Formula
Electric power equals voltage times current: P = UI.
ENT-level problem
A resistor of resistance $R = 10$ ohm is connected to a source of voltage $U = 20$ V. Find the current flowing through the resistor.
Ohm's law for a circuit segment: $I = U/R$.
$$I = \frac{U}{R} = \frac{20}{10} = 2 \text{ A}$$
Power shows how much energy a device consumes or produces per unit of time. The unit of power is the watt (W): 1 W = 1 J/s. At 220 V and 5 A a lamp uses 1100 W.
With the resistance known, power can also be found as P = I²R and P = U²/R — handy when the current or the voltage is unknown. The total power of household appliances decides whether the wiring and the fuse can handle it.
Use P = I²R. For I = 2 A and R = 10 Ω the power is 40 W.
Power is the energy consumed per unit of time: a 1000 W device uses 1 kWh of electricity per hour.
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