
Current causes heating in wires
Resulting in power loss
Increasing voltage reduces current
Smaller current causes less heating in wires
Less power is lost
What is created when there is a current in a wire or solenoid?
A magnetic field around the wire
What do magnetic field lines created by a current in a wire or solenoid look like?
Concentric circles around the wire
What do strength and direction of a magnetic field created by a current depend on?
Direction and magnitude of current
Exam tip ; )
The strength and direction of the field depend on the size and direction of the current:
- If the current is increased, the field will get stronger
- If the direction of the current is changed, the direction of the field will change
The strength of the magnetic field is related to the distance between the field lines:
As the field lines spread out, the field gets weaker
The field lines around a wire get further apart the further they are from the wire
What is the direction of a magnetic field line at a point?
The direction of the force on the N pole of a magnet at that point
What is the effect of reversing current on a current carrying conductor in a magnetic field?
Direction of force reverses
What is the effect of reversing direction of field on a current carrying conductor in a magnetic field?
Direction of force reverses
How does a magnetic field deflect a charged particle?
- Moving charges forms a current
- The field exerts a force on the particle, deflecting it
How can the deflection of charged particles be demonstrated?
- Using a cathode ray tube and a pair of magnets
- Passing a collimated beam of beta particles between the poles of a horseshoe magnet
How can the turning effect of a current carrying coil in a magnetic field be increased?
Increasing the number of turns on the coil
Increasing the current
Increasing the strength of the magnetic field
Describe the action of a d.c. motor
Current passes through the coil
Current creates a magnetic field around the coil
This interacts with the field of the magnets
Force is exerted on the coil
This pushes one side of the coil up and the other down
Creating a turning effect
The split-ring commutator reverses the direction of current in the current every half turn
Why does the split-ring commutator reverse the direction of current in a d.c. motor every half turn?
To reverse the direction of force and keep the coil spinning
What materials are magnets attracted to?
iron
steel
nickel
cobalt
What are the forces between magnets?
Same poles repel
Opposite poles attract
Magnetic field lines important points:
- when the magnetic field lines are close together what does this indicate?
- what does the direction of the magnetic field show?
- what should field lines never do?
- Magnetic field is strongest when field lines are closer together
- The direction of the magnetic field shows which direction a compass would point
- Magnetic field lines point away from north and towards south
- Field lines never cross
What is a uniform field?
constant magnetic field - constant direction + strength
arrows point in same direction, field lines are equally spaced apart, magnetic field strength = constant
can be made in the space between opposite poles of a magnet
Describe the properties of magnetically hard and soft materials
permanent magnets are made of magnetically hard materials eg steel, which retain their magnetism once magnetised
temporary magnets eg electromagnets are magnetically soft eg iron, which lose their magnetism once they are no longer exposed to a magnetic field and are easy to magnetise and de-magnetise
What does a magnetic field line do?
Indicate the size and direction of the magnetic force
Describe the method for investigating the magnetic field pattern for a single magnet
Place a bar magnet on the centre of a piece of paper
Bring a plotting compass close to end of North Pole of magnet but stop it touching magnet
Mark where the compass needle is pointing next to compass
Lift up plotting compass and place the centre of it on the mark made on the paper
Repeat until the South Pole is reached, join up dots
Repeat for at least another 5 lines, starting each line near the North Pole but at a different place
Describe the method for finding the magnetic field pattern for two interacting bar magnets (north to south)
Place a sheet of paper on a horizontal surface and place two bar magnets North Pole to South Pole next to each other
Leave a gap between them at least four times width of plotting compass
Bring plotting compass close to end of North Pole and draw a small dot next to where arrowhead is pointing
Move and repeat
Repeat for at least another four lines from the North Pole
What happens when some materials are placed in a magnetic field?
