Electromagnetism exam q's Flashcards
1
Q
What principle operation do d.c motor and galvanometers have in coomon?
A
An external magnetic field exerting a force on a current carrying wire.
2
Q
What is the principal energy conversion that takes place in a d.c motor?
A
Electrical energy to kinetic energy.
3
Q
What material is normally used in the carbon brushes? Give two properties of this material that make it suitable for use in a motor?
A
- Carbon
- Lubricant and soft
4
Q
If the motor jammed, a larger current than normal would flow through the motor. Explain why.
A
No opposing emf
5
Q
List three factors that affect the torque(couple) acting on the coil.
A
- Magnetic flux density
- Size of current
- Number of turns
6
Q
What would be the effect on the motor if it jammed?
A
Motor would overheat
7
Q
What changes can be made to a d.c motor to convert it to an a.c. generator?
A
Split ring commutator replaced by slip rings.
8
Q
State Faraday’s Law of electromagnetic induction.
A
If the magnetic flux lines passing through a circuit change then an emf is induced. The induced emf is proportional to the rate of change of magnetic flux.
9
Q
It takes much longer for a magnet to fall through a copper pipe than a glass pipe explain why.
A
- Falling magnet creates changing magnetic flux and emf induced
- current flows in copper only, generating magnetic field which opposes the motion of the falling magnet.
10
Q
What is meant by potential difference? State its unit.
A
Work done per unit charge
- Volt
11
Q
Define electric field strength.
A
Force per unit charge
12
Q
Describe how an insulated spherical conductor can be charged positively by induction.
A
- Negatively charged rod close to it
- Earth sphere
- Remove earth
13
Q
Explain how point discharge occurs.
A
- Charge accumulates at a point, air around point is ionised and opposite charges attracted, like charges repelled.
14
Q
Describe how point discharge can be demonstrated in the laboratory.
A
- Charged point, candle at point
- Flame is blown away from the point
15
Q
What is electromagnetic induction?
A
When a change in magnetic flux induces an emf.
16
Q
State the laws of electromagnetic induction.
A
- Magnitude of the induced emf is proportional to rate of cutting flux
- induced emf in such a direction as to oppose the change that causes it
17
Q
A magnet is attached to a string and allowed to swing and a copper sheet is then placed underneath the magnet. Why does the amplitude of its swing decrease rapidly?
A
- Induced emf in copper
- Current flows in copper sheet, generating a magnetic field, opposing motion of magnet.
18
Q
A magnet is attached to a string and allowed to swing and a copper sheet is then placed underneath the magnet. What is the main energy conversion that takes place as the magnet slows down?
A
Kinetic energy → heat energy
19
Q
List the primary colours of light.
A
Red, green, blue
20
Q
What is a magnetic field?
A
Region where magnetic forces are felt.
21
Q
State Faraday’s law of electromagnetic induction.
A
Emf induced is proportional to the rate of change of magnetic flux.
22
Q
Where in the eye is light detected?
A
Retina
23
Q
List three invisible parts of the electromagnetic spectrum that have a shorter wavelength than visible light.
A
- Ultraviolet
- X-rays
- gamma rays
24
Q
State one law of electromagnetic induction.
A
Faraday’s Law states that the size of the induced emf is proportional to the rate of change of flux.
25
Define magnetic flux.
- Magnetic flux is the product of the average magnetic field times the perpendicular area that it penetrates.Φ=BA
26
Explain the principle involved when a small metal cylinder is dropped through one end of the copper tube, it falls freely under gravity. If the small cylinder is first magnetised, it then takes much longer to fall through the tube.
2018 electromagnetic induction cabidéal
27
Define magnetic flux.
Magnetic flux is the product of the magnetic flux density times the area through which magnetic flux density is acting .Φ=BA
28
Explain how the principle that a current-carrying conductor is used in the definition of the ampere.
Current flows in two parallel conductors, 1 metre apart in a vacuum, force of 2 x 10(-7)
29
What particles are used for transatlantic communication in (i) telegraph cables, (ii) in satellite signals, (iii) optical fibres.
(i) Electrons
(ii) Photons
(iii) Photons
30
State one law of electromagnetic induction.
Faraday's law states that the size of the induced emf is proportional to the rate of change of flux.
31
Define magnetic flux.
Magnetic flux is the product of the magnetic flux density times the area through which magnetic flux density is acting .Φ=BA
32
Derive an expression for the effective resistance of two resistors in parallel.
- Apparatus: magnet, conductor, meter
- Move magnet, reading on meter
- Move magnet faster, larger reading
33
Define magnetic flux
Magnetic flux density multiplied by area through which magnetic flux is acting.
34
State Faraday's law of electromagnetic induction.
The induced emf is proportional to the rate of change of magnetic flux passing through the circuit.
35
Define the tesla.
Force of 1 N when 1 A flows through a wire a length 1 m.
36
Explain the term electromagnetic induction.
Emf induced by changing magnetic flux.
