Part 08 · Circuits and magnetism · Card 4 of 12
Electric power
Rate of energy use in a circuit, P = IV = I squared R = V squared / R. The unit is the watt.
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Analogy
How fast a battery drains.
Example
Power use
P = I VP = 3 x 12P = 36 W
In practice
A 12 V, 3 A device uses 36 W, or 0.036 kWh per hour.
Practice
Flashcards
- Write the power formulas for a resistor.P = IV = I^2 R = V^2/R.
Interview questions
- A 12 V battery is connected to a 6 ohm resistor in series with a parallel pair of 6 ohm and 3 ohm resistors. Find the current from the battery and the power used.
- Parallel pair is 2 ohm
- Total is 8 ohm
- I = 1.5 A, P = 18 W
Questions people ask
How do I work out the cost of running a device?
Energy in kilowatt-hours equals power in kilowatts times hours. A 2 kW kettle used for 0.1 hours uses 0.2 kWh. Multiply by the price per kWh to get the cost. Power companies bill in kWh, and 1 kWh equals 3.6 million joules, so it is a large unit.
Why are power lines high voltage?
Power lost in a cable is I squared R. For the same delivered power P = IV, raising V lowers I, which cuts losses sharply. Doubling the voltage halves the current and reduces heating loss by a factor of four. Transformers then step the voltage down for homes.