P15 Flashcards
Electromagnetism (58 cards)
1
Q
Where on a magnet are magnetic forces the strongest ?
A
at the poles
2
Q
What do magnetic poles do if they are alike/opposite ?
A
like poles repel whilst opposite poles attract
3
Q
Why are permanent magnets made of steel rather than iron or other magnetic materials ?
A
doesn’t lose its magnetism as quickly as other magnetic materials
4
Q
What is a permanent magnet ?
A
a magnet that is always magnetic
- it produces its OWN magnetic field
[] bar magnets are good examples of these
5
Q
What is an induced magnet ?
A
an unmagnetised magnetic object that becomes a magnet when placed in a magnetic field
- these magnets lose their magnetism when taken out of the magnetic field
- induced magnetism always causes a force of attraction
6
Q
Which are the key four magnetic materials ?
A
- iron
- steel
- cobalt
- nickel
7
Q
What is a magnetic field ?
A
a region around a magnet where the non-contact force of magnetism acts on other magnets or magnetic materials
- the strength of the magnetic field depends on the other magnet/magnetic material’s distance from the magnet
8
Q
Explain how to plot the magnetic field of a bar magnet using a compass
A
- place bar magnet in middle of piece of paper
- place compass near north pole of magnet
- draw x at north pole of compass
- place compass so its south pole is on the cross
- again mark where the compass’ north pole is
- repeat until one complete magnetic field line has been plotted, and join with a smooth curve
- draw an arrow on the line to represent the direction of the magnetic field
[] ALWAYS runs from north to south pole - repeat whole process starting at different points around the north pole of the bar magnet
9
Q
How can you tell that the Earth has its own magnetic field due to its core ?
A
- compasses have a small bar magnet inside
- if you hold the compass away from any magnets, the needle always points in the north-south direction
10
Q
When is a magnetic field produced around a CONDUCTING wire ?
A
when a current flows through it
11
Q
How can you prove that there is a magnetic field around a conducting wire with a current through it ?
A
- place a plotting compass next to the wire with the current off
[] the pin should point in the north-south direction due to the Earth’s magnetic field - turn on the current and observe that the needle has deflected, proving the magnetic field produced by current through a conducting wire
12
Q
What happens to the direction of the magnetic field around a conducting wire if the direction of current is reversed ?
A
the direction of the magnetic field also reverses
13
Q
What shape are the lines of force around a straight conducting wire with a current ?
A
(from above) a series of concentric (centred on the wire) circles
14
Q
What does the strength of the magnetic field of a straight conducting wire depend on ?
A
the size of the current and distance from the wire (stronger nearer)
15
Q
How can you tell the direction of the magnetic field of a wire ?
A
- the right hand grip rule
[] make a thumbs-up with your right hand
[] place your right hand in this shape on the paper with your thumb pointing in the direction of the current
[] MAKE SURE FINGERS ALWAYS FACE UP - YOU SHOULD NOT SEE THE BACK OF YOUR HAND AT ANY TIME
[] the magnetic field is in the direction that your fingers point
16
Q
What is a solenoid ?
A
long coil of insulated wire
17
Q
Describe the magnetic fields in and outside of a solenoid
A
inside:
- strong and uniform (in the same direction)
- parallel to the axis of the solenoid
outside:
- similar to a bar magnet BUT with the exception that each field line is a complete loop as it passes through the inside of the solenoid also
18
Q
What is an electromagnet ?
A
a solenoid with current through it and an iron core
- the iron core is magnetised by the magnetic field of the solenoid/wire
- can be turned on/off
19
Q
How can you increase the strength of an electromagnet/solenoid’s magnetic field ?
A
- increase the size of the current
- increase number of turns/coils of solenoid
20
Q
Give 4 examples of devices where electromagnets are used
A
- scrapyard crane
- electric bell
- circuit breaker
- relay
21
Q
What is a relay and how does it work ?
A
- device used to switch dangerously high voltage circuits on and off
how it works: - two circuits; high voltage and low voltage
- in the low voltage circuit, there is a switch and an electromagnet
- in the high voltage circuit, there are metal contacts in place of a switch, connected to an iron block and a spring keeping the two contacts apart
- when the low voltage circuit is switched on, the electromagnet is switched on and produces a magnetic field
- the iron block is attracted to the electromagnet, making the metal contacts touch and closing the high voltage circuit, turning it on
- once the low voltage circuit is turned off, there is no magnetic field and the spring breaks the high voltage circuit
22
Q
Describe how an electric doorbell works
A
- in the circuit, an iron contact connected to the clapper that hits the bell is kept in place to complete the circuit by a spring, and there is also an electromagnet
- when the buzzer is pressed, the circuit closes and current flows through the circuit
- the current activates the electromagnet
- the electromagnet now produces a magnetic field and attracts the iron contact
- this makes the clapper hit the bell, and it sounds
- however, this breaks the circuit, so the electromagnet turns off and the field goes away
- the spring then pulls the iron contact back into place completing the circuit again
- the cycle repeats very quickly until the buzzer is released
23
Q
How does a circuit breaker work ?