Magnetism is induced
Why does magnetism occur?
movement of electrons around atom
therefore a wire carrying a current (conductor) produces a magnetic field around it
Why does magnetism occur?
movement of electrons around atom
therefore a wire carrying a current (conductor) produces a magnetic field around it
Describe the construction of solenoids
soft iron core wrapped in a coil of wire
when current flows through coil of wire it becomes magnetic
How can a coil be strengthened?
more turns in coil
iron core
increase current
What happens when you reverse the current in a solenoid?
reversing the current reverses the poles of the electromagnet
Why is iron used for the core in a solenoid, not steel?
iron is magnetically soft/ gains/loses magnetism easily
Describe how a relay works
Relay = switch controlled by electromagnet
Electromagnet with small current passing through it closes switch by pushing an armature into the contacts
Closing the switch using an electromagnet reduces chance of electric shock
Describe how a circuit breaker works
Current passes through electromagnet
Magnetic field around electromagnet not strong enough to attract contact
If there is a surge in current strength of magnetic field increases and electromagnet attracts contact
Breaks circuit
Describe how a loudspeaker works
- how can the pitch and loudness of the sound be changed?
Alternating current is passed through electromagnet
Changes polarity of electromagnet
Electromagnet attracts and then repels permanent magnet
Causes magnet and cone to vibrate creating a soundwave
pitch = change frequency of alternate current which change frequency of vibration
loudness = increase current in electromagnet, stronger magnetic field around it, larger force of attraction and repulsion between it and fixed magnet -> increased amplitude of vibration
How does an electric bell work?
1.button pressed
2. Current flows
3. Electromagnet operates
4. Attracts armature
5. Strikes gong
6. Circuit broken at screw
7. Electromagnet turns off
What is a conventional current?
Current that flows from + to - battery terminals
What is the motor effect?
force on charged particle (eg in a wire carrying an electric current) when it moves in a magnetic field as long as its motion is not parallel to the field
this is because magnetic field produced by moving charges interacts w/ magnetic field of permanent magnet
How can the force on a current carrying conductor in a magnetic field be increased?
Increasing size of magnetic field
Increasing size of current
Making angle between conventional current and magnetic field closer to 90’’ (if current (conducting wire) was moving parallel to field then there would be no force)
Use longer wire
How are alpha beta and gamma deflected by force which is perpendicular to a magnetic field?
alpha deflected in opposite direction (have opposite charge)
gamma not deflected
beta is deflected the other way to alpha
What happens to a coil in a magnetic field?
When a current flows in the coil a force is applied to it
Current in each side of the loop is in opposite directions
Force on one side is upwards and on other side it is downwards
One side of the loop will be forced to the top, the other to the bottom (rotate clockwise)
The coil will then oscillate and eventually line up vertically with one side at the top and one at the bottom
How can an electric motor’s strength be increased?
increase current
increase number of turns in coil
increase strength of magnetic field
increase length of wire
How does a simple DC motor work?
Current flows in a coil of wire
Creates a magnetic field around coil of wire
Interacts with field from permanent magnet
Causes force on loop of wire which causes it to start rotating
Split ring commutator changes direction of current every half turn as it spins, reversing direction of current and reversing direction of force so it can keep spinning
What is the function of the coil of wire in dc motor
Directs current through magnetic field
What do the brushes do
Prevents wires getting tangled allowing coil of wire to rotate
What does the split ring commutator do
changes direction of current every half turn
changes direction of force, allowing it to rotate for more than half a turn
What happens when a conductor is moved across a magnetic field? (electromagnetic induction)
conductor/wire cuts through magnetic field lines
potential difference is induced
if wire is part of complete circuit a current will flow
wire and magnetic field must move perpendicular to each other (no field lines are cut if move parallel to each other, no potential difference is therefore induced)
How can a larger potential difference be induced?
More turns in coil
Stronger magnetic field
Faster movement
How can the direction of the current be changed?
Changing direction of magnetic field
Changing direction of movement
What is a generator?
