P4.2 Uses of magnetism

Question 1

What is an electromagnet?

Answer 1

● A solenoid with an added iron core.

● Adding the iron core increases the strength of the magnetic field.

Question 2

What is the motor effect?

Answer 2

When a force is exerted between a magnetic field and a current-carrying conductor placed in that field.

Question 3

What rule is used to determine the force experienced due to the motor effect?

Answer 3

Fleming’s Left-Hand Rule

Question 4

What do the fingers in Fleming’s left hand rule represent?

Answer 4

The forefinger points in the direction of the magnetic field

The second finger points in the direction of current flow in the conductor

The thumb points in the direction of the force produced, and therefore the motion produced by the force.

Question 5

What factors affect the size of the force on a current-carrying wire in a magnetic field?

Answer 5

1. Magnitude of current flowing through conductor
2. Strength of the magnetic field that the conductor is
   placed in

Question 6

If the direction of current in a current-carrying wire
placed in a uniform magnetic field is reversed, what
happens to the force?

Answer 6

The direction of the force is reversed.

Question 7

If the strength of the current in a current-carrying wire placed in a uniform magnetic field is increased, what
happens to the force?

Answer 7

Increases

Question 8

What criteria must be met for the equation linking
force, magnetic flux density, current and length to
hold?

Answer 8

The conductor must be at right-angles to

the magnetic field it is placed in.

Question 9

What equation is used to calculate the force that a

current carrying wire experiences in a field?

Answer 9

Force (N) = Magnetic Flux Density (T) x Current (A) x Length (m)
F = BIL

Question 10

How does an electric motor work?

Answer 10

● A coil of wire, carrying a current, is placed in a magnetic field.
● The forces on the two sides perpendicular to the field experience forces in opposite directions.
● This causes a rotational effect.

Question 11

How do loudspeakers make use of the motor effect?

Answer 11

The motor effect is used to convert variations in the current of an electrical circuit into the pressure variations which produce audible sound.

Question 12

Explain how a loudspeaker works.

Answer 12

● A cone with a wire wrapped around it is connected to an a.c power supply and is placed in a permanent
magnetic field.
● When current flows through the wire, it creates a
second magnetic field, which interacts with the permanent field.
● This produces a force which causes the cone to vibrate.

Question 13

How is the pitch of the sound from a loudspeaker

changed?

Answer 13

● The frequency of the a.c current is altered.

● This creates a different frequency of vibration in the cone.

Question 14

What happens when an electrical conductor moves

relative to a magnetic field?

Answer 14

A potential difference is induced across the ends of the conductor.

Question 15

What happens to an electrical conductor when there

is a change to the magnetic field that it is placed in?

Answer 15

A potential difference is induced across the ends of the conductor.

Question 16

f there is a change in the magnetic field around a
conductor, explain what changes occur in the
conductor.

Answer 16

● A potential difference is induced across its ends.
● This drives a current.
● The direction of the induced current is such that it
creates a magnetic field that opposes the change
in the existing field.

Question 17

What are two ways that the generator effect is used to generate different types of current?

Answer 17

● In an alternator to produce alternating-current.

● In a dynamo to produce direct-current.

Question 18

What electromagnetic effect does a microphone take advantage of and how?

Answer 18

● The generator effect.

● It converts the pressure variations in sound waves into alternating current in a circuit.

Question 19

Describe the makeup of a basic transformer.

Answer 19

A primary coil and a secondary coil of wire wrapped around an iron core

Question 20

Explain how a transformer works.

Answer 20

● An alternating current flows through the primary coil.
● This induces a changing magnetic field in the core.
● This changing magnetic induces a current to flow in the secondary coil

Question 21

Why must the current flowing through the primary

coil of a transformer be alternating?

Answer 21

● For current to be induced in the secondary coil, the magnetic field in the core must be continuously changing.
● For the magnetic field to be changing, the current in the primary coil must be alternating.

Question 22

What equation is used to link the voltage and

number of turns on the coils of a transformer?

Answer 22

v1/v2 = n1/n2