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Intensity calculator

A-level Waves

Intensity is the power of a wave that passes through each unit area.

I = P / A

This free calculator is for A-level physics. Choose the quantity you want to find, enter the other values with their units, and it shows the working in the layout used in exams, with the answer to the number of significant figures you choose. The values shown are an example. Change them to your own.

Quantity to find

500 W/m² W/m²

Working

  1. Write the equation
    I = P / A
  2. Substitute the values
    I = 60 ÷ 0.12
  3. Calculate
    I = 500 W/m²
  4. Give the answer to 3 significant figures
    I = 500 W/m²

Worked example

Find I, the intensity. Given: power P = 60.0 W; area A = 0.120 m².

  1. Write the equation
    I = P / A
  2. Substitute the values
    I = 60 ÷ 0.12
  3. Calculate
    I = 500 W/m²
  4. Give the answer to 3 significant figures
    I = 500 W/m²

Symbols and units

SymbolQuantitySI unit
IIntensityW/m²
PPowerW
AAream²

Intensity: rearranged

  • To find intensity: I = P / A
  • To find power: P = I A
  • To find area: A = P / I

Exam tip

The area must be at right angles to the direction of the wave.

Common questions

What is the formula for intensity?

The equation is I = P / A, where I is the intensity (W/m²); P is the power (W); A is the area (m²).

What units should I use for intensity?

In SI units: intensity in W/m², power in W, area in m². The calculator also accepts common alternatives such as kilometres, grams or minutes, and converts them for you.

How do I rearrange I = P / A?

Rearranging gives these forms: to find intensity: I = P / A. to find power: P = I A. to find area: A = P / I.

Practise intensity questions

Press the button for a question with random numbers. Work it out on paper first, then type your answer.

Related

More waves equations: Wave speed, Frequency and time period, Magnification, Refractive index, Snell’s law, Critical angle, Double-slit fringe spacing, Diffraction grating. See the full list on the a-level physics equation calculator page.