K223 / K224 / 2027
Electric charge and current of electricity overview

Lesson 1 of 4

Electric charge and interactions

  • I can state both charge signs and the coulomb unit.Syllabus

    Syllabus K223 / K224, 10(a). State both charge signs and the coulomb unit.

  • I can state that like charges repel and unlike charges attract.Syllabus

    Syllabus K223 / K224, 10(b). State that like charges repel and unlike charges attract.

Make a guess. You are not marked.

A plastic rod rubbed with a cloth becomes negatively charged. What has moved?

  1. Protons, which have moved from the rod onto the cloth.
  2. Negative charge made by friction on the rod's surface.
  3. Electrons, which have moved from the cloth onto the rod.
Show the answer

Electrons, which have moved from the cloth onto the rod.

Rubbing transfers electrons. The rod gains electrons and becomes negative; the cloth loses them and is left equally positive.

In short

  • There are two kinds of charge, positive and negative.
  • Like charges repel; unlike charges attract.
  • Rubbing moves electrons from one object to the other.

Electric charge has two signs: positive and negative. Charged objects can exert forces on one another without touching.

Charge is represented by q or Q and is measured in coulombs, C. A positive sign means a net positive charge; a negative sign means a net negative charge.

Neutral does not mean no charged particles

Atoms contain positively charged protons in their nuclei and negatively charged electrons. A neutral object has equal total positive and negative charge, so its net charge is zero.

An object with an excess of negative charge has a negative net charge. An object with an excess of positive charge has a positive net charge. Positive and negative describe the charge balance, not whether an object is made only of protons or only of electrons.

Predict attraction and repulsion

Like charges repel. Unlike charges attract.Two positive charges repel, two negative charges repel, and a positive and a negative charge attract.

Predict the electric force on each charged object

These small objects carry the shown net charges. Only their electric forces on one another are shown; orange arrows are forces, not motion.

Like charges repel and unlike charges attractThree rows show separately charged objects A and B. Two positive objects have force arrows pointing apart, as do two negative objects. Opposite-sign objects have force arrows pointing towards one another. Each object has its own force arrow, acting along the line joining the objects. Force direction alone does not specify velocity.Two positive charges repel++ABForce on AForce on BTwo negative charges repel--ABForce on AForce on BUnlike charges attract+-ABForce on AForce on B
Each force arrow acts on the object beside it. Like-sign objects are pushed apart; opposite-sign objects are pulled towards each other in this simple isolated charged-object model.

What the observation establishes

Two charged balls repel

They have the same sign. They could both be positive or both be negative; repulsion alone does not distinguish those possibilities.

If a ball repels a known positive charge in this model, that additional comparison identifies the ball as positive.

Attraction has more than one possible explanation. A charged object can attract an oppositely charged object, but it can also attract a neutral object. Attraction alone does not prove that both objects carry opposite net charges.

Check your understanding

Two small charged balls, separately suspended and isolated from other charged objects, repel. What can be concluded about their charge signs?

  1. They must both be positively charged, because like charges repel each other.
  2. Opposite signs, because the two objects push each other away.
  3. Same sign, but this test cannot tell positive from negative.
Show the answer

Same sign, but this test cannot tell positive from negative.

Like charges repel. Both positive and both negative fit the observation. A further comparison with a known charge would be needed to identify the sign.

When charge moves through a wire, the rate at which it passes a point is the electric current.