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**Electricity & Magnetism**

Lecture 4: Coulomb’s Law

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**Summary: Lecture 3 Newtons Laws of motion Vectors Units**

1. if F = 0, a = 0 2. F = m a (N.B. F, a are vectors) 3. F12=-F21 Vectors Vectors & Scalars Vector addition Geometric Unit vectors Vector components & addition of components Units

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

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Coulomb’s Law

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**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

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**Coulomb’s Experiments**

F Line Fr-2 r

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**Coulomb's Law Coulomb determined**

Force is attractive if charges are opposite sign Force proportional to the product of the charges q1 and q2 along the lines joing them Force inversly proportional square of the distance I.e. |F12| |Q1| |Q2| / r122 or |F12|= k |Q1| |Q2| / r122

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**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 = x109 Nm2C-2 k is normally expressed as k = 1/40 where is the permittivity of free space

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Coulomb's Law

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**Vector form of Coulomb’s Law**

+ - + r12 Q2 + Q1

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**Quiz A: FAB=-3FBA B: FAB=-FBA C: 3FAB=-FBA D: FAB=12FBA**

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

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**Force from many charges**

Superposition

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**Force from many charges**

- Q2 + Q1 - Q3 Force on charge is vector sum of forces from all charges Principle of superposition + Q4

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Coulomb vs Newton

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**Coulomb’s Law vs Newton’s Law of Gravity**

Attractive or repulsive 1/r2 very strong only relevant relatively local scales Always attractive 1/r2 very weak important on very large scales, planets, the Universe Two spheres

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**Summary: Lecture 4 Coulomb's Law Coulomb’s Law (vector) form**

Electrostatic Force between charges Coulomb’s Law (vector) form Coulomb force vs Gravity Electrostatic force is in general much stronger Superposition

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