Question 11
(a)
(i) Define capacitance of a capacitor.
(ii) What is the reason for putting a dielectric between the plates of a capacitor?
(iii) State two applications of capacitors.
(iv) The potential difference between the plates of a capacitor is 135 V and the plate separation is 1.5 cm. If a 4.5 x 10-15 kg charged oil drop between the plates is stationary, calculate the magnitude of the charge on the oil drop
(b)
(i) Define electric field intensity at a point.
(ii) State two disadvantages of a moving-coil galvanometer.
(c)
Two point charges of magnitudes +5.0 x 10-5 C and +3.0 x 10-5 C situated in a vacuum, are separated by a distance of 0.2 cm. Determine the:
(i) electric field intensity at a point, P, midway between the charges;
(ii) force on a +2.0 x 10-6 C charged body at P.
Diagram
The diagram above illustrates a galvanometer of full-scale-deflection 20mA and internal resistance of 10, converted to an ammeter reading up to 6A. Calculate the value of R.
(d) State two:
(i) applications of static electricity;
Observation
Part (a) Well attempted by most candidates.
Part (b) Many candidates attempted this question correctly.
Part (c) Candidates performance was average.
The expected answer is:
11.
(a)(i) Fleming’s Left Hand Rule
If the thumb and the first two fingers of the left hand are held mutually at right angles to each other, the first finger pointing the direction of the Field, the second finger in the direction of Current, then the thumb points in the direction of Motion.
(ii) Factors that enhance the sensitivity of a moving coil galvanometer
- weaker (phosphor) hair spring
- larger number of turns in the coil
- increase area of coil
- strongest magnet
(b) (i) Advantages of moving coil galvanometer
- Very sensitive to small changes in current
- Can be upgraded to measure higher currents
- Well damped/robust
- Has uniform / linear scale
(ii) Disadvantages of moving coil galvanometer
- Can only measure direct current
- Affected by current change
- Their internal resistance affects the current and being measured
(c) Calculation of the value of R
I = Is + Ig
Is = 6 – 0.02
= 5.98 A
But Igrg = IsRs
R = 
(d) (i) Applications of static electricity
Used in
- xerography/electrophotography
- air filters
- photocopiers
- paint sprayers
- (medical) theatres
- powder testing
- printers
- static bonding
- aircraft refuelling
- lightning arrester
(ii) Industrial hazards of static electricity
- Explosions in flour mill
- Explosions when fuelling aircraft
- Damage to integrated circuits
- Electric shock