I/GCSE Physics - Transformers

TRANSFORMERS

• A transformer is a device that steps-up or steps-down an alternating voltage from the primary coil to the secondary coil, which is a concept of I/GCSE Physics.

Circuit symbol for a transformer:

How a transformer works:

• The primary coil is supplied with alternating voltage which sets up a varying magnetic field around the coil and extends to the secondary coil. When the varying field cuts across the secondary coil, alternating voltage is induced in the secondary coil.
• *Transformers only work with a.c.

A step-up transformer:

• Secondary coil has more turnsthan primary coil
• Converts low voltage to high voltage
• Converts high current to low current

A step-down transformer:

• Secondary coil has fewer turns than primary coil
• Converts low current to high current

The ratio of the voltage of the secondary coil over the voltage of the primary coil to the number of turns in the secondary coil over the number of turns in the primary coil is the same.

i.e. Vs/Vp = Ns/Np

Energy losses in transformers:

• Heat loss from the resistance in the winding of the coil: as current passes through the wire, heat is released.
• Eddy currents losses in the core: eddy currents are circulating currents induced in the core by the magnetic field from the primary coil, producing heat.
• Leakage of magnetic field: not all of the magnetic field from the primary coil reaches the secondary coil so energy is lost.
• Magnetization and demagnetization of core: magnetizing and demagnetizing the core require work to be done, which uses up energy.

How to reduce these losses:

• Resistance is directly proportional to length of conductor over cross-sectional area of conductor. Therefore using thick coils will reduce the resistance in windings, reducing heat loss.
• By laminating the core with thin magnetic strips or “lamina” which are insulated from each other will reduce eddy currents in the core.
• We can decrease the leaking of the magnetic field by (a) bringing the primary and secondary coils closer together and (b) removing the air gaps between the turns in the coils.
• If we use a soft iron corewhich is easy to magnetize and demagnetize, less energy will be lost.

Uses of transformers:

1. To step down voltage from the mains to power systems like radios, television sets, door bells etc.
2. For resistance welding: a step down transformer is used to produce low voltage and therefore high current at the secondary coil to melt and therefore weld things together. The turns ratio = 1/1000.
3. In an induction furnace: step down transformer to produce excessive heat to melt solder or boil water.
4. In the National Grid system to transport electricity from power stations to consumers by stepping up and stepping down voltage. It consists of:

• A power station
• Step-up transformers
• Pylons and transmission cables
• Sub-power stations
• Step-down transformers

How energy is transported:

The power station generates electricity with low voltage and high current, which is then stepped up to be transported with high voltage and low current to reduce heat production in the transmission cables, which are made from copper as it is a cheap and good conductor. This voltage is then stepped down for domestic use.

Why transmission of electricity is cheaper if voltage is very high:

< >High voltage means low current so less heat loss from cables will occur.Since there is less heat in the cables, they wouldn’t melt easily and as a result will last longer.

Power loss = current x voltage

OR

Power loss = resistance x current^2

*Voltage = resistance x current

That's the end of the topic!

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Drafted by Kin (Physics)