| a. |
State
the Coulomb’s Law and describe an experiment to verify the correctness
of the law. |
5
marks |
|
|
|
|
Coulomb's Law |
|
|
For two point charges Q1
and Q2 at distance r apart, the electric force
between them is |
|
|
 |
|
|
where e is a constant
called the absolute permittivity of the intervening medium. |
1 |
|
|
|
|
Investigating Coulomb's
Law |
|
|
 |
1 |
|
When the thread suspending the sphere (of mass
m) makes an angle q with the vertical,
the electric force is given by |
|
|
 |
|
|
where T in (2) and (3) is the tension
in the thread. |
|
|
For small angle of deflection, we have |
|
|
 |
1 |
|
Thus, the electric force is |
|
|
 |
1 |
|
When a graph of d is plotted against
1/r2, a straight line would be obtained. |
1 |
|
|
|
| b. |
Describe,
with the aid of a diagram, a van de Graaff generator, explaining the physical
principles of its action. |
6
marks |
|
|
|
|
 |
2 |
|
A van de Graaff generator makes use of the fact that excessive
charge resides only on the outer surface of a conductor. |
1 |
|
As the rubber belt is driven into motion, it would be positively
charged by friction. This induces a negative charge on the spray point.
Discharge occurs due to the action of points. Negative charges are then
spread on the belt. |
1 |
|
Inside the dome, discharge due to action of points occurs
again. This would cause the negative charge on the belt to transfer to
the dome surface. |
1 |
|
Charge can accumulate to a very large amount. |
1 |
|
|
|
| c. |
A
copying machine has an aluminium drum which is coated with a photoconductor,
selenium. Describe how the operations of a copying machine make use of
the principles in electrostatics. |
5
marks |
|
|
|
|
 |
2 |
|
The heart of a copying machine is the selenium
coated drum. The drum is made of aluminium which is an excellent electric
conductor. Selenium, on the other hand, is a photoconductor. It is an insulator
in the dark but becomes a conductor when exposed to light. |
1 |
|
After the drum is coated with electric charge,
light reflected from the original document falls onto it. Light area on
the drum becomes neutral. Charges retain in the dark area. |
1 |
|
Powder with opposite charge is then spread
onto the drum. They would adhere to the dark area. |
|
|
The drum is then rolled over the copy paper.
The paper is then heated and the toner fused onto it permanently. |
1 |
|
|
|