Electric Motors

A current carrying wire establishes a magnetic field around the wire. How can the magnetic field be increased?

• change the direction of the current

• add in a second wire

• increase the current through the wire

• use a different material

When a current carrying wire is placed in an external magnetic field, it experiences a force. Which statement best describes why this happens?

• The wire is magnetic.

• There is a magnetic field around the wire which interacts with the external magnetic field.

• The electric field is attracted to the magnetic field.

• There is not enough information.

It is possible to use Fleming’s left-hand rule to work out the direction of the force experienced by a wire in an external magnetic field. Which of the following correctly explains how to use the rule?

• first finger field (N to S), second finger current (neg to pos), thumb movement

• first finger field (N to S), second finger current (pos to neg), thumb movement

• first finger current (neg to pos), second finger field (N to S), thumb movement

• first finger movement, second finger field (S to N)

How can the speed of an electric motor be increased?

• increase the strength of the external magnetic field

• increase the number of coils

• increase the current passing through the wire

• all of the above

A copper wire is moved inside a magnetic field. Which of the following will induce the biggest current in the wire?

• The wire is moved parallel to the field lines.

• The wire is moved diagonally through the field lines.

• The wire is moved at right angles to the field lines.

• It does not matter how it is moved, the current will be just as big.

A copper wire is moved at right angles to a magnetic field. How can the induced current in a wire be increased?

• move the wire in and out of the field more quickly

• use a thicker wire

• swap the poles of the magnetic field

• increase the length of the wire

The primary coil on a step-up transformer has 500 coils and is connected to a 12 V supply. What is the output voltage if the secondary coil has 2000 turns?

• 48 V

• 0.25 V

• 84 000 V

• 2000 V

The secondary coil of a step-down transformer has 100 turns and produces 2 V. What is the number of turns required for a transformer connected to a 10 V supply?

• 250

• 2000

• 1000

• 500

A transformer is said to be 100% efficient. Which of the following correctly describes the relationship between power output and power input for the transformer?

• power in is greater than power out

• power in is less than power out

• power in is equal to power out

• more information is needed

A transformer is connected to a 230 V mains supply, 0.25 A. The voltage output on the secondary coil is 12 V. What is the current through the secondary coil?

• 700 A

• 0.25 A

• 4.8 A

• 6 A