Device that electrically transfers kinetic store of generator into other stores (opposite of electric motor)
Produce electricity
Describe how an AC generator works
coil of wire is spun in magnetic field
coil cuts through magnetic field lines so potential difference is induced
as coil spins each part of wire sometimes movs up and sometimes down
because it changes direction current that is induced is an alternating current (changes direction)
How does a DC generator (dynamo) generate a direct current?
split ring commutator converts alternating current into direct current
How does a bike dynamo work?
spins magnet in a coil as wheels move instead of coil of wire
magnetic field lines cut through sides of coil inducing potential difference in coil
magnetic field constantly changing direction as it rotates, output current is also alternating - so unlike normals dynamos it produces a ac not dc current
What do the slip rings do in an AC generator?
connect coil to brushes, maintains a/c inside coil to a/c in outer circuit
what kind of materials are electrical conductors?
silver, copper, alumininum, steel (delocalised electrons)
what kind of materials are insulators?
rubber, plastic, glass, wood
but can conduct a bit of static electricity, just not a flow of charge
what happens when a cloth and rod are rubbed together?
initially neutral
force of friction causes electrons to move
if the cloth transfers some electrons to the rod, what charge will the objects now have?
cloth: more protons -> positive
rod: more electrons -> negative
what charge does polythene get when charged with cloth?
negative
why are charges evenly distributed in a conductor?
electrons are repelled by other electrons, and these are attracted to protons
is it possible to move positive charges?
no
is it possible to move negative charges?
yes! electrons are not within nucleus
charging via induction
What happens if you bring a negatively charged rod near a piece of paper (neutral)?
rod approaches paper, electrons in paper are repelled away from rod
makes one side of paper negative and other positive
positive side is closer to rod
resultant force = attractive as opposite charges are closer
paper sticks to rod
works the same if rod is positive -> electrons attracted to one side
charging by contact
How can a neutral object become charged by touching a charged object?
electrons flow onto (if object positively charged) or off of object (if negatively charged, as the elctrons repel each other and move away from each other)
what is earthing?
charge on object is neutralised by transfer of electrons
why would electrons try to flow off of a negatively charged object?
they are repelled
why would electrons try to flow onto a positively charged object?
they are attracted
what is the purpose of a photocopier?
to print copies of an image/document
how does a photocopier work?
- what is done to the drum before toner is added?
image of document is projected on to positively charged drum
drum loses charge in light areas and keeps positive charge in dark areas (light areas conduct electricity)
negatively charged toner sticks to where there is a positive charge on drum
transferred onto paper
how do inkjet printers work?
ink droplets positively charged via charging electrodes
pass between oppositely charged plates
computer controls plate voltage
droplets attracted towards negatively charged plate + repelled from positive one (the deflection can be changed by altering strength of plate charges)
direction changed by changing the charge of the plates
how does electrostatic paint sprayer work?
paint droplets given charge by paint sprayer so repel each other-> fine mist and covers large area
oppositely charge car
less waste, more even coverage
how does a dust precipitator work?
reduces pollution
dust becomes charged by passing thru negatively charged plates
dust removed from air as attracted to positively charged plates
dust removed from collecting plates as it is neutralised upon touching the plates, falls into dust collection
how is lightning made?
droplets + ice crystals rub against each other in cloud during storm, creates static electrical charge
clouds have positive and negative side
negative side is at the bottom -> when there is enough charge in the cloud it is released via lightning bolt, which is goes to anything oppositely charged (eg lightning conductor, positively charged, earths it)
why is refuelling airplane potentially dangerous?
build up of static charge : friction between fuel and pipe, fuel gains charge -> could spark and fuel could ignite and explode (also friction with air whilst flying can cause a charge)
explosion prevented by fuel tank being connected to earth w conductor
carriers charge through to earth to reduce sparking risk
what is the electric current?
Rate of flow of charge
what is the equation to calculate current from time and charge?
give units
current = charge(c)/time(s)
I=q/T
what is the definition of potential difference?
