Power & the Grid
Watts, joules and the wires that carry them. Master the four electrical power and energy equations, then see why the National Grid pushes electricity across the country at hundreds of thousands of volts.
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Power & the Grid ⚡
Every appliance transfers energy at a certain rate — its **power**, measured in **watts (W)**. For an electrical device the power is: **P = V I** (power = potential difference × current) with P in watts, V in volts and I in amps.
Find the power
An interactive activity.
Power and resistance 🔥
Power is also linked to resistance: **P = I² R** (power = current² × resistance) This is the power **dissipated as heat** in a component of resistance R carrying current I. It is exactly why wires warm up when a current flows — and the key to the National Grid later.
Find the heating power
An interactive activity.
Energy transferred 🔋
Power is the *rate* of transfer. For the total **energy transferred (in joules)**, use: **E = P t** (energy = power × time, with time in **seconds**) **E = Q V** (energy = charge × potential difference, with charge in coulombs)
Energy from power and time
An interactive activity.
Energy from charge
An interactive activity.
Match each equation to what it finds
- P = V I
- P = I² R
- E = P t
- E = Q V
- Power, from potential difference and current
- Power dissipated as heat in a resistance
- Energy transferred, from power and time
- Energy transferred by a charge through a p.d.
The National Grid 🏭
The **National Grid** carries electricity from power stations to homes through **cables and transformers**. The problem: current in the cables wastes energy heating them (**P = I² R**). The fix: **step-up transformers** transmit at **very high voltage**, so the **current is low** — and a low current means far less heat loss. **Step-down transformers** then lower the voltage for safe use.
Label the National Grid
An interactive activity.
Grid transmission
Select ALL THREE statements about National Grid transmission that are TRUE.
- Transmitting at high voltage means the current is low
- A low current wastes less energy heating the cables
- Step-down transformers lower the voltage for safe use in homes
- High voltage is used to make appliances run faster
- A high current in the cables reduces heat loss
- Step-up transformers are used just before homes
Why so high?
Why does the National Grid transmit electricity at very high voltage?
- So the current is low, reducing the energy wasted as heat in the cables
- Because high voltage is safer to have inside homes
- To make the electricity travel faster along the wires
- Because transformers can only work at low current
Power summary
The power of an appliance is P = V I, and the power wasted heating a resistance is P = I² _____. Energy transferred is E = P _____ or E = Q V. The National Grid transmits at high voltage so the _____ is low, wasting less energy as heat; a _____-down transformer then lowers the voltage for homes.