Electric Charge Examples in Physics

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Electric Charge.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Physics.

Concept Recap

A fundamental property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).

Some particles carry an invisible 'label' — positive or negative — that makes them push or pull on each other.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Charge comes in two types (positive and negative) and is conserved — it can move but cannot be created or destroyed.

Common stuck point: Protons don't move in circuits — it's electrons (negative charges) that flow through wires.

Sense of Study hint: When solving a charge problem, first identify whether charge is being transferred, conserved, or creating a force. Use Q = It to relate charge to current and time. If calculating force between charges, use Coulomb's law. Always check that total charge is conserved — charge cannot be created or destroyed.

Worked Examples

Example 1

easy
An object has an excess of 3.0 \times 10^{13} electrons. What is the total electric charge on the object? Use e = 1.6 \times 10^{-19} \text{ C}.

Solution

  1. 1
    Each electron carries a charge of -1.6 \times 10^{-19} \text{ C}.
  2. 2
    Total charge: Q = ne = 3.0 \times 10^{13} \times (-1.6 \times 10^{-19})
  3. 3
    Q = -4.8 \times 10^{-6} \text{ C} = -4.8 \text{ } \mu\text{C}

Answer

Q = -4.8 \text{ } \mu\text{C}
Electric charge is a fundamental property of matter. It is quantized — it always comes in multiples of the elementary charge e = 1.6 \times 10^{-19} \text{ C}. Excess electrons give a negative charge.

Example 2

medium
Two charged spheres carry charges of +5 \text{ } \mu\text{C} and -3 \text{ } \mu\text{C}. They are brought into contact and then separated. What is the charge on each sphere afterward?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

medium
A current of 2 \text{ A} flows through a wire for 30 \text{ s}. How many electrons pass through the wire? Use e = 1.6 \times 10^{-19} \text{ C}.

Example 2

hard
A Van de Graaff generator accumulates charge on a metal sphere of radius 0.15 \text{ m}. If the electric field at the surface reaches 3 \times 10^6 \text{ V/m} (air breakdown), what is the maximum charge on the sphere? Use k = 9 \times 10^9 \text{ N m}^2/\text{C}^2.
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