What potential increase vac must an electron be accelerated through if the most energetic photon it can emit will scatter off of a stationary electron at an angle ϕ=60∘ with wavelength 8.4×10−12m? express your answer in kilovolts to two significant digits?

Referring to Compton scattering 
Δλ = h/m₀c (I- cos Ф)
λ' =λ = (0,0242×10⁻¹⁰) (1- cos 60°)
λ= λ' -(0.0242 × 10⁻¹⁰) (1- cos 60°)

7.19 ˣ 10⁻¹²m

The increased potential is given by 
Vₐc = hc/eλ = 6.625 × 10 ⁻³⁴ J,s) ( 3× 10⁸ m/s ( 1.6 ˣ 10 ⁻¹⁰C)
(7.19 ˣ 10⁻¹²m)

173kV.

The answer is 4f======since electrostatic force is given by    we let f be the electrostatic force when  r = 1 m    if we let  denote the electrostatic force when the distance is decreased so that r = 0.5m, then    we established earlier that f = kq₁q₂. therefore    the force will now be 4f.

Answer///(d) atoms contain small,negatively charged particles;