Rearranging Equations for IGCSE Physics

Many IGCSE Physics questions require you to rearrange an equation before substituting values. The method is the same for every equation.

The cover-up method

For equations with three variables (like V = I R), write them in a triangle:

• V on top
• I and R on the bottom

Cover the variable you want to find:

• Cover V: what remains is I x R, so V = I R
• Cover I: what remains is V over R, so I = V / R
• Cover R: what remains is V over I, so R = V / I

The balance method

For more complex equations, treat the equation like a balance. Whatever you do to one side, do to the other.

Example: Rearrange E = m c (T2 - T1) to find c.

Step 1: Divide both sides by m: E / m = c (T2 - T1) Step 2: Divide both sides by (T2 - T1): E / [m (T2 - T1)] = c

So c = E / [m (T2 - T1)].

Practice equations

Try rearranging each of these for the variable shown:

1. v = f x lambda. Find lambda.
2. p = F / A. Find F.
3. W = F x d. Find d.
4. P = E / t. Find t.
5. rho = m / V. Find V.
6. Ep = m g h. Find h.
7. p = h rho g. Find h.

Answers

1. lambda = v / f
2. F = p x A
3. d = W / F
4. t = E / P
5. V = m / rho
6. h = Ep / (m g)
7. h = p / (rho g)

Common mistakes

• Forgetting to flip the fraction when moving a term from denominator to numerator.
• Not applying the operation to every term on both sides.
• Substituting before rearranging, which makes algebra harder.