AC & High Voltage Transmission (OCR GCSE Physics A (Gateway)): Revision Note

AC & High Voltage Transmission

Higher Tier Only

• Remember, in a transformer, an alternating current is supplied to the primary coil

• The current is continually changing direction

  • This means it will produce a changing magnetic field around the primary coil

  • A changing magnetic field leads to the generation of an alternating potential difference

  • If connected in a complete circuit then an electric current will flow

Advantages of High Voltage Transmission

• When electricity is transmitted over large distances, the current in the wires heats them, resulting in energy loss

• To transmit the same amount of power as the input power the potential difference at which the electricity is transmitted should be increased

  • This will result in a smaller current being transmitted through the power lines

  • This is because P = IV, so if V increases, I must decrease to transmit the same power

• A smaller current flowing through the power lines results in less heat being produced in the wire

  • This will reduce the energy loss in the power lines

Electricity is transmitted at high voltage, reducing the current and hence power loss in the cables

Structure of a Transformer

• A basic transformer consists of:

  • A primary coil

  • A secondary coil

  • An iron core

• Iron is used because it is easily magnetised

Structure of a transformer

How a Transformer Works

• An alternating current is supplied to the primary coil

• The current is continually changing direction

  • This means it will produce a changing magnetic field around the primary coil

• The iron core is easily magnetised, so the changing magnetic field passes through it

• As a result, there is now a changing magnetic field inside the secondary coil

  • This changing field cuts through the secondary coil and induces a potential difference

• As the magnetic field is continually changing the potential difference induced will be alternating

  • The alternating potential difference will have the same frequency as the alternating current supplied to the primary coil

• If the secondary coil is part of a complete circuit it will cause an alternating current to flow

The Ideal Transformer

• An ideal transformer would be 100% efficient

• Although transformers can increase the voltage of a power source, due to the law of conservation of energy, they cannot increase the power output

• If a transformer is 100% efficient:

Input power = Output power

• The equation to calculate electrical power is:

P = VI

• Where:

  • P = power in Watts (W)

  • V = potential difference in volts (V)

  • I = current in amps (A)

• Therefore, if a transformer is 100% efficient then:

V_p × I_p = V_s × I_s

• Where:

  • V_p = potential difference across primary coil in volts (V)

  • I_p = current through primary coil in Amps (A)

  • V_s = potential difference across secondary coil in volts (V)

  • I_s = current through secondary coil in Amps (A)

• The equation above could also be written as:

P_s = V_p × I_p

• Where:

  • P_s = output power (power produced in secondary coil) in Watts (W)