KinexiaLEARN PHYSICS

Electric Field — From Measured Force (Worked Example)

Easy
Problem

A proton (q = 1.6 × 10⁻¹⁹ C) experiences a force of 3.2 × 10⁻¹⁵ N in an electric field. What is the field strength?

Given

  • F = 3.2 × 10⁻¹⁵ N

  • q = 1.6 × 10⁻¹⁹ C

Find

The field E.

Principle

Field is defined as force per unit charge: E = F/q. Measuring the force on a known charge reveals the field.

Formula

Solution

  1. Write the definition: E=F/qE = F/q

  2. Substitute: E=(3.2×10−15)/(1.6×10−19)E = (3.2 \times 10^{-15})/(1.6 \times 10^{-19})

  3. Divide: E=2.0×104 N/CE = 2.0 \times 10^4\ \mathrm{N/C}

2.0 × 10⁴ N/C

Unit check

N ÷ C = N/C — correct.

What this means physically

2.0 × 10⁴ N/C — a modest lab field. This is how field strengths are actually measured: put a known charge in and read the force.

Common mistake

Dividing q by F instead. E = F/q — force on top, charge below; check that units come out N/C.