Question 11
(a) (i) Define the electric potential at a point in an electric field.
(ii) An uncharged body, A, was charged electrostatically by a test charge, B, using the method of induction and the method of contact. State two differences between the two methods.
(b) An important precaution during an electricity experiment is to open the circuit when no readings are being taken. Give two reasons for the stated precaution.
(c) Fig. 11.0 is a circuit diagram in which a coil of inductance, L, and a resistor of resistance, R, are connected to a variable alternating source of frequency, f.
The table shows the square of the impedance, Z2; corresponding to each value of f2.
- Write down the equation for Z in terms of f2, R2 and L2.
- Plot a graph of Z2 against f2 and use it to determine the values of:
- L;
- R [π2 = 10]
Observation
In question 11a, a significant number of candidates failed to make any reference to materials A and B, resulting in them losing out on full marks. However, in question 11b, the majority of candidates were able to provide the correct answers. When it came to question 11c, most candidates struggled to determine the values of L and R from the graphs they plotted.
(a) (i) Electric potential at a point in an electric field
The work done per unit positive charge in bringing the charge from infinity to the point.
(ii) Difference between induction and contact methods
Induction |
Contact |
B never touches A |
B touches A |
The polarity of B is opposite to that of A eventually |
The polarity of B is the same as A eventually |
Charge on B is repelled to the opposite end of A |
Charge on B is transferred to the same side of the contact |
Requires earthing |
Does not require earthing |
(b) Reason for precaution
- To prevent/minimize overheating (in the load)
- To prevent the cell from running down/draining quickly.
- To allow the depolarizer to recover
(c) (i) 