1. Electricity Dr Chris Booth

2. Organisation Two lectures per week (for 6 weeks):
Mondays 10:00 Hicks Building LT07
Fridays 10:00 Diamond LT05
Syllabus (see booklet)
7 topics (plus revision session)
1 or 2 lectures per topic

3. TOPIC 1 Electric Charge & Coulomb’s Law

4. Introduction Electric charge is everywhere! Atoms: positive nucleus
negative electrons
Unit of charge: Coulomb (C) – very large!
Charge on proton = e = 1.610–19 C
Charge on electron = –e
(So > 61018 electrons to make 1 C!)

5. Normally, matter is neutral – equal numbers of e & p
Separated charge  electrostatics
Moving charge  magnetic effects (as well)
Rub plastic (with fur)  plastic gains negative charge
Rub glass (with silk)  glass gains positive charge
In each case – transfer of electrons.
(Positive charge = deficit of electrons)

6. Conductors & Insulators
Conductors – e.g. metals
Some atomic electrons free to move within body of conductor
Insulators – electrons tightly bound to atom or molecule
(Semiconductors – small number of free charges – depends on impurity level)

7. Forces & Induction Like charges ←+ +→ ←– –→ repel
Unlike charges +→ ←– attract
Conductor: Add extra electrons at one end. They repel and spread throughout material
Induction: Use one charged body to charge another – see illustrations.

8. Insulating rod Isolated conducting sphere
Macroscopic view: charge on rod repels positive charge on sphere, attracts negative.
Microscopic view: some free (negative) electrons in sphere are attracted, leaving excess positive charge behind.

9. Connect sphere with wire to ground
Macroscopic view: positive charges escape to ground
Microscopic view: electrons are attracted from ground, neutralising positive charge.

10. Coulomb’s Law Force between 2 charges Q and q:
Proportional to product of charges Qq
Proportional to 1 / square of separation
Q r q
Const. of proportionality k = 8.99109 N m2 C–2
(Check the units!)

11. Normally written
0 = 8.8510–12 C2 N–1 m–2
(Why 4? – See later!)
Vector form:
Force on q due to Q if points from Q to q.

12. Superposition The force due to one charge is unaffected by others.
 Just add vectors
Ftot = F1+F2+F3+...
Example
Charges +Q, +Q and –Q are arranged at the vertices of an equilateral triangle as shown. What would be the force on a charge q at the centre of the triangle?

13. Example 2
Charges +9Q and –Q are a distance  apart. At what point would there be no net force on a test charge q?
Example 3
What is the ratio of the gravitational to the electrostatic force between the electron and proton in a hydrogen atom?
Mass of proton = 1.6710–27 kg; mass of electron = 9.1110–31 kg
Magnitude of electron & proton charges = 1.610–19 C