37
Why is it inefficient to use low voltage when transmitting electricity?
Large current - more heat lost
38
Explain why it is necessary to use the rms voltage when comparing a.c. and d.c. electricity.
So as to make the power output equivalent between a.c. and d.c.
39
Give one advantage and one disadvantage of electric cars.
Ad: Fewer carbon emissions
Disa: Expensive batteries
40
2014
2014
41
Describe an experiment to show that a current-carrying conductor in a magnetic field experiences a force.
A strip of aluminium foil is placed at right angles to a U-shaped magnet. The foil is connected in series with a battery and a switch. When the switch is closed the aluminium foil experiences an upward force.
42
Explain how such a motor could be converted to an A.C. generator.
In an AC generator the two ends of the coil are each attached to a slip ring that makes contact with brushes as the coil turns. The two slip rings of the AC generator allow the coil to turn without breaking the connections to the load circuit.
43
State Faraday's law.
The induced emf is proportional to the rate of change of magnetic flux passing through the circuit.
44
Describe an experiment to demonstrate Faraday's law.
Magnet, coil, miliivoltmeter
Move the magnet towards the coil and an emf is induced in the coil. The size of the emf is read from the millivoltmeter.
The faster the magnet the greater the induced emf.
45
What is magnetic flux?
Magnetic flux density x area through which magnetic flux density is acting. Φ=BA
46
State Faraday's law of electromagnetic induction.
The induced emf is proportional to the rate of change of magnetic flux passing through the circuit.
47
Describe an experiment to demonstrate Faraday's law.
Magnet, coil, miliivoltmeter
Move the magnet towards the coil and an emf is induced in the coil. The size of the emf is read from the millivoltmeter.
The faster the magnet the greater the induced emf.
48
State the principle on which a moving-coil galvanometer is based.
A current carrying conductor experiences a force in a magnetic field.
49
Name another device based on the same principle as the moving-coil galvanometer.
d.c. motor
50
Give an application of the induction coil.
Electric fence
51
In an a.c. generator, why is a voltage induced in the coil?
Coil experiences in a change in magnetic flux.
52
State three factors that determine the magnitude of the induced voltage.
- Magnetic flux density
- Speed
- Numbers of turns of coil
53
Energy companies transmit electricity at very high voltages even though these high voltages are dangerous. What is the reason for the high voltages?
High voltage means low current means huge saving on energy losses as heat.
54
Give two factors that lead to energy losses in transformers.
- Ohmic resistance in coils
| - Heat
55
Why would higher frequency tuning forks lead to less accuracy in this experiment?
Higher frequency corresponds to smaller values of length and smaller value give greater percentage errors.
56
Name and state the two laws of electromagnetic induction.
Faraday: The induced emf is proportional to the rate of change of magnetic flux passing through the circuit.
Lenz: The induced emf is proportional to the rate of change of magnetic flux passing through the circuit.
57
Explain the principle involved when a small metal cylinder is dropped through one end of the copper tube, it falls freely under gravity. If the small cylinder is first magnetised, it then takes much longer to fall through the tube.
- Falling magnet gives changing magnet flux
- Emf induced in copper
- Current flows in copper
- Current opposes falling magnet
58
Name and state the two laws of electromagnetic induction.
Faraday: The induced emf is proportional to the rate of change of magnetic flux passing through the circuit.
Lenz: The induced emf makes a current flow in such a direction as to oppose the change that caused it.
59
Name the Irish scientist who shared the Nobel prize with J.D. Cockcoft in 1951.
E T S Walton
60
Name the Irish scientist who invented an induction coil in the 919th century.
N. Callan
61
Name the Irish scientist who invented an induction coil in the 19th century.
N. Callan
62
What is the function of an induction coil? State the principle on which it is based.
- Used to get a high voltage from a low voltage d.c. source
| - Mutual induction
63
Give one application of the induction coil.
Spark plugs in a car
64
Name the Irish physicist who invented the induction coil.
N Callan
65
A transformer and an induction coil can both be used to change the size of a voltage. What is the basic difference in the operation of these two devices?
T: Ac input
I: DC input
66
Give two factors that affect the efficiency of a transformer.
- Heat losses in coils
| - Coil resistance
67
The a.c. generator is similar to a motor in reverse. Explain this statement.
- Motor is suppied with current, generates a couple that causes rotation.
- Generator is supplied with rotation, generates a current.
68
State one law of electromagnetic induction.
Lenz: The induced emf is proportional to the rate of change of magnetic flux passing through the circuit.
69
State one law of electromagnetic induction.
Lenz: The induced emf makes a current flow in such a direction as to oppose the change that caused it.
70
State Lenz's law
The induced emf makes a current flow in such a direction as to oppose the change that caused it.
71
State the principle on which the moving coil galvanometer is based and name one other device which is based on the same principle.
- A current-carrying conductor in a magnetic field experiences a force
- Motor / loudspeaker
72
A transformer and an induction coil can both be used to change a small voltage into a larger voltage. What is the basic difference in the operation of these two devices?