(voltage)
energy gained or lost by each electron (energy transferred per unit charge)
how do you calculate potential difference from energy transferred and charge?
potential difference = energy transferred (J) /charge (C)
v= e/q
what does an ammeter do?
measure current
what does a voltmeter do and where should it be in a circuit?
measures voltage
should be paralell to component
what is resistance?
what determines/reduces flow of current
what happens to the current when the potential difference increases?
increases (electrons gain more energy, move faster)
what happens to the current when the resistance of a circuit increases?
decreases
what is the equation v=ir?
potential difference = current x resistance
how do you convert from mA to A?
divide by 1000
there are 1000 milliamps in one amp
what is the unit for resistance?
ohm
what affects the resistance of a wire?
just list 4 things
material
length
thickness
temperature
why does length affect resistance?
length of wire is increased, electrons have to travel further
chance of collisions increases
causes resistance to increase
why does thickness affect resistance?
increasing thickness of wire increases area that electrons can flow through
decreases chance of collisions w metal ions
causes resistance to decrease
why does temperature affect resistance?
ions vibrate faster
harder for electrons to flow
temp increases -> resistance increases
what is ohms law?
current is directly proportional to poterntial difference if temp is constant
thus component that obeys ohms law has constant resistance (the gradient)
why does the resistance of a wire increase when the current increases?
current increases -> more FREQUENT collisions of electrons w ions
rate of energy transer to ions increases
ions vibrate faster
resistance of wire increases
what can a diode or LED be used to determine?
whether a current is flowing
what are the similarities between resistors and bulbs?
reduce the current in a circuit
have some resistance
energy is transferred
what are the similarities between resistors and bulbs?
reduce the current in a circuit
have some resistance
energy is transferred
what are the differences between resistors and bulbs?
resistor has fixed resistance
bulb has a changing resistance
bulb emits light
what are the similarities between bulbs and light emitting diodes?
emit light
resistance isn’t constant
energy transferred
what are the differences between bulbs and light emitting diodes?
current can only flow in one direction
bulb -> current can flow in either direction
what is the use of a thermistor?
sense diff temps and switch off an oven when it gets too hot
why does the resistance of a thermistor drop with temperature?
as it gets hotter -> more electrons get excited + become available for conduction
resistance drops
what are LDRS used for
sense light levels and switch on streetlamps when it gets dark
why does the resistance decrease with the increase of light intensity?
as it gets brighter, more electrons get excited and become available for conduction -> R drops
why would a larger potential difference increase total amount of energy that is transferred?
each electron transfers more energy to the bulbs
why would a larger current increase the total amount of energy that is transferred?
more electrons pass through bulb each second
what causes heating?
electric current
what is the equation for total electrical energy transferred?
energy transferred= current x voltage x time
E=IVT (energy in joules)
what is the equation for electrical power?
P=IV
power in watts
what is the equation for electrical power?
P=IV
power in watts
what equation links power energy and time?
power = energy/time
in a series circuit there is ____ possible _ for an electron to ___
there is only one path
in a paralell circuit
the electrons can…
take different paths around the circuit
why is a paralell circuit used for normal room lights?
so they can be switched on and off independently
if one bulb blows in a paralell circuit…
the other bulbs will still work
whereas in a series they would all go out
it is possible to turn on individual bulbs in a _ circuit
paralell
What are the rules in a series circuit?
current is the same throughout circuit
potential difference is shared between components (bulbs will get dimmer the more you add)
total resistance = adding resistances of individ components
what are the rules for a paralell circuit?
potential difference is same throughout circuit
current splits at each junction (more current flows through the branch with smaller resistor)
use v=ir to calculate current
what happens when more components are added to a paralell circuit?
increases current
resistance of circuit decreases
what is alternating current?
electrons continually change direction
mains electricity is ac
what type of current is battery?
