The diagram shows the gamma rays produced when positrons and electrons collide and annihilate - Edexcel - GCSE Physics - Question 5 - 2015 - Paper 1

To find the total mass of the positron and electron before annihilation, we can use the energy-mass equivalence formula:

$E = mc^2$

Given that the total energy of the gamma rays is $1.6 × 10^{-13} J$ and using the speed of light $c = 3.0 × 10^8 m/s$:

1. Substituting into the equation: $1.6 × 10^{-13} = m (3.0 × 10^{8})^2$

2. Solving for mass $m$: $m = \frac{1.6 × 10^{-13}}{(3.0 × 10^{8})^2}$

3. This calculation gives: $m ≈ 1.77 × 10^{-30} kg$.

Before annihilation, the positron has a charge of +1 and the electron has a charge of -1. Therefore, the total charge is: $+1 - 1 = 0$ After annihilation, gamma rays have no charge, so: - gamma rays have charge = 0.