magnetism, electromagnetism, emi

Question 1

What is the direction of conventional current?

Answer 1

Positive to negative (Long to short in circuit diagram of cell)

Question 2

Magnetic field lines point from ____ to ____

Answer 2

North to South

Question 3

Right hand grip rule?

Answer 3

Curl fingers in the direction of current flow, thumb points to North
OR Curl fingers in the direction of magnetic field lines, thumb points to direction of current (into the plane / out of the plane)

Question 4

What does x and . represent?

Answer 4

x: into the plane
.: out of the plane

Question 5

Force between 2 parallel current-carrying conductors (current in OPPOSITE direction)?

Answer 5

Wires repel

Question 6

Force between 2 parallel current-carrying conductors (current in SAME direction)?

Answer 6

Wires attract

Question 7

Fleming’s left hand rule?

Answer 7

Thumb: force (father)
Second finger: magnetic field (mother)
Third finger: current (child)

Question 8

How to explain the force on a current carrying conductor?

Answer 8

Magnetic field due to the current interacts with magnetic field due to the magnet, resulting in a resultant magnetic field which exerts force on current carrying conductor.

Question 9

Why does galvanometer register a deflection when magnet is moved in and out of solenoid?

Answer 9

As the magnet moves towards the solenoid, its magnetic field lines ‘cut’ the coils of the solenoid. The magnetic flux linking the coils have changed.
When this happens, a galvanometer connected to the coil registers a momentary deflection (an e.m.f. is induced).

Question 10

Why does galvanometer not register a deflection when magnet is stationary solenoid?

Answer 10

When the magnet is stationary in the solenoid, there is no deflection as there is no change in the magnetic flux linking the coils (no e.m.f. induced).

Question 11

Lenz’s Law states that…

Answer 11

Lenz’s Law states that the direction of the induced e.m.f., and hence the induced current in a closed circuit, is always such that its magnetic effect opposes the motion or change producing it.

Question 12

Faraday’s law tells us the _____ of the induced e.m.f.
Lenz’s law tells us the _____ of the induced e.m.f.

Answer 12

Faraday’s law tells us the magnitude of the induced e.m.f.
Lenz’s law tells us the direction of the induced e.m.f.

Question 13

By Lenz’s Law, when an S pole approaches a solenoid, ___ pole will form at that end of the solenoid.

Answer 13

By Lenz’s Law, when an S pole approaches a solenoid, an S pole will form at that end of the solenoid.

Question 14

By Lenz’s Law, when an S pole moves away from a solenoid, ___ pole will form at that end of the solenoid.

Answer 14

By Lenz’s Law, when an S pole moves away from a solenoid, a N pole will form at that end of the solenoid.

Question 15

By Lenz’s Law, when an S pole moves away from a solenoid, ___ pole will form at that end of the solenoid.

Answer 15

By Lenz’s Law, when an S pole moves away from a solenoid, an N pole will form at that end of the solenoid.

Question 16

Lenz’s law is a consequence of the Law of energy conservation. What is the energy conversion?

Answer 16

Mechanical work is transformed into electrical energy.

Question 17

Differences between DC motor and AC generator?

Answer 17

DC motor
- requires split ring commutator (reverse current every half cycle)
- fleming’s left hand rule

AC generator
- slip rings (to transfer alternating induced e.m.f. to the external circuit)
- uses electromagnetic induction
- fleming’s right hand rule

Question 18

Doubling the number of turns of the coil…

Answer 18

Doubling the number of turns of the coil doubles output voltage

Question 19

Doubling the frequency of rotation…

Answer 19

Doubling the frequency of rotation doubles output voltage and frequency

Question 20

How is voltage and number of coils and current in transformer related?

Answer 20

VS/VP = NS/NP = IP/IS

Question 21

Efficiency =

Answer 21

output power∕input power × 100%

Question 22

What are the causes of power loss?

Answer 22

• Heat loss due to the resistance of the coils
• Leakage of magnetic field lines between the primary and secondary coils
• Heat loss due to eddy current induced in the iron core

Question 23

Electricity from power station to homes?

Answer 23

Electricity produced in power stations is stepped up by step-up transformers.
High voltage electricity is transmitted through transmission cables.
Step-down transformers reduce the voltage to suitable values for use.

Question 24

Faraday’s law states that…

Answer 24

Faraday’s Law of electromagnetic induction states that the magnitude of the induced e.m.f. in a circuit is directly proportional to the rate of change of magnetic flux in the circuit.