Electricity Magnetism Lecture 4 Coulombs Law Summary Lecture

Electricity & Magnetism Lecture 4: Coulomb’s Law

Summary: Lecture 3 • Newtons Laws of motion – 1. if F = 0, a = 0 – 2. F = m a (N. B. F, a are vectors) – 3. F 12=-F 21 • Vectors – – Vectors & Scalars Vector addition Geometric Unit vectors Vector components & addition of components • Units

Today’s Topics • Coulomb’s Law • Vector form of Coulomb’s Law • Coulomb vs Newton • Superposition

Coulomb’s Law

Coulomb's Torsion Balance This dial allows you to adjust and measure the torque in the fibre and thus the force restraining the charge This scale allows you to read the separation of the charges

Coulomb’s Experiments F Line F r-2 r

Coulomb's Law • Coulomb determined – Force is attractive if charges are opposite sign – Force proportional to the product of the charges q 1 and q 2 along the lines joing them – Force inversly proportional square of the distance • I. e. – |F 12| |Q 1| |Q 2| / r 122 – or – |F 12|= k |Q 1| |Q 2| / r 122

Coulomb's Law • Units of constant can be determined from Coulomb's Law • Colomb (and others since) have determined this constant which (in a vacuum) in SI units is – k = 8. 987. 5 x 109 Nm 2 C-2 • k is normally expressed as k = 1/4 0 – where is the permittivity of free space

Coulomb's Law

Vector form of Coulomb’s Law r 12 + + Q 1 + Q 2 -

Quiz Object A has a charge of +2 C and Object B has a charge of +6 C. Which statement is true? FBA? A +2 C FAB? B +6 C • • A: FAB=-3 FBA B: FAB=-FBA C: 3 FAB=-FBA D: FAB=12 FBA

Force from many charges Superposition

Force from many charges Q 2 - Q 1 + - Principle of superposition + Q 4 Q 3 Force on charge is vector sum of forces from all charges

Coulomb vs Newton

Coulomb’s Law vs Newton’s Law of Gravity • • Attractive or repulsive 1/r 2 very strong only relevant relatively local scales • • Two spheres Always attractive 1/r 2 very weak important on very large scales, planets, the Universe

Summary: Lecture 4 • Coulomb's Law – Electrostatic Force between charges • Coulomb’s Law (vector) form • Coulomb force vs Gravity – Electrostatic force is in general much stronger • Superposition