Magnetism Flashcards
(63 cards)
1
Q
What will happen to a magnet that is free to move?
A
It will always line up in a North-South direction
2
Q
What are the poles of a magnet?
A
There are 2 poles: North and South
-they are where the magnetic forces which the magnet can exert are the strongest
3
Q
What happens to 2 magnets that are placed together?
A
- They exert a force on one another
- the LIKE poles will REPEL
- the UNLIKE poles will ATTRACT
4
Q
What affect does distance have on the forces between 2 magnets
A
The forces are strongest when the magnetic poles are closer together/ weakens as the distance between them increases
5
Q
What is a compass? What is it used for?
A
A magnetic compass consists of a bar magnet pivoted about its center in a case USES: -Navigation -Identifying the poles of a magnet -Tracing the pattern of magnetic fields
6
Q
What are the rules of drawing magnetic field lines?
A
- Start on North poles and end on South poles, or form closed loops
- Cannot start or end in space
- Cannot cross one another
- Arrowheads point in the direction of force that would be exerted on a free North pole (North to South)
- Are closer together where the field is stronger
7
Q
What are soft magnetic materials? Give an example
A
Materials that
-are easy to magnetise
-easily lose their magnetisation
E.g, Iron
8
Q
What are hard magnetic materials? Give an example
A
Materials that are difficult to magnetise, but once they are magnetised, they are difficult to demagnetise. For e.g, Steel
9
Q
What is a permanent magnet?
A
A magnet that produces its own magnetic field AND can be both attracted to and repelled by another magnet
10
Q
What is an induced magnet?
A
A material that only becomes magnetic when it is placed into a magnetic field AND it will only ever be ATTRACTED to a permanent magnet
11
Q
How can a nonmagnetic material be induced so it can be used as a bar magnet?
A
- The material must be a hard magnetic material
- Place it is a strong magnetic field (e.g, a solenoid)
- The nonmagnetic material will be induced into a permanent magnet and retain it magnetisation
- Can be used as a bar magnet
12
Q
Do electric currents create their own magnetic field?
A
Yes
13
Q
What factors affect the magnetic field created by an electric current?
A
- The direction of the magnetic field depends on the direction of the current
- The strength of the magnetic field depends on the size of the electric current
14
Q
What causes an electric current to create a magnetic field?
A
The magnetic field created by electric current is due to the moving charges- not by the material through which they are moving e.g, the wire.
15
Q
Under what conditions can a beam of charged particles create their own magnetic field?
A
If the beam of charged particles are moving through a vacuum
16
Q
What are some of the characteristics of the magnetic field patterns around a straight current-carrying wire?
A
- Consists of concentric circles
- Circles become father apart at greater distance from the wire
- Circles/ field lines have a direction given that can be predicted using a right- hand grip rule
17
Q
What happens when current-carrying wires are wound into a coil?
A
-The magnetic field created by each part of the coil adds together, creating a strong field through the center of the coil
18
Q
How is a long coil/ solenoid formed?
A
Many narrow coils being wound close together
19
Q
What happens when the narrow coils are wound together in a solenoid?
A
-The magnetic field created by each narrow coil adds together to create a very uniform field through the center of the solenoid
20
Q
What are the characteristics of the field lines formed from a solenoid?
A
- The magnetic field pattern is similar to that of a bar magnet
- The end of the solenoid where the field lines emerge from, acts as a magnetic North pole and the other end acts as a magnetic South pole
- The field at the sides/ outside of the solenoid is weak and points in the opposite direction to the field inside the solenoid
- The field inside the solenoid is uniform in the center (constant strength/ direction)
- The field can be controlled by controlling the current
21
Q
How can you identify which end of the Solenoid is the North pole?
A
-See which end the field lines emerge from
OR
-See whether the current circulates clockwise or anti-clockwise at the end of the coil
if CLOCKWISE, that end is the SOUTH pole
if ANTI-CLOCKWISE, that end is the NORTH pole
22
Q
What affects the strength of the magnetic field around a wire? How do these factors affect it?
A
- The current in the wire: increasing the current, increases the magnetic field strength
- The distance from the wire: the farther the field is from the wire, the weaker it gets
- The medium surrounding the wire
23
Q
When a wire/coil passes through Iron, why is their an increase in strength of the wires magnetic field?
A
- Iron is a ferromagnetic material (each of its atoms can act like a tiny bar magnet)
- So when a current-carrying wire passes through Iron, it’s atoms line up with the external field to create a much stronger resultant field
24
Q
What factors affect the strength of the magnetic field inside and at the ends of a solenoid?
A
- Increasing the No. of turns in the same length increases the strength of the magnetic field
- using a soft iron core inside the coil increases the strength of the magnetic field
- Increasing the current in the coil increases the strength of the magnetic field
25
What is an Electromagnet?
An electromagnet consists of many turns of insulated wire wound around a core made from a soft magnetic material such as Iron
26
What are some of the differences between an Electromagnet and a Permanent magnet?
