# 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.

- Read and record the diameter
**d1**and**d2**. - Measure and record
**xmi**and the corresponding**xni**for**W1**. - Evaluate
**R1 = x R**in**each**case. - Measure and record
**xmi**and the corresponding**xni**for**W2**. - Tabulate your readings
- Plot a graph with
**R2**on the vertical axis and**R1**on the horizontal axis. - Determine the slope,
**s,**of the graph. - 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:

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

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

- Six values of
**Xm**correctly converted - Six values of
**Xn**= (100 –**Xm)**correctly evaluated for**W1** - Six values of
**R1**correctly evaluated to at least to 2 d.p - Six values of
**Xm**for**W2**correctly

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

- Six values of
**Xm**correctly converted - Six values of
**Xn**= (100 –**Xm)**correctly evaluated for**W2** - 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

Repeated readings/repeat readings

b(i) A metal consist of free electrons in random motion. When the temperature of the metal is

increased, the vibration of the atoms is increased. This increases the number of collisions of the electrons with the atoms and therefore reduces the average velocity of electrons (conductivity). This results in the increase in the resistance of the metal.

**OR**

When the temperature of a metal is increased, the thermal vibrations of the atoms in the metal increases, this increases the interaction of the electrons with the crystal lattice, and therefore reduces the mobility of the electrons. The reduction in mobility of the electrons in the metal increases the resistance of the metal.

**