Electromagnetic Induction (Module 25)

Question 1

What is electromagnetic induction?

Answer 1

The generation of an emf when the magnetic flux linkage through a coil changes or a conductor cuts across magnetic field lines.

Question 2

What can induced emf be increased by?

Answer 2

• Moving the wire faster.
• Making a wire into a coil, and pushing the magnet in or out of the coil.
• Using a stronger magnet.

Question 3

How can no emf be induced in the wire? (without being disconnected)

Answer 3

If the wire is parallel to the magnetic field lines as it moves in the field.

Question 4

Give two other methods of generating an induced emf.

Answer 4

• Using an electric motor in reverse

- Using a cycle dynamo.

Question 5

What is the dynamo rule?

Answer 5

The direction of the induced current can also be worked out using Fleming’s right hand rule, also referred to as the dynamo rule.

Question 6

What is Lenz’s Law?

Answer 6

This states that the direction of the induced current is always such as to oppose the change that causes the current.

Question 7

How do you calculate magnetic flux?

Answer 7

This is the product of the magnetic flux density and the area. (BA)

Question 8

What is magnetic flux linkage?

Answer 8

Through a coil of N turns, = NΦ = NBA where B is the magnetic flux density perpendicular to area A.

Question 9

What is the unit of magnetic flux linkage?

Answer 9

Weber (Wb) –> 1 Tm^2

Question 10

What is Faraday’s Law of electromagnetic induction?

Answer 10

This states that the induced emf in a circuit is equal to the rate of change of flux linkage through the circuit.

Question 11

If you have length, and speed what else would you need to work out the induced emf in a moving conductor in a magnetic field?

Answer 11

Magnetic Flux Density (B)

E = Blv

Question 12

Define back emf.

Answer 12

emf induced in the spinning coil of an electric motor or in any coil in which the current is changing (e,g, the primary coil of a transformer). A back emf acts against the change of applied pd.

Question 13

Since the induced emf is proportional to the speed of rotation of the motor, the current changes. What happens to the current when there are low speeds?

Answer 13

The current is high due to the induced emf is low.

Question 14

What does (electrical power transferred to mechanical power IE) + (Electrical power wasted due to circuit resistance I^2R) equal?

Answer 14

Electrical power supplied by the source IV

Question 15

What is an alternating current?

Answer 15

This is a current that repeatedly reverses its direction. In one cycle of an alternating current, the charge carriers move one way in the circuit, then reverse direction, then re-reverse direction.

Question 16

Define the root mean square value of an alternative current.

Answer 16

This is the value of direct current that would give the same heating effect as the alternating current in the same resistor.

Question 17

What is a step-up transformer?

Answer 17

This has more turns on the secondary coil than on the primary coil. So the secondary voltage is stepped up compared with the primary voltage.

Question 18

what is a step-down transformer?

Answer 18

This has fewer turns on the secondary coil than the primary coil. So the secondary voltage is stepped down compared withe the primary voltage.

Question 19

What is a eddy current?

Answer 19

Induced current t in the metal parts of ac machine.

Question 20

How do you calculate the efficiency of a transformer?

Answer 20

( power delivered by the secondary coil / power supplied to the primary coil. ) x 100

Question 21

Are transmissions of electrical power over long distances more efficient at high voltages or low voltages?

Answer 21

High voltages as there is less current so less is lost to the surroundings.