ELECTROMAGNETS PERMANENT MAGNET
-can be switched on/ off -Continuous
-Strength of its magnetic -Strength is constant but
field can vary may decay with time/
heat
-Polarity can be reversed - Polarity is constant
by reversing the direction
of the current
-Core made from soft - Magnet made from
magnetic material (Iron); hard magnetic material
can magnetise and (e.g, steel); once
demagnetise quickly magnetised, it remains
magnetised
-Limit to how much current -No limit
can pass through- heat
generated by high current
melts the insulation
27
What will happen if a conductor, carrying current, is placed into a magnetic field?
The conductor and the magnet(s) producing the field will exert a force on one another (unless the conductor is parallel to the field lines)
28
How can you determine the force exerted on a conductor that is placed across a magnetic field?
Use Fleming's left-hand rule:
F - force exerted on conductor represented by the thumb
B - magnetic field represented by the index finger
I - current represented by the middle finger
29
What factors affect the magnitude of the force on a wire in a magnetic field? How do these factors affect it?
- Magnitude of current: the greater the current, the greater the force
- Strength of the magnetic field: the greater the magnetic field, the stronger the force
- Length of wire in the magnetic field: the greater the length, the greater the force
- Angle b/t magnetic field and current: force is greatest at 90 degrees and 0 at 0 degrees (if it is parallel)
30
What equation is used to calculate the force exerted on the current-carrying wire (The Motor Effect Force)?
F = BIL
F: Motor effect force in newtons (N)
B: The magnetic field strength in tesla (T)
I: the current in the wire in amps (A)
L: the length of wire at 90 degrees tot he magnetic field in metres (m)
THE DIRECTION OF THE FORCE IS GIVEN BY THE LEFT HAND RULE
31
What other units can be used for magnetic strength along with tesla (T)?
"force per unit current-length"
1T = 1Nm-1 A-1
32
What is a DC motor?
A motor powered by a direct power source
33
How does a DC motor work?
- A current-carrying coil is placed in a magnetic field at 90 degrees to the magnetic field lines
- the current flowing through one side of the coil is in the opposite direction to the current flowing on the other side of the coil
- Therefore, either side experiences forces in opposite directions: one is pushed DOWN and one is pushed UP (using Flaming's left hand rule)
- this creates a turning effect
- As the coil turns, the distance b/t the 2 motor effect forces changes and so the turning effect changes
- It is at is max when the 2 forces are further apart and 0 when the two forces are in the same vertical plane
34
What is the role of a split-ring commutator? How does it work?
- To allow the motor to work constantly
- The split-ring commutator rotates with the coil between the carbon brushes
- Each time the coil passes the vertical position, the direction in which current flow around the coil reverses
- It acts as a rotating switch, reversing the connections to the coil every half rotation
- allowing the coil to rotate continuously
35
What factors affect the turning effect on the coil in a DC motor? How?
- Current in the coil: greater current creates a greater turning effect (achieved by increasing the voltage of the supply)
- Size of the coil: the larger the size of the coil, the greater the turning effect
- Magnetic field strength: a stronger magnetic field creates a greater turning effect
- Number of turns on the coil: the larger the number of turns, the greater the turning effect
- Winding the coil onto a soft iron core: this increases the magnetic field strength and increases the turning effect
- Angle of coil in field: maximum turning effect when the coil is in the plane of the field and 0 when perpendicular to the field
36
Give 2 examples where electromagnets are used in everyday life
LOUDSPEAKER:
- Voice coil is connected to a movable speaker cone
- placed in radial magnetic field created by a permanent magnet
- current flows through voice coil- a motor effect pushes it forwards or backwards
- the cone creates compressions and rarefactions in the air- creating a sound
LIFTING MAGNETS:
- Coil used- consisting of many turns of thick insulated copper wire
- there is a large current in the coil when it is switched on
- Soft Iron core intensifies the field
- Lifted objects can be released by switching off the current
37
What is an electromagnetic induction?
It is the p.d. induced in a conductor when:
-it cuts across the lines of a magnetic field
-the magnetic field passing through it changes
(There is no induced p.d. when there is no relative motion between the magnet and the conductor
38
Can electromagnetic induction produce current, if so how?
Yes, if there is a closed circuit, current will flow
39
What factors affect the magnitude of an induced voltage?
An induced potential difference will increase if:
- The speed of the rotation of the magent increases
- The size of the magnetic field increases
- The No. of turns of wire on the coil increases
- The area of the coil increases
40
What factors affect the direction of the induced potential difference?
The direction is reversed when:
- The bar magnet is moved out of the coil
- The other pole of the magnet is moved into the coil
41
What is an AC generator?
A device that consists of a coil of wire rotating in a magnetic field, used to produce a potential difference
42
What factors affect the amplitude of the output ac voltage?
