Book Reference : Pages 83-85 & 80-81Book Reference : Pages 83-85 & 80-81

1.1. To look at the work of Charles CoulombTo look at the work of Charles Coulomb

2.2. To understand electric potentialTo understand electric potential

Around 1784 Coulomb devised an Around 1784 Coulomb devised an experiment to establish the force between experiment to establish the force between charged object using a torsion balancecharged object using a torsion balance

1.1. Like charges repelLike charges repel2.2. A pair of charged pith A pair of charged pith

ballsballs3.3. Suspended from a vertical Suspended from a vertical

wirewire4.4. 22ndnd charged ball charged ball5.5. Wire twisted until it Wire twisted until it

balanced the repulsionbalanced the repulsion6.6. Distance between balls Distance between balls

variedvaried

Here are some of Coulomb’s results can Here are some of Coulomb’s results can you spot a pattern between F and r?you spot a pattern between F and r?

Distance, r

36 18 8.5

Force, F 36 144 567

Note both results were measured in Note both results were measured in degrees and so the units are relativedegrees and so the units are relative

Halving the distance from 36 to 18 makes Halving the distance from 36 to 18 makes the force increase by x4 and again from 18 the force increase by x4 and again from 18 to 8.5to 8.5

F F 1/r 1/r22

The force is also proportional to the size of The force is also proportional to the size of the two forces involvedthe two forces involved

F F Q Q11QQ22

Bringing this togetherBringing this together

F F Q Q11QQ2 2

rr22

As usual we can turn a proportionality into As usual we can turn a proportionality into an equation by introducing a suitable an equation by introducing a suitable constant of proportionalityconstant of proportionality

F = F = 11 QQ11QQ2 2

4400 r r22

Definition :Definition :The magnitude of the force F between The magnitude of the force F between two electrically charged bodies, which two electrically charged bodies, which are small compared to their separation r are small compared to their separation r is inversely proportional to ris inversely proportional to r22 and and proportional to the product of their proportional to the product of their charges Qcharges Q11 and Q and Q22

Compare the form of Coulomb’s law with Compare the form of Coulomb’s law with Newton’s law of gravitation Newton’s law of gravitation

Not required for A2Not required for A2

PermittivityPermittivity describes how an electric field affects, and is describes how an electric field affects, and is affected by, a dielectric medium, and is determined by affected by, a dielectric medium, and is determined by the ability of a material to polarize in response to the the ability of a material to polarize in response to the field, and thereby reduce the total electric field inside field, and thereby reduce the total electric field inside the material. the material. Thus, permittivity relates to a material's Thus, permittivity relates to a material's ability to transmit (or "permit") an electric fieldability to transmit (or "permit") an electric field..

PermeabilityPermeability is a constant of proportionality that exists is a constant of proportionality that exists between magnetic flux density and magnetic field between magnetic flux density and magnetic field strength in a given mediumstrength in a given medium

Experimentally it can be shown that c = 1 / Experimentally it can be shown that c = 1 / 0000

Calculate the force between an electron Calculate the force between an electron andand

a.a.A proton at a distance of 2.5x10A proton at a distance of 2.5x10-9-9mmb.b.The nucleus of a nitrogen atom (atomic The nucleus of a nitrogen atom (atomic number 7) at a distance of 2.5x10number 7) at a distance of 2.5x10-9-9m m

[3.7 x 10[3.7 x 10-11-11N]N] [2.6 x 10 [2.6 x 10-10-10N]N]

ee = -1.6 x 10 = -1.6 x 10-19-19 C C00 = 8.85 x 10= 8.85 x 10-12 -12 F/mF/m

Two point charges QTwo point charges Q11 is +6.3nC & Q is +6.3nC & Q22 is - is -2.7nC exerts a force of 3.2x102.7nC exerts a force of 3.2x10-5-5N when they N when they are d metres apartare d metres apart

a.a. Find dFind db.b. Find the force if d increases to 3dFind the force if d increases to 3d

[69mm][69mm] [3.6 x 10 [3.6 x 10-6-6N]N]

ee = -1.6 x 10 = -1.6 x 10-19-19 C C00 = 8.85 x 10= 8.85 x 10-12 -12 F/mF/m

We know like charges repel. To bring two We know like charges repel. To bring two like charges X & Y together, work must be like charges X & Y together, work must be done. The field around Y must be overcomedone. The field around Y must be overcome

As we move X from As we move X from towards Y the towards Y the electric electric potential energy potential energy of X increases from 0of X increases from 0

Definition : Definition : The electric potential at a certain point in any The electric potential at a certain point in any electric field is defined as the work done per unit electric field is defined as the work done per unit charge on a “small +ve test charge” when it is charge on a “small +ve test charge” when it is moved from moved from to that point to that point

For a +ve test charge in a field where the For a +ve test charge in a field where the electric potential energy is Eelectric potential energy is Epp, the electric , the electric potential V is shown by:potential V is shown by:

V = EV = Epp / Q / Q

Where Q is the charge in Coulombs, EWhere Q is the charge in Coulombs, Epp is the is the electric potential energy (J) & V is the electric potential energy (J) & V is the electric potential in Volts or J/Celectric potential in Volts or J/C

If test charge Q is moved from a point in the If test charge Q is moved from a point in the field where the potential is Vfield where the potential is V11 to a point to a point where the potential is Vwhere the potential is V22, then the work , then the work done done W is given byW is given by

W = Q(VW = Q(V22 –V –V11))