Power: P = V I

The idea

P = V I is the symbolic form of the power equation: power equals potential difference multiplied by current. P is power in watts, V is potential difference in volts and I is current in amperes. The equation shows that a component transfers more energy each second when either the potential difference across it or the current through it increases.

For example: A heater on a 230 V supply drawing 4.0 A has power P = 230 × 4.0 = 920 W. The same relationship rearranges to V = P ÷ I and I = P ÷ V. Because the equation is given in symbols, you must match each symbol to its quantity and unit before substituting, and you should quote the unit W with a numerical answer.

What earns the marks

• Identify P as power in watts, V as potential difference in volts and I as current in amperes.
• Substitute values into P = V I correctly, for example 230 V × 4.0 A = 920 W.
• Rearrange to V = P ÷ I or I = P ÷ V and calculate the unknown accurately.
• Give the answer with the correct unit, converting to kilowatts if the question requires it.
• Interpret the result as the rate of energy transfer, for example 920 W means 920 J transferred each second.

Common mistakes and how to put them right

• Confusing the symbol I with the number 1 or with current in milliamperes.

  Put it right: I stands for current in amperes, so convert mA to A first.

• Substituting power for potential difference or current.

  Put it right: Check that the value placed in V is in volts and the value placed in I is in amperes.

• Forgetting to square or otherwise adjust when using related equations.

  Put it right: P = V I applies directly, while P = I²R and P = V² ÷ R are different forms for different known quantities.

Exam tips

• Underline the quantities given in the question and label them P, V or I before choosing the rearrangement you need.
• Show the substituted equation with units, then the answer with its unit, so method and final value are both clear.
• Estimate first: 230 × 4 is about 900, so an answer of 920 W is reasonable and a value of 92 W signals a slip.