transformers (topic 7)

Question 1

Answer 1

A - iron core
B - primary coil
C - secondary coil

Question 2

state if this is a step up or a step down transformer

Answer 2

step down transformer

Question 3

explain the operation of a step down transformer (4)

Answer 3

• alternating current in the primary coil produces a magnetic field
• causing an alternating magnetic field in the core
• the core strengthens the magnetic field
• the secondary coil experiences a changing magnetic field
• which induces a potential difference in the secondary coil

Question 4

transformers are used to increase the voltage output of power stations before distribution via the National Grid, explain why electricity is transmitted at high voltages (3)

Answer 4

• when a current flows through a wire it causes it to heat up
• increasing the output voltage reduces the output current
• minimising the energy lost as heat

Question 5

what is the purpose of a transformer (1)

Answer 5

to decrease or increase the voltage of the alternating current

Question 6

what happens in a step-down transformer (1)

Answer 6

the voltage decreases and the current increases

Question 7

Answer 7

a battery powered lamp

Question 8

explain why step-up transformers are used at the output of power stations (3)

Answer 8

• to increase the voltage output and decrease the current
• to minimise the heating effect of the current in the transmission wires
• increasing efficiency

Question 9

draw a labelled diagram of a transformer

Answer 9

Question 10

explain how a transformer works to increase the voltage of an alternating current input (5)

Answer 10

• alternating current sets up an alternating magnetic field in the primary coil
• alternating magnetic field is induced in the core
• iron core strengthens magnetic field
• alternating magnetic field at the secondary coil
• induces an alternating current at the secondary coil
• step up transformer if the secondary coil has more turns than the primary coil

Question 11

state the equation linking number of turns on each coil, input voltage and output voltage

Answer 11

Question 12

the primary coil has 400 turns, calculate how many turns are on the secondary coil if the voltage is reduced to 115 V from 230V

Answer 12

Question 13

the current in the primary coil is 2.5 A, calculate the power dissipated in the primary coil with a voltage of 230

Answer 13

• power = voltage x current
• power = 230 x 2.5 = 575 W

Question 14

a transformer has 600 turns on the primary coil and an input voltage of 230 V, the output voltage is 575 V, calculate the number of turns on the secondary coil

Answer 14

Question 15

state the equation linking input power and output power for a transformer

Answer 15

input power = output power

Question 16

the transformer has 1,500 turns on the primary coil and 120 turns on the secondary coil, calculate the output voltage of the transformer

Answer 16

Question 17

state the input voltage to the transformer from the mains supply

Answer 17

230V

Question 18

explain why the output power is less than the input power of a transformer (3)

Answer 18

• the transformer is not 100% efficient
• when a current flows through the transformer wires, it causes them to heat up
• energy is lost to the surroundings