Electric Charge and Electric Field: Coulomb, Induction, Field Lines

In the paper

Unit 5 carries 4 of the 50 physics marks; with three chapters sharing them, this one averages a little over one (MEC publishes weights by unit, not by chapter). It is the cheapest of the three to secure. One law, one definition and a short set of rules for drawing lines cover most of what can be asked, and none of it depends on another unit — unlike the next chapter, which needs everything here.

The MEC scope lines are: Field due to point charges; Electrostatic induction; Coulomb's law and its applications. The three headings below are those points, in MEC's order.

Three ways it comes:

• Recall — the value of 1/4πε₀, which of four listed quantities is a vector, the rule that two field lines never cross, the sign a glass rod takes when it is rubbed with silk.
• Understanding — why an uncharged scrap of paper is attracted to a charged comb, why a charged body always attracts a neutral one, why the field is zero somewhere between two like charges but nowhere between two unlike ones.
• Application — the force between two charges when the medium is changed from air to water, the position of the neutral point on the line joining two charges, the torque on a dipole in a uniform field.