T: a.c. input, a.c. output, current flows through secondary]
I: d.c. input, d.c. output, sparks across gap in secondary
73
Give two factors that affect the efficiency of a transformer.
- Heat losses in coils
| - Poor flux linkage
74
An electrical device bought in America, where the n=mains electricity is supplied at 110V, is to be used in Ireland, where the mains voltage is 230V. What type of transformer is needed?
Step-down transformer
75
Describe an experiment to illustrate Lenz's law.
App: Aluminium ring, long thread, strong magnet.
P: Move magnet towards suspended ring
O: Ring swings away
C: Induced current tries to reduce the relative motion between magnet and ring.
76
What is electromagnetic induction?
When a change in magnetic flux induces an emf.
77
Who invented the induction coil?
N Callan
78
What is the function of the induction coil?
Changes low voltage d.c. to high voltage d.c.
79
Why are there a large number of turns in the secondary coil in an induction coil?
Emf induced proportional to number of turns.
80
Explain why the primary coil in an induction coil has thick wire.
Has low resistance
Greater efficiency
Reduced energy losses
81
Why are both coils wrapped on the same soft-iron core in an induction coil.
- Greater efficiency
| - Less energy losses
82
List two types of electromagnetic waves with less energy than that of light waves.
- Microwaves
| - infra red
83
Give one property that is common to all types of electromagnetic waves.
Same speed, can be polarised
84
What is an electromagnetic relay?
Switch in electric circuit turned on by electromagnet.
85
State two examples where an electromagnetic relay is used in a car and explain the advantage of using electromagnetic relay in these circuits.
- Starter motor and wipers
| - Devices draw large current but switch takes only very small current.
86
State the principle on which the moving coil galvanometer works and describe an experiment to demonstrate this principle.
- Force on a current-carrying conductor in a magnetic field.
| - Aluminium foil strip and magnet. Turn on current and aluminium strip moves.
87
Explain the role of electromagnetic induction in damping the oscillation of the pointer in a moving coil galvanometer.
Induced currents oppose oscillation by Lenz's law.
88
What is meant by electromagnetic induction.
When a change in magnetic flux induces an emf.
89
Distinguish between Faraday's law and Lenz's law.
F: Size of induced emf
L: direction of induced current
90
The ring could could be made of aluminium but not of iron. Explain why.
Iron is magnetic
| Aluminium is non-magnetic
91
What happens to the ring when the magnet moves towards it?
The rings moves away
92
What happens to the ring when the magnet moves towards it? Explain how this demonstrates Lenz's Law.
- The rings moves away
| - Induced current opposes movement of magnet - ring moves away
93
State what is meant by self induction.
Induced current in a coil due to changing current in the coil itself.
94
What is electromagnetic induction?
When a change in magnetic flux induces an emf.
95
State the laws of electromagnetic induction.
Faraday: The induced emf is proportional to the rate of change of magnetic flux passing through the circuit.
Lenz: The induced emf makes a current flow in such a direction as to oppose the change that caused it.
96
State the principle of conservation energy.
Energy cannot be created or destroyed it may only change from one form to another.
97
What is the main energy conversion that takes place in an electric motor?
Electrical energy to kinetic energy
98
What is the function of (i) the commutator, (ii) carbon brushed, (iii) magnet, in an electric motor.
(i) So that coil rotates in one direction
(ii) To link power supply to coil
(iii) To provide a magnetic field
99
Give an advantage of an induction motor over a d.c. motor.
Less friction, no sparking
100
Describe an experiment to demonstrate the principle on which the induction motor operates.
A: Aluminium disc, magnet
- Place disc on pivot
- Rotate magnet over disc
- Disc rotates in same direction as magnet.
101
Define magnetic flux.
Magnetic flux density x area through which magnetic flux density is acting. Φ=BA
102
State Faraday's law.
The induced emf is proportional to the rate of change of magnetic flux passing through the circuit.
103
List three factors that affect the force on a current-carrying conductor placed near a magnet.
- Magnetic flux density
- Current
- Length
104
What energy transformation takes place in an electric motor?
Electrical energy to kinetic energy.
105
What is the function of a commutator in a dc motor?
Allows current to always flow into the same end of the coil.
106
How are the slip rings connected to an external circuit in an a.c. generator?
Carbon brushes
107
A transformer and an induction coil can both be used to change a small voltage into a larger voltage. What is the basic difference in the operation of these two devices?
T: a.c. input, a.c. output, current flows through secondary
I: d.c. input, d.c. output, sparks across gap in secondary
108
Name the Irish physicist who invented the induction coil.
N Callan
109
Give two factors that affect the efficiency of a transformer.
- Heat losses in coils
| - Poor flux linkage
110
A resistor is connected in series with an ammeter and an ac power supply. A current flows in the circuit. The resistor is then replaced with a coil. The resistance of the circuit does not change. What is the effect on the current flowing in the circuit?
Current is reduced as back emf induced in coil.