A
- electromagnet in series with switch held closed by a spring
- when current is too large, electromagnet activated and switch is pulled open by the magnetic field
- stays open, breaking the circuit until reset manually
24
Q
What is the motor effect ?
A
when an upwards force acts on a wire with a current through it whilst it is in a magnetic field
- in other words, the force acting on a wire produced by the interaction of the wire’s magnetic field due to current and another magnetic field
25
What can increase the force produced in the motor effect ?
- longer conducting wire
- stronger magnet used
- more current passed through the wire
- making sure the wire is perpendicular to the magnetic field
[] if parallel instead, the force is zero
26
Give the equation for calculating the size of the force produced in the motor effect (with units)
force (N) = magnetic flux density (T) x current (A) x length of conductor (m)
NOTE: only applies to a wire perpendicular to the magnetic field
27
What is the magnetic flux density of a magnetic field ?
measure of the strength of a magnetic field in the unit tesla
28
Describe how to determine the direction of the force experienced by a wire in the motor effect
Fleming's left hand rule
- first finger = F for magnetic FIELD direction
- second finger = C for CURRENT
- thumb = M for MOVEMENT/direction of force experienced
hand should be in a scissor shape
29
Describe how the motor effect is used by electric motors
- there is a rectangular coil of insulated wire with a current through it in a magnetic field
- coil connected to battery as is fixed to split ring commutator connected to two metal/graphite brushes connected to the circuit
- when current passed through coil forces act in opposite directions on either side of the rectangle because current runs in opposite directions on either side
[] there is now a moment on either side
[] so it spins
- when the coil rotates 90 degrees (one half-turn), the split in the metal ring breaks the circuit momentarily
- the coil keeps rotating due to momentum
- when the ring again contacts the brushes and current passes through, the current is reversed
- the split-ring commutator reverses the current and forces the coil to keep turning in the same direction
30
Describe how the motor effect is used in moving-coil loudspeakers and headphones
- inside the speaker, there is a cone, with a conducting wire wrapped around the thin, tubular end towards the inside
[] this coil of wire is connected to an AC electrical supply
[] inside the tubular end of the cone, there is the middle protrusion of an E-shaped permanent magnet (the NORTH pole)
- magnetic field generated when current passes through coil of wire and interacts with permanent magnet's magnetic field
- attraction/repulsion of both magnetic fields on one another create resultant force causing the cone to move
- when current reverses direction due to AC, direction of force on cone also reverses
- cone moves in and out
- these vibrations create soundwaves
- by changing AC supply frequency, can change frequency of vibration of cone
[] higher freq. = higher pitch sound
[] lower freq. = lower pitched sound
- increasing size of current supplied increases amplitude of soundwaves, thus increasing the volume
31
What is the generator effect ?
effect of inducing p.d or current by using a magnetic field (thus generating p.d/electricity)
32
Describe how to make the generator effect stronger (creating a larger induced p.d)
- use a coiled wire (with lots of turns)
- use a stronger magnetic field
- move the wire more rapidly through the magnetic field
33
Describe how the generator effect works
FOR COMPLETE CIRCUIT ONLY !!
- CIRCUIT moved through the magnetic field between two poles (direction of movement perpendicular to direction of field)
- when wire is moving, CURRENT is induced
- if circuit stops moving, induction stops
- when direction of circuit's movement reverses (eg. going down instead of up), direction of current reverses also
34
What is induced potential ?
the p.d induced at the ends of a wire when moved through a magnetic field
35
Describe how the generator effect works in a circuit with a coiled loop of wire (solenoid)
- instead of moving circuit through field, move bar magnet in and out of solenoid
- when north pole of magnet pushes into solenoid, current is induced and the solenoid has its own magnetic field (opposes magnetic field of magnet so becomes a north pole)
- like poles repel, so bar magnet is pushed outwards
- when bar magnet moves outwards, solenoid becomes south pole and ATTRACTS the north pole of the bar magnet again
- because direction of current reverses when magnet moves out
- so it goes back into the solenoid
36
Why is work done when a magnet is inserted/pulled out of a solenoid ?
- the induced current creates a magnetic field opposing the magnetic field of the bar magnet
- so it is harder to pull out/push in the magnet
- so work is done because energy is transferred from the movement of the magnet into the movement of the current
37
What is an alternator ?
