# WAEC Standard for Physics Alternative to practical III.

A standard resistor of resistance 2 Ω is connected in the left gap of the metre bridge and a length l1 of wire W1 is connected across the right gap of the bridge as shown in the diagram above. The balance point B is determined by touching the metre wire with the jockey. The balance length xm and xn are measured and recorded.

The procedure is repeated for five other values of length l.

The entire procedure is repeated using a second wire W2.

The diameters d1 of W1 and d2 of W2 are measured and recorded

Fig. 3(a) and fig. 3(b) represent sections of a micrometer screw guage indicating the
readings of diameters d1 and d2 respectively.

Fig. 3(c) and fig. 3(d) show the positions Bi of the jockey on the metre wire for W1 and W2 respectively where i = 1, 2, 3, 4, 5 and 6.

1. Read and record the diameter d1 and d2.
2. Measure and record xmi and the corresponding xni for W1.
3. Evaluate R1 =  x R in each case.
4. Measure and record xmi and the corresponding xni for W2.
6. Plot a graph with R2 on the vertical axis and R1 on the horizontal axis.
7. Determine the slope, s, of the graph.
8. Determine the intercept, c, on the vertical axis.
• State two precautions that are necessary to ensure accurate results when performing this experiment.

(b)  (i) Explain why the resistance of a metallic conductor increases with increase in temperature.

(ii)  The rating of an electrical heater is 1000 W, 200V. Calculate its resistance.

### Observation

Part (a) This was popular question among the candidates. However, many candidates could not follow the given instructions in the question. measurement for w1 were mixed with w2. Also, some candidates invented their own values for xn and xm for the two wires.
Part (b) Candidates performance was poor. Most candidates could not explain why the resistance of a metallic conductor increases with increase in temperature.
The expected response is as stated below:

1. Value of d1 correctly read and recorded to 2 d.p in mm
2. Value of d2 correctly read and recorded to 2 d.p in mm
3. Six values of Xm for W1 correctly measured and recorded

to at least 1 d.p and within tolerance of ± 0.1 cm

1. Six values of Xm correctly converted
2. Six values of Xn = (100 – Xm) correctly evaluated for W1
3. Six values of R1 correctly evaluated to at least to 2 d.p
4. Six values of Xm for W2 correctly measured and recorded to

at least 1 d.p and within tolerance of ± 0.1 cm

1. Six values of Xm correctly converted
2. Six values of Xn = (100 – Xm) correctly evaluated for W2
3. Six values of R2 correctly evaluated to at least 2 d.p
• Plot a graph using reasonable scales
• draw line of best it
• determine the slope of the graph
• state any two of the following precautions.

e.g.

Ensured tight terminals/tight connections

Ensured clean terminals/clean terminals

Avoid parallax error in reading metre rule/galvanometer

Zero error noted/corrected on micrometer screw gauge/metre rule/galvanometer

Open key when not taking readings

Avoid sliding jockey on wire