D.2 Electric and magnetic fieldsIB Physics SL: Revision notes

There are two types of electric charge: positive and negative. Like charges repel; unlike charges attract. Charge is measured in coulombs (C).

The conservation of electric charge: the total charge of an isolated system never changes. Charge can be moved from one body to another, but it is never created or destroyed. When two identical conductors touch, their total charge is shared equally between them.

The force between two point charges is F = kq₁q₂/r², where k = 1/(4πε₀) = 8.99 × 10⁹ N m² C⁻². It acts along the line joining the charges; it is repulsive for like charges and attractive for unlike charges. Like gravity, it is an inverse-square law.

In Millikan's experiment, tiny charged oil drops are held between horizontal plates. Adjusting the potential difference until a drop is stationary gives qE = mg, so q = mg/E. Every charge measured is a whole-number multiple of about 1.6 × 10⁻¹⁹ C, so charge is quantised: it comes in units of the elementary charge e = 1.60 × 10⁻¹⁹ C.

• Friction: rubbing two insulators transfers electrons from one to the other; they get equal and opposite charges.
• Contact: touching a charged object to a conductor lets charge flow onto it; the conductor ends with charge of the same sign.
• Electrostatic induction: a charged rod held near a conductor separates the charges in it. Grounding (earthing) the conductor while the rod is near lets electrons flow to or from earth. Removing the earth first, then the rod, leaves the conductor with charge of the opposite sign to the rod.

In metals only electrons move; the positive nuclei stay fixed.

The electric field strength at a point is the force per unit charge on a small positive test charge: E = F/q (N C⁻¹ or V m⁻¹). Electric field lines show the direction of the force on a positive test charge: they start on positive charges and end on negative charges, are radial around a point charge, and never cross. Field line density represents field strength: closer lines mean a stronger field.

Between two parallel plates the field is uniform, with parallel, equally spaced lines, and E = V/d. Near the edges the lines curve outwards and the field weakens.

Magnetic field lines show the direction of the force on a north pole. Around a bar magnet they leave the north pole and enter the south pole outside the magnet, then continue inside from south to north, so they form closed loops. A long straight wire has circular field lines centred on the wire. A solenoid has a uniform field inside (parallel, equally spaced lines) and a bar-magnet pattern outside. Line density again represents field strength.