dc (direct, electrons flow in same direction)
what are the differences between batteries and mains?
cells have chemical stores which are transferred electrically, mains depends
constant low voltage, alternating high voltage
DC, AC
50Hz is typical freq for UK mains
why is the wire in the cable made from copper?
conductor, flexible
why is the wire casing made from flexible plastic?
insulator, flexible
why are the plug pins made from brass?
conductor, rigid
why is the plug casing made from stiff plastic?
insulator, rigid
what colour is the live wire and what does it do (plug)
brown
high voltage
carries current that alternates between neg and pos volatge
what colour is the earth wire and what does it do?
green
safety wire used to earth appliances w a metal case
what colour is the neutral wire?
blue
completes circuit
kept at 0 voltage
how do fuses work?
why are they needed and explain fuse ratings
surge in current could damage an electrical appliance
fuse contains thin wire -> melts if current too hgh
this breaks cirucit and elecrticity unable to flow
fuse rating gives current for which the fuse will melt eg 3A fuse
should be slightly larger than normal operating current
what is earthing?
reduces chance of shock
earth wire connected to casing of metal device to provide alternative path for current if fault occues
if live wire becomes loose and touches metal case, v large current flows (low resistance) to eath and melts the fuse wire breaking circuit
what are double insulated appliances?
plastic casess
case cannot become live, doesn’t conduct
if one layer of insulation damaged the appliance is still safe
what are circuit breakers?
circuit broken by using an electromagnet to pull two wires apart
How does an induction cooker work?
use alternating magnetic fields to induce currents in saucepan, which heats saucepan
little energy transferred to thermal store of surroundings, making it v efficient
Describe the structure of a transformer
Alternating current in primary coil produces continually changing magnetic field within iron core
Iron core guides magnetic field into secondary coil
Field lines continually moving because magnetic field is changing strength
Potential difference is induced across secondary coil because secondary coil cuts through field lines
If secondary coil is attached to a circuit then current will flow
What is a transformer used for?
transforming alternating voltage from a larger/smaller value to a smaller/larger value
What does a step up transformer do to voltage current and power output, and why
more turns in secondary coil
increases voltage
decreases current
no effect on output power (because P=IV)
What does a step down transformer do to voltage, current and power?
fewer turns in secondary coil
decreases voltage
increases current
power stays same
What happens when voltage is increased using step up transformer?
Because power = current x voltage, current decreases
What is the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer?
Vp/Vs = Np/Ns
P= primary
S= secondary
N= number of turns
What is the relationship between input and output power when a transformer is 100% efficient?
input power (IpVp) = output power (IsVs)
How do you calculate efficiency of transformer?
Power in secondary/power in primary x 100
How do you calculate efficiency of transformer?
Power in secondary/power in primary x 100
Why do transformers not work with direct current?
In direct current, magnetic field would be constant so no field lines would be cut and there would be no potential difference generated
How is energy wasted in transformers (3 ways)
Heat is produced in coils, since current is flowing through them
Magnetic field in core produces current which also causes heating
Magnetic field may leak out of core if there is an air gap between primary and secondary
How do you make transformers efficient?
heating in wires -> use low resistance wires
eddy currents in iron core -> laminate iron core
leaking magnetic field -> use an iron core to direct current through coils of wire
How are transformers used on an industrial scale?
used to increase voltage of electricity supply after it has left power station -> lower current, less heating of wires, less energy wasted
voltage reduced at sub stations before reachers consumer, as this is safer
Ion
A charged atom
Rubbing a polythene rod with a dry cloth…
Transfers electrons to the surface atoms of rod from the cloth, so the rod becomes negatively charged.
Electrons gained in the polythene rod.
Atoms in dry cloth becomes positive as a result of losing electrons.
Rubbing a perspex rod with a dry cloth…
Transfers electrons from the surface atoms of the rod to the cloth, so the rod becomes positively charged.
