Electrostatics: Charge, Force, Field, and Potential

A rubbed balloon can cling to a wall because charge moves and the wall’s charges rearrange. Electrostatics turns that everyday effect into a model of force, field, energy, and potential.

This lesson supports Chapter 16 of High School Physics.

Charge is conserved and quantized

Like charges repel, and opposite charges attract. Charge is transferred, not created from nothing. In ordinary matter it comes in whole-number multiples of the elementary charge. Conductors allow charge to move freely, while insulators restrict that motion.

Coulomb’s law is an inverse-square law

The electric force magnitude between point charges is \[F=k\frac{|q_1q_2|}{r^2}.\] Doubling the separation reduces the force to one fourth. The electric field is force per unit positive test charge, \(\vec E=\frac{\vec F}{q}\), so its direction is the direction a positive test charge would accelerate.

Potential describes energy per charge

Electric potential is \(V=\frac Uq\). A potential difference can change a charge’s kinetic energy. Field lines and equipotential lines give different information: field lines point in the force direction for a positive test charge, while moving along one equipotential requires no electric work.

Worked example

Two charges are moved from separation \(r\) to \(2r\). Coulomb’s law contains \(r^2\) in the denominator, so the new force is \(\frac F4\). This conclusion does not require the charge values.

Watch the idea in action

Khan Academy gives a focused explanation of the chapter’s central idea. Pause before each calculation and predict the next step on paper.

Practice questions

1. How do like charges interact?
2. Is electric charge conserved?
3. Write Coulomb’s law for force magnitude.
4. How does force change when distance doubles?
5. How is electric field defined?
6. What does electric potential measure?

Answers

1. They repel.
2. Yes.
3. \(F=k\frac{|q_1q_2|}{r^2}\).
4. It becomes one fourth as large.
5. Force per unit positive test charge.
6. Electric potential energy per unit charge.

Keep studying

High School Physics includes 12 full-length practice tests: two in the book and ten online.

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