AC & High Voltage Transmission (OCR Gateway GCSE Physics)

• 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)

Step 1: List the known quantities

• • Voltage in primary coil, V_p = 115 V
  • Voltage in secondary coil, V_s = 230 V
  • Current in secondary coil, I_s = 5 A

Step 2: Write the equation linking the known values to the current drawn from the supply, I_p

V_p × I_p = V_s × I_s

Step 3: Substitute in the known values

115 × I_p = 230 × 5

Step 4: Rearrange the equation to find I_p

Step 5: Calculate a value for I_p and include the correct unit

I_p = 10 A