Atoms become positive in the perspex rod as a result of losing electrons.
Electrons gained by dry cloth.
Battery symbol
Battery.
A cell is necessary to push electrons around a complete circuit. A battery consists of two or more cells.
Switch symbol
Switch.
A switch enables the current in a circuit to be switched on or off.
Indicator symbol
Indicator.
An indicator is designed to emit light as a signal when a current passes through it or as a light source such as a bulb.
Ammeter symbol
Ammeter.
Used to measure current.
Resistor symbol
Resistor.
Limits the current in a circuit.
Variable Resistor symbol
Variable Resistor.
allows the current to be varied.
Voltmeter symbol
Voltmeter.
Used to measure voltage.
Diode symbol
Diode
allows current through in one direction only.
LED symbol
LED
Emits light when a current passes through it.
Fuse symbol
Fuse
designed to melt and break the circuit if the current through it is greater than a certain amount.
Heater symbol
__ITTTTTTI__
LLLLLLLI
Heater.
Designed to transfer electrical energy to heat the surroundings.
What way in a circuit does current flow?
+ to -
The size of an electric current…
… is the rate of flow of an electric charge.
What is the unit of electric charge?
Coulombs (C)
Current =
Charge flow (C) / Time taken (s)
Potential difference across the component (V) =
Energy transferred (J) / Charge (C)
What causes resistance?
Electrons having to push past lots of vibrating ions in a metal filament.
resistance (Ω) =
Voltage / Current
Define Ohm’s Law
The current through a resistor at constant temperature is directly proportional to the voltage across the resistor.
2 Non-Ohmic Conductors
Filament Bulb
Diode
1 Ohmic Conductor
Resistor
Thermistor symbol
Thermistor
A resistor that depends on temperature.
Light Dependent Resistor symbol
Light Dependent Resistor
A resistor that depends on the intensity of light.
In a series circuit, how much current passes through each component?
The same amount.
In a series circit, what happens to the voltage?
It is shared between all of the components.
In a parallel circuit, what happens to the current?
It is split up, and runs through separate components.
In parallel, what is the voltage like for each component?
the same.
Direct Current
Electric current in a circuit that is in one direction only.
Alternating Current
Electric current in a circuit that repeatedly reverses its direction.
What is the mains frequency in the UK?
50Hz (AC)
Live wire
The mains wire that has a voltage that alternates between +325 and -325v (in Europe)
Neutral Wire
The wire of a mains circuit that is earthed at the local substation so its potential is close to zero.
Half wave rectification
The use of a diode in a circuit with an alternating supply pd to allow current in only one direction every other half cycle of the supply pd.
Socket
A mains socket is used to connect the mains plug of a mains appliance to the mains circuit.
What are plug pins made of?
Brass (Its a good conductor, and it doesn’t rust or oxidise)
What is the longest pin of a three-pin plug and why?
The earth plug, because it is the first pin that connects with the wall socket, so the plug is earthed even before the other pins connect.
Fuse
A fuse contains a thin wire that melts and cuts the current off if too much current passes through it.
What does the earth wire protect?
The user
What does the fuse protect?
The appliance and the wires of a circuit.
Circuit Breaker
An electromagnetic switch that opens and cuts the current off off if too much current passes through it.
Why are circuit breakers better than fuses?
They work faster
They can be reset
Residual Current Circuit Breaker
(RCCB)
An RCCB cuts off the current in the live wire when it is different from the current in the neutral wire.
More sensitive than fuses/ normal circuit breakers.
A fuse is always connected in…
…series.
Power Supplied (W) =
Current (A) x Voltage (V)
When charge flows through a resistor…
energy is transferred to the resistor, so the resistor becomes hotter.
Kilowatt-hour
(kwh)
Electrical energy supplied to a 1kW device in 1 hour.
What is a kWh?
A unit of energy
Are low power lamps more or less efficient than high power ones?
More, because they give the same amount of light energy for the same input of electrical energy.
