Electrical Power and EnergyAQA GCSE Physics: Revision notes
Section 1
Calculating Electrical Power
Electrical power can be calculated using either of two equations: P = VI and P = I^2 R (power in watts, potential difference in volts, current in amperes, resistance in ohms). Both equations give the same power for the same component - use whichever variables you are given.
A component with current 2 A and resistance 5 Ω has power P = I²R = 2² × 5 = 20 W.
Section 2
Calculating Energy Transferred by Electricity
The energy transferred by an electrical appliance can be calculated using: E = Pt and E = QV (energy in J, power in W, time in s, charge in C, potential difference in V).
Section 3
How Domestic Appliances Transfer Energy
Domestic appliances transfer energy from the battery or mains supply to other useful stores, depending on their purpose:
- A kettle: electrical energy → thermal energy (heating water)
- A motor/fan: electrical energy → kinetic energy
- A lamp: electrical energy → light (and some wasted thermal) energy
Some of the input energy is usually wasted, often as thermal energy dissipated to the surroundings.
Section 4
The National Grid and Transformers
The National Grid is the system of cables and transformers that distributes electrical power around the country. Step-up transformers increase the voltage (and correspondingly decrease the current) for transmission along power lines, while step-down transformers decrease the voltage back down to safe levels for use in homes and businesses.
Section 5
Why the National Grid Uses High Voltage
Transmitting electricity at a high voltage (and therefore low current) greatly reduces energy losses. Since power loss due to resistance in the cables depends on I²R, lowering the current (by raising the voltage for the same power) reduces the wasted power significantly, making the National Grid an efficient way to transfer energy over long distances.
If asked why the National Grid uses high voltage, always mention that power loss depends on current squared (I²R) - halving current cuts losses to a quarter.
Must Know
- P = VI and P = I²R both calculate electrical power
- E = Pt and E = QV both calculate energy transferred
- Domestic appliances transfer electrical energy into useful stores (thermal, kinetic, light) plus some wasted energy
- Step-up transformers raise voltage for transmission; step-down transformers lower it for safe use
- High voltage transmission reduces current, which reduces I²R power losses in the cables
That's the notes covered.
Carry on to the next subtopic.