- output voltage increases if:
- The coil is rotated more rapidly
- The magnetic field is stronger
- The coil has greater area
- there are more turns on the coil
43
Draw the graph for the output voltage of a simple ac generator and explain the 5 different positions
y = sinx graph
A) at 0 degrees: The coil is moving parallel to the direction of the magnetic field, so no p.d. is induced
B) at 90 degrees: The coil is moving at 90 degrees to the direction of the magnetic field, so the induced p.d. is at its MAX
C) at 180 degrees: The coil is moving parallel to the direction of the magnetic field, so no p.d. is induced
D) at 270 degrees: The coil is moving at 90 degrees to the direction of the magnetic field, so the induced p.d. is at its MAX. Here the induced p.d. travels in the opposite direction to what it did at B
E) at 360 degrees: It is back at its starting point, having done a full rotation. The coil is moving parallel to the direction of the magnetic field, so no p.d. is induced
44
What are the 2 most important applications of electromagnetic induction?
- Generation of ac electrical energy- generators
| - Transmission of ac electrical energy- transformers
45
What are generators?
Generators transfer mechanical work (to rotate the generator) to electrical energy in the form of ac electricity. The energy source for the mechanical work comes from:
-chemical energy
OR
-kinetic energy
46
What are transformers?
Transformers use electromagnetic induction to increase or decrease the voltage of an ac supply
47
What the difference between a step up and step down transformer?
- A step-up transformer increases the voltage
| - A step-down transformer decreases the voltage
48
Describe the structure of a transformer
- Consists of 2 coils wound onto a soft Iron core
- The coil on the input side of the transformer is called the PRIMARY coil
- The coil on the output side is called the SECONDARY coil
49
Why are transformers used?
- They are used to step down mains ac voltages (230 V) down to the lower voltages used to power common electrical devices.
- They are used to step up the ac output voltage from power stations to very high voltages for long distance transmission
50
Why do transformers need alternating current?
For voltage to be induced in the secondary coil:
- the current in the primary coil must be changing continuously
- to generate a continuous changing magnetic field
- in order to induce a continually changing ac voltage in the secondary coil
51
What is the equation liking the voltage ratio and turns ratio in an ideal transformer?
Vp/ Vs = Np/ Ns
Vp: (ac) voltage across the primary coil
Vs:(ac) voltage across the secondary coil
Np: Number of turns on the primary coil
Ns: Number of turns on the secondary coil
52
What is the relationship between the voltage ratio and the turns ratio in a step-up transformer?
step-up:
-Ns > Np so that Vs > Vp
(The number of turns on the secondary coil is greater)
(The voltage across the secondary coil is greater)
53
What is the relationship between the voltage ratio and the turns ratio in a step-down transformer?
step-down:
-Ns < Np so that Vs < Vp
(The number of turns on the secondary coil is smaller)
(The voltage across the secondary coil is smaller)
54
When is the equation liking the voltage ratio and turns ratio valid?
-In "IDEAL transformers"
= one that is 100% efficient
= [The electrical power (P-in) transferred to the primary coil, is equal to the electrical power (P-out) transferred from the secondary coil to external components
55
What are the equations for P-in and P-out?
```
Electrical power = p.d. x current
SO
P-in = Vp x Ip
P-out = Vs x Is
Vp: voltage across the primary coil
Vs: voltage across the secondary coil
Ip: current in the primary coil
Is: current in the secondary coil
```
56
If P-in = P-out for an ideal transformer, what does that mean for the voltage and current?
VpIp = VsIs
57
Real transformers aren't always 100% efficient, Why?
Some of the input power is transferred to heat as a results of:
- The resistance in the wires of the coils
- Heating effects in the core as it magnetises and demagnetises
- Currents induced in the ore by the changing magnetic field
58
What is meant by the term power transmission?
Electrical power that is transmitted over long distances using transmission lines
59
What are the conditions needed for power transmission? why is this?
- High voltages are used
- To keep the current in the transmission lines low
- To reduce the losses due to heating of the cables
60
What equation can be used to calculate the power transmitted?
P = IV
Power: watts(W)
Current: amps(A)
Voltage: volts (V)
Note- the same power can be transmitted by HIGH current and LOW voltage or LOW current and HIGH voltage
61
Why must current be made as small as possible during power transmissions?
- Transmission lines are made from conductors that have electrical resistance
- When there is a current in the transmission Iines, this resistance (R) causes some of the electrical power to be transferred to heat
- To minimise wasted energy, step-up transformers are used to step up the voltage to compensate
- so a low current level can be used
62
What equation can be used to calculate the power wasted in a transmission line?
P = I X I X R
(current squared)
Power:watts(W)
current:amps(A)
Resistance:ohms
63
Why is alternating current required for power transmissions?
- Transformers are needed to step up and step down the voltage so that transmission losses are minimised
- Transformers work by electromagnetic induction
- requires a changing magnetic field
- therefore, it requires continually changing current
- i.e, ac current