- USES THE GENERATOR EFFECT
- coil of wire rotating in a magnetic field
- connected to two metal ring commutators (NOT split ring)
- the left side of the coil ALWAYS connects to ring A
- the right side of the coil ALWAYS connects to ring B
38
What do commutators do ?
allow current to pass out of the coil in an alternator
39
When is the maximum p.d in an alternator achieved ?
when coil is horizontal AS wire is sweeping DIRECTLY through magnetic field at fastest possible rate
- left side moves downwards, and right side moves upwards here
40
What position is an alternator's coil in when p.d is zero ?
vertical
- no movement occurs through the magnetic field, so no p.d is induced
41
What position is an alternator's coil in when p.d is lowest ?
next time coil is horizontal AFTER passing the vertical position
- because the sides of the coil are now moving in different directions than before and thus current moves in the opposite direction
- so p.d is negative
42
Why does an alternator produce alternating current and p.d ?
because the two sides of the coil are attached to two different ring commutators
43
What is increased if the rotation speed of an alternator is increased ?
- frequency of AC
- size of AC
44
What is a dynamo ?
- coil of wire rotating through a magnetic field
- the two sides of the coil touch a different side of a SPLIT RING commutator
45
Describe how a dynamo works (360 turn)
- wire moves through magnetic field (left side moving up, right side moving down)
- left side touches left side of split ring commutator, right side touches right side
- when coil reaches vertical position, current stops, as the coil ends are between the gap in the split ring commutator, so no conduction occurs
- after vertical position, right side of coil moves upwards, and left side of coil moves downwards
- right side of coil now touches LEFT side of split ring commutator, and left side of coil now touches RIGHT side of split ring commutator until the next vertical position
46
Explain why a dynamo produces DC
at all times, the side of the coil moving upwards touches the left side of the split ring commutator, and the side of the coil moving downwards touches the right side of the split ring commutator
- THUS direction of current is never reversed, producing DC
47
When does a dynamo produce p.d at 0 volts ?
when coil is in vertical position
48
What type of current do dynamos and alternators produce respectively ?
- dynamo = DC
- alternator = AC
49
Describe the internal structure of a microphone
- E shaped permanent magnet (north pole is middle protrusion)
- coil of wire wrapped around middle protrusion and attached to thin sheet of plastic called diaphragm
50
Explain how a microphone works with an amp and moving-coil loudspeaker to amplify sound
- soundwaves hit the diaphragm of the microphone
- this makes the diaphragm and thus coil move inwards, then outwards between waves (reverses direction)
- this means that the coil of wire moves in and out of the magnet's magnetic field
- this induces ALTERNATING p.d at the ends of the coil of wire at the SAME FREQUENCY of the soundwaves hitting the diaphragm
- this alternating p.d is passed through an amplifier, then into a moving-coil loudspeaker to increase the volume of the initial soundwaves
51
Describe the structure of a transformer
- primary coil and secondary coil wrapped around opposite sides of an iron core
- primary coil connected to AC
- THERE IS NO WAY FOR CURRENT FROM THE PRIMARY COIL TO PASS INTO THE SECONDARY COIL
52
Describe how a transformer works if both the primary and secondary coils have the same amount of turns
- AC through primary coil means that the coil creates a magnetic field running through the iron core and thus through the secondary coil
- because current is ALTERNATING, the direction of the magnetic field from the primary coil also alternates
- when the alternating magnetic field passes through the secondary coil, p.d is induced
- the direction of the p.d thus also alternates
- because there are the same number of turns in both coils, the p.d in each coil is also the same
53
Why is an iron core used in a transformer ?
- easily magnetised
- increases strength of primary coil's magnetic field
54
Why is AC used in a transformer ?
- AC needed to create alternating magnetic field, which is needed to induce p.d
- DC produces constant magnetic field, so no p.d would be induced if using it
55
Explain how a step-up transformer works
- primary coil has less turns than secondary coil
- so p.d induced in secondary coil is greater than p.d in primary coil
56
Explain how a step-down transformer works
- primary coil has more turns than secondary coil
- so p.d induced in secondary coil is smaller than p.d in primary coil
57
power in transformers MUST be conserved, so:
power in primary coil = power in secondary coil
NOTE: this only applies if the transformer is 100% efficient
58
Explain why p.d is used in the national grid rather than current
- the power wasted as heat through a wire is proportional to the square of the current
- if using a large current, this means that a large amount of power is wasted as heat
- thus, by using p.d instead, the amount of power lost as heat is greatly reduced, so the national grid is much more efficient
