Physics · Fields. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
Objects become charged when electrons move, by friction, by contact or by induction. Charge is never created or destroyed, so the total charge stays the same. Millikan's oil drop experiment showed that charge is quantized: it comes only in whole-number multiples of e = 1.6 × 10−19 C.
Coulomb's law gives the force between two point charges. Like charges repel and unlike charges attract. Electric field strength is the force per unit positive charge on a small test charge. Electric field lines run from positive to negative.
Magnetic field lines form closed loops: circles around a straight current-carrying wire, and a uniform field along the axis inside a solenoid.
Students forget to square the distance in Coulomb's law, or leave charges in μC or nC and distances in cm. Convert to coulombs and metres first: 1 μC = 10−6 C, 1 nC = 10−9 C.
Answer: 4.5 N, repulsive
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