MID-SEMESTER TEST PHYS1022 will be on Wednesday 10 November at 11 am It will consist of two questions taken from Wolfson chapters 20-23. Please prepare one A4 sheet of revision notes which you may bring with you. This will be handed in with your answers but will not be marked. The test is worth 10% of your final mark for this course. The main purpose of the test is to assess your progress and provide feedback.

Potential difference is path independent.

the field of an electric dipole a uniform field the field of a point charge the field of an electric dipole Equipotential surfaces are always perpendicular to electric field lines and thus to E, which is always tangent to these lines.

CHECKPOINT: A family of parallel equipotential surfaces (in cross section) and five paths along which we move an electron from one surface to another. What is the direction of the electric field associated with the surfaces? For each path, is the work we do positive, negative or zero? (c) Rank the paths according to the work done, greatest first. Answers: (a) to the right (b) positive for 1, 2, 3 and 5; negative for 4 (c) 3 most work; 1, 2 and 5 equal; 4 is negative

Calculating potential V for continuous charge distributions Read Wolfson p 373-375

EXERCISE: What is the potential at P due to charge Q at distance R? at centre of arc when Q as been spread out uniformly over the arc? at centre of ring when Q has been spread over whole ring uniformly? Rank the three according to the magnitude of the electric field set up at P, greatest first. They all have the same potential Q/4πε0R (a) then (b) then (c) zero

EXAMPLE 22.6 Potential of a charged ring A total charge Q is distributed uniformly around a thin ring of radius a. Find the potential on the ring’s axis. Now extend this to a charged disk; see Example 22.7 which involves much more maths!

Calculating the field from the potential Cross sections of equipotential surfaces, with potential difference between each pair dV. The field E at any point P is perpendicular to the surface through P. Test charge q0 moves through distance ds from one surface to the next. What is the work done by the electric field on q0?

Calculating the field from the potential The component of E in any direction is the negative of the rate of change of the electric potential with distance in that direction.

Values of field and potential aren’t related; rather, field is the negative rate of change of potential with position

22.4 Charged conductors An uncharged conductor in an external field What difference would it make if there was an excess charge on the conductor?