Electric Motors + Transformers

Question 1

What are motors used for?

Answer 1

Many electrical appliances that make things move.

Question 2

What is there around a wire carrying electric current?

Answer 2

A circular magnetic field.

Question 3

What is an electromagnet?

Answer 3

If the wire is made into a coil, the magnetic field pattern becomes similar of that of a bar magnet.

Question 4

What is the motor effect?

Answer 4

When a current flows in a wire that is in a magnetic field, the wire experiences a force: if wire is free to move, it moves.

Question 5

When is the force largest?

Answer 5

When the current is at right angles to the magnetic field lines: direction of force is always at right angles to both the current in the wire and the magnetic field lines.

Question 6

When is no force experienced?

Answer 6

When the current is parallel to the magnetic field lines.

Question 7

When is the direction of the force reversed?

Answer 7

If either the current or the magnetic field is reversed.

Question 8

What happens when a simple motor is placed in a uniform magnetic field?

Answer 8

One side of the rectangular current-carrying coil is forced upwards: the other is forced downwards to produce rotation.

Question 9

When will the motor turn faster?

Answer 9

• if current is increased
• number of turns of coil is increased
• magnetic field is stronger
• soft iron core in the coil.

Question 10

Why does the motor effect work?

Answer 10

Because the interaction between the magnetic field around the current carrying wire and the magnetic field of the permanent magnets.

Question 11

What does the commutator do?

Answer 11

Connects the coil of the motor to the power supply: swaps contacts with the coil every half turn to reverse the current though the coil: keeps motor turning.

Question 12

What does a transformer do?

Answer 12

Changes the voltage of an a.c power supply.

Question 13

What does a transformer consist of?

Answer 13

Two separate coils around an iron core.

Question 14

What is fed into the primary coil?

Answer 14

The input voltage.

Question 15

What is across the secondly coil?

Answer 15

The output voltage.

Question 16

What does a step up transformer do?

Answer 16

Converts a low voltage input to a higher voltage output: primary coil has fewer turns than the secondary coil.

Question 17

What does the step down transformer do?

Answer 17

Converts a high voltage input to a lower voltage output: primary coil has more turns than the secondary coil.

Question 18

What does the alternating current in the primary coil create?

Answer 18

An alternating magnetic field around it: magnetic, soft iron core channels the magnetic field though the secondary coil.

Question 19

When is the alternating voltage induced across the secondary coil?

Answer 19

When the alternating magnetic field will continuously cut through the wires in the secondary coil.

Question 20

What happens if the number of turns in the secondary coil is doubled?

Answer 20

The output voltage will be doubled to.

Question 21

The turns ratio is equal to the voltage ratio: word equation:

Answer 21

Voltage across primary coil/voltage across secondary coil = number of turns in primary coil/number of turns in secondary coil.