Filament Bulb
+ and -‘s
very inefficient
heat from the bulb makes the plastic casing go brittle and crack.
CFL’s
+ and -‘s
More efficient that filament bulbs
LED’s
+ and -‘s
Use a low voltage and low power
V. Efficient
Last much longer.
Step Up transformers
raise voltage
lower current
Step Down transformers
Lower voltage
Raise current
Describe the magnetic field patterns with like poles facing each other.
Bend away from the midpoint.
Describe the magnetic field patterns with unlike poles facing each other.
The field between the poles is uniform.
What is special about a uniform field?
The magnetic field lines are parallel to each other.
What metal is used for the core of electromagnets?
Iron (Easily magnetised and demagnetised)
Name for metals that can be magnetised/ demagnetised easily
ferrous metals
4 examples of electromagnets
> Scrapyard cranes > Circuit breakers > Electric bells > Relays
Increasing _____ makes the magnetic field of an electromagnet stronger.
Current
What surrounds an electromagnets core?
A coil of insulated wire
How can the size of the electromagnetic force be increased
Increasing the current Using a stronger magnet
In the motor effect, when is the force greatest?
When the wire is perpendicular to the magnetic field.
What are the magnetic field lines around a wire?
Circles centred on the wire in a plane perpendicular to the wire.
What are the magnetic field lines around a wire?
Circles centred on the wire in a plane perpendicular to the wire.
Reversing the direction of the current…
…reverses the magnetic field lines
The Motor Effect
When a CURRENT is passed along a wire in a magnetic field, A FORCE IS EXERTED on the wire by the magnetic field.
In the motor effect, when is the force zero?
When the wire is parallel to the magnetic field.
Fleming’s Left Hand Rule
Movement
Field (N to S)
Current
Commutator for DC
Split Ring- Reverse the current on every half turn to keep the motor spinning constantly.
Commutator for AC
Slip-ring - Doesn’t alternate the current because it alternates already.
Electromagnetic Induction
The process of inducing a potential difference/voltage in a wire by moving the wire so it cuts across the lines of force of a magnetic field.
The Generator Effect
The production of voltage using a changing magnetic field.
What never changes in a DC generator?
The Voltage’s Polarity
What is a simple AC generator made up of?
A coil that spins in a uniform magnetic field.
When is the size of an induced p.d/voltage on an AC generator zero?
When the coil is perpendicular to the magnetic field lines.
When is the size of the induced p.d/voltage on an AC generator greatest?
When the coild is parallel to the direction of the magnetic field.
What types of current does a generator work with?
AC
The Solenoid Rule
Direction of coil tells us which pole it corresponds to.
North- iN
sOUTh- OUT
4 facts about electromagnetic induction.
No current when the wire is stationary
A current is generated when the magnet moves but not the wire.
Faster movement = More current
Current reverses when the direction of motion reverses.
What must there be for electromagnetic induction to occur?
A changing magnetic field.
3 Acronyms for Transformers
Vicky needs 5 A’s
Voltage Can Make Vicky Cry
PC Ian Can’t Solve Crime
A A A A A
V C M V C
PC IC SC
3 steps of a transformer based on the acronyms
An alternating voltage in the primary coil creates an alternating current there.
This creates an alternating magnetic field in the iron core
This induces an alternating voltage in the secondary coil, which induces an alternating current.
3 steps of a transformer based on the acronyms
An alternating voltage in the primary coil creates an alternating current there.
This creates an alternating magnetic field in the iron core
This induces an alternating voltage in the secondary coil, which induces an alternating current.
A step up transformer…
Has more turns on the secondary coil, so increases voltage.
A step down transformer…
Has less turns on the secondary coil, so decreases voltage.
The higher the grid voltage…
… the better the efficiency of the transformer.
