Physics P5

Answer 1

A step-up transformer increases the voltage.
has more turns on the secondary coil than on the primary coil

A step-down transformer decreases the voltage

Question 2

transformer structure

Answer 2

Transformers consist of two coils wound onto a soft iron core. The coil on the input side of the transformer is called the primary coil and the coil on the output side of the transformer is called the secondary coil.

Question 3

how do transformers work

Answer 3

Current in primary coil changes

↓

Magnetic field in core changes

↓

Voltage is induced in the secondary coil

Question 4

dc or ac current for transformers

Answer 4

ac

Question 5

The soft iron core

Answer 5

Iron is a magnetically soft material. This means it can be magnetised and demagnetised easily and quickly

This allows the magnetic field in the core to change both in magnitude and direction as the ac current in the primary coil changes. The iron core loops through both coils linking them magnetically so that the changing magnetic field in the primary also passes through the secondary.

Question 6

Conservation of energy

Answer 6

Whilst a transformer can be used to increase the voltage from a supply it cannot create energy.

A consequence of increasing voltage is that the current in the secondary decreases and since energy transferred is equal to E=IVt
, the decrease in current balances the increase in voltage so that energy is conserved.

Question 7

ideal tranformers

Answer 7

An ideal transformer is 100% efficient.

This means that the electrical power Pin transferred to the primary coil = to the electrical power Pout transferred from the secondary coil to external components:

Question 8

Real transformers

Answer 8

Whilst large transformers do come close to 100% efficiency, all real transformers fall short of this figure. This is because some of the input power is transferred to heat as a result of:

the resistance in the wires on the coils

heating effects in the core as it magnetises and demagnetises

currents induced in the core (eddy currents) by the changing magnetic field.

Question 9

NOT READ - Understand power transmission, including calculating losses during transmission and the need for high voltage