Describe positive and negative charge and that like charges repel and unlike charges attract, and use Coulomb's law qualitatively (force proportional to the charges and inversely proportional to the square of the distance) (MA STE Introductory Physics, Motion and Forces, HS-PS2-4).

What this topic is asking

This topic opens Module 6 and sits in the Motion and Forces reporting category of the Massachusetts Introductory Physics MCAS, under standard HS-PS2-4. You must describe electric charge (positive and negative), the rule that like charges repel and unlike charges attract, and use Coulomb's law qualitatively: the electric force is proportional to the product of the charges and inversely proportional to the square of the distance between them. The crosscutting idea is cause and effect together with patterns: the electric force follows the same inverse-square pattern as gravity.

Electric charge

The MCAS expects you to know that there are two charges and how objects get them. Ordinary matter is neutral because it has equal positive and negative charge. Rubbing certain materials together transfers electrons from one to the other, leaving one positively charged and the other negatively charged, which is why a balloon rubbed on hair sticks to a wall. No charge is created or destroyed in the process; it is only moved from one object to the other, which is what "charge is conserved" means.

Like repels, unlike attracts

This is the single most-tested fact in the topic, and it has a clear parallel and contrast with gravity:

• Electric force can be a push (repulsion) or a pull (attraction), depending on the signs of the charges.
• Gravity (covered in gravitation and Coulomb's law) is always a pull, because mass comes in only one kind.

So a key difference the MCAS likes: the electric force has two possible directions, gravity has one.

Coulomb's law: how the force depends on charge and distance

At the introductory level the MCAS tests Coulomb's law through proportional reasoning, not a constant-filled calculation:

• Charge. If one charge doubles, the force doubles. If both double, the force quadruples.
• Distance. If the separation doubles, the force becomes one quarter ($1/2^2$). If it triples, the force becomes one ninth ($1/3^2$). This is the inverse-square behavior.

The same inverse-square form appears in Newton's law of gravitation, so the framework groups them: both are forces that act at a distance through a field and weaken with the square of the separation. The electric force, though, is enormously stronger than gravity for everyday charges and can act in either direction.

Worked example

Reference-sheet note

The reference sheet does not print Coulomb's law (it gives the electrical formulas $V = IR$ and $P = IV$ for circuits, not the electrostatic force). So on this test the electric force is handled qualitatively: state the like-repel, unlike-attract rule and use proportional reasoning for how the force depends on the charges and the square of the distance. What you recall is the two kinds of charge, conservation of charge, and the inverse-square dependence.

Try this

Q1. State whether the force between two negative charges is attraction or repulsion. [1]

• Cue. Repulsion; they are like charges, and like charges repel.

Q2. Two charges feel a force $F$. The distance between them is halved. State the new force. [2]

• Cue. Inverse-square: halving the distance gives a factor of $1/(1/2)^2 = 4$, so the force becomes $4F$.