Electromagnetic induction

Question 1

What is electromagnetic induction?

Answer 1

It is the study of energy conversion from mechanical to electrical energy

Question 2

What is the basic principle of electromagnetic induction?

Answer 2

When an electrical conductor is moved perpendicularly in a uniform magnetic field such that it cuts the magnetic lines of force, a current will be generated

Question 3

How do we determine the direction of the induced current?

Answer 3

Using Flemings right hand rule (father(force-thumb), mother(magnetic field direction-pointy finger), child(current-middle finger))

Question 4

What happens when a metal rod is moved perpendicular to the lines of force?

Answer 4

There will be a momentary deflection on the voltmeter

Question 5

What factors affect the magnitude of induced current? (3)

Answer 5

1) It increases with the length of the wire
2) It increases with the strength of the magnetic field
3) It increases with the speed at which the wire moves in the magnetic field

Question 6

What is the law of electromagnetic induction?

Answer 6

It states that the magnitude of induced current is directly proportional to the change in the number of magnetic lines of force per unit time

Question 7

Equation for E.M.F

Answer 7

E.M.F = change in number of lines of force ÷ time taken

Question 8

What will be observed when inducing a current in a solenoid?

Answer 8

When the magnet is pushed towards the solenoid, the magnetic lines of force cut changes with time. According to the law of electromagnetic induction, a current will be induced in the solenoid. The current meter shows a momentary deflection in one direction

Question 9

How do we determine the direction of induced current in a solenoid?

Answer 9

By using Lenz’s law

Question 10

What is Lenz’s law?

Answer 10

It states that the direction of induced current in a coil is such that it opposes the movement of the magnet

Question 11

What will be observed if the magnet is at rest near the solenoid?

Answer 11

When the magnet is at rest, there will be no change in the magnetic lines of force being cut so no current will be induced

Question 12

What is the effect of moving the magnet more rapidly towards the solenoid?

Answer 12

When the magnet is moved rapidly, the number of magnetic lines of force cut by the solenoid changes in a shorter time interval. Thus the current is greater and the current meter will show a greater momentary deflection

Question 13

If a more powerful magnet is pushed towards the solenoid at the same speed. What will be observed?

Answer 13

A powerful magnet has a larger number of magnetic lines of force surrounding it. There will be a greater change in magnetic lines of force cut by the solenoid in the same time interval. Therefore the current meter will show a greater momentary deflection

Question 14

What is the effect of pulling the magnet away from the solenoid?

Answer 14

The number of magnetic field lines of force cut by the solenoid changes from a maximum to a minimum and so a current is induced in the solenoid. The current meter will show a momentary deflection in the opposite direction

Question 15

If the bar magnet is moved both towards and away from the solenoid what will happen?

Answer 15

An a.c voltage or current will be generated and the current meter will fluctuate in both directions

Question 16

What are the applications of electromagnetic induction? (2)

Answer 16

1) In an a.c generator

2) In an a.c transformer

Question 17

What is the principle of an a.c generator?

Answer 17

When the coil is rotated by external agents it cuts the field lines and a current is induced in it

Question 18

What is the purpose of the copper slip rings in an a.c generator?

Answer 18

It is used to maintain a continuous rotation of the coil inside the magnetic field without twisting the terminals of the coil

Question 19

What happens when the coil is vertical in an a.c generator?

Answer 19

When the coil is vertical, force and field are parallel so no current is induced

Question 20

What are the factors affecting the magnitude of the maximum induced current? (4)

Answer 20

1) It increases with the speed of the rotation of the coil
2) It increases with the strength of the magnetic field
3) It increases with the surface area of the coil
4) It increases with the number of turns in the coil

Question 21

What is an a.c trnasformer?

Answer 21

It is a device used to alter the magnitude of an a.c voltage

Question 22

What does a step-up transformer do?

Answer 22

It increases the voltage

Question 23

What does a step-down transformer do?

Answer 23

It decreases the voltage

Question 24

Principle of an a.c transformer

Answer 24

When a voltage is applied to the primary coil, a magnetic field is produced in its surroundings. Since the secondary coil is found very close to it, the magnetic lines of force will be cut by the secondary coil. Hence, a current will be generated at the terminals of the secondary coil. As an a.c voltage is applied, there is a continuous change in magnetic lines of force cut by the secondary coil and an a.c voltage is generated at the output

Question 25

Why is iron used in an a.c transformer?

Answer 25

Since iron is magnetic, it will concentrate all the magnetic lines of force towards the secondary coil

Question 26

Why do use a laminated core in a.c transformer?

Answer 26

It reduces the amount of heat produced in the core

Question 27

Why should the input current be AC in an a.c transformer?

Answer 27

So that there is a continuous change in the number of field lines being cut by the secondary coil

Question 28

Formula to relate voltage and number of turns of the coil?

Answer 28

output voltage÷input voltage = number of secondary coils÷number of primary coils

Question 29

Formula for efficiency

Answer 29

Efficiency = output power ÷ input power

Question 30

What is an ideal transformer?

Answer 30

It is one where efficiency is 1 or 100%

Question 31

Formula for ideal transformer

Answer 31

Output voltage x output current = input voltage x input current