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Electricity & Magnetism

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

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Electricity & Magnetism

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  1. Electricity & Magnetism Lecture 4: Coulomb’s Law

  2. Summary: Lecture 3 • Newtons Laws of motion • 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

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

  4. Coulomb’s Law

  5. 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 Torsion Balance

  6. F r Coulomb’s Experiments Line Fr-2

  7. 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

  8. 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 = 8.987.5x109 Nm2C-2 • k is normally expressed as k = 1/40 • where is the permittivity of free space

  9. Coulomb's Law

  10. - + + + Vector form of Coulomb’s Law Q2 r12 Q1

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

  12. Force from many charges Superposition

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

  14. Coulomb vs Newton

  15. Always attractive 1/r2 very weak important on very large scales, planets, the Universe Attractive or repulsive 1/r2 very strong only relevant relatively local scales Coulomb’s Lawvs Newton’s Law of Gravity Two spheres

  16. 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

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