High Voltage Transmission Equations
Higher Tier Only
- When a current passes through a wire, the current creates a heating effect which means the wires warm up
- This means they lose electrical energy as heat which reduced the efficiency of the transformer
- This is due to electrical resistance which is present in all wires
- The power (energy per second) lost in the wire is given by the following equation
P = I2R
- Where:
- P = power in watts (W)
- I = current in amps (A)
- R = resistance in ohms (Ω)
- Since the power is the energy lost per second, the total energy lost in a time t will be:
E = P × t
- Where:
- E = energy in joules (J)
- t = time in seconds (s)
- A step-up transformer may be used to increase the voltage of a power supply from the power station to the transmission wires
- The number of turns and voltage for the transformer is related by the following equation:
- Where:
- Vp = potential difference (voltage) across the primary coil in volts (V)
- Vs = potential difference (voltage) across the secondary coil in volts (V)
- np = number of turns on the primary coil
- ns = number of turns on the secondary coil
- A step-up transformer has more turns on the secondary coil, Ns, than on the primary coil, Np
- Since a transformer cannot output more power than is put into it, increasing the voltage must result in the current being lowered
IpVp = IsVs
- Where:
- Ip = current in the primary coil in amps (A)
- Is = current in the secondary coil in amps (A)
- Lower current results in less power and energy loss in the cables
- This makes the transfer of electrical energy through the wires more efficient
Examiner Tip
You will be expected to remember what each variable means for the equations above and their appropriate units.