Equasion for transformer efficiency:
Vp / Ip = Vs / Is
V = Voltage
I = Current
The transformer equation:
Vp / Vs = (No. of turns) Np / Ns
The faster the coil rotates…
…The greater the frequency of the AC current
+
The larger the peak value of the AC current.
Switch mode transformer.
A transformer that works between 50 000 Hz and 200 000 Hz
What is the core of a switch mode transformer made of?
Ferrite
Advantages of a switch mode transformer
> Lighter and Smaller
> Higher frequency
Voltage of the National Grid
132 000 V
Why does the voltage of the national grid being higher make it more efficient?
If the voltage was lower more curren would be needed for the same amount of power to be transferred. The cables would overheat and result in more wasted energy.
What direction are the magnetic field lines in?
North to South
Which equation do we tend to use to compare the power of resistors that are connected in series?
P= I^2 R
Which equation do we tend to use to compare the resistors that are connected in parallel?
P= V^2/ R
What is direct current?
Current flowing in one direction
What is alternating current?
Changes direction
What is a variable resistor?
It is a controlled resistor and it is used as volume control and as potentiometer- controls the current.
What does a diode do?
Allows current to flow in one way only
How should the diode be placed for the circuit to work?
The straight line should be on the side of the negative - terminal of the battery
What is diode made to do?
Change ac to dc by rectification
What are short circuits?
Is a low resistance path for the current to follow
What is the earthing wire used for in our homes?
Safety precautions to prevent electric shock
What are dangers of electricity?
Damaged insulation
Overheating of cables
Damp conditions since water is a conductor
What is a transducer?
A device for converting a non-electrical input into an electrical signal
What does the input sensor do?
Detects changes in the environment and converts them to electrical signal
Give examples for input devices
Thermistor
LDR
Thermocouple
Give examples for output devices
LED’s
Heater
Bell
Loudspeaker
What are analogue signals?
Handled information which is represented by continuous change and flow, such as voltage and current
What are digital signals?
Represent data in discrete units
Give the truth table for an OR gate
0 1 1
1 0 1
1 1 1
0 0 0
Give the truth table for an AND gate
0 0 0
0 1 0
1 0 0
1 1 1
Give the truth table for a NOT gate
0 1
1 0
Give the truth table for a NAND gate
0 0 1
1 0 1
0 1 1
1 1 0
Give the truth table for a NOR gate
0 0 1
1 0 0
0 1 0
1 1 0
Draw on a piece of paper ALL of the logic gates
.
List magnetic materials
Iron
Steel
Nickel
Cobalt
What are ferromagnetic materials?
Iron
Steel
Give an example of a hard magnet material
Steel
What does a hard magnetic material mean?
Hard to magnetize- long time to magnetize
Permanent magnet
What does a soft magnetic material mean?
Easy to magnetise- short time to magnetize
Temporary magnet
Why are some materials attracted to magnets?
Because of the existence of tiny magnets inside. When a magnetic is brought near it, its tiny magnets are arranged uniformly
How can magnetism be made?
By induction
By stroking
By electrically
How can you magnetise electrically?
Place magnetic material in a solenoid connected to dc current
How can magnetism be destroyed?
Hammering a magnet
Heating
Dropping a magnet
Demagnetising by using reduced AC through a coil of wire wrapped round a magnet
What is the magnetic field?
The area around a permanent magnet or a wire carrying a current.
Describe the straight wire’s magnetic field
Concentric circles
What can a solenoid be used for?
Circuit breaker
Magnetic relay
What is the purpose of a relay?
Control high voltage system with low voltage or control high current system with low current
Describe the relay process
The low voltage circuit is on so there is a current in the solenoid. The magnetic field in the solenoid attracts the armature and the current flows through the contacts so the high voltage is on.
What is a circuit breaker?
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit.
What increases the magnetic force?
Increasing current
Stronger magnet
Increasing length of wire in the field
What are parts of an electrical motor
Coil of wire
Two magnets
Split ring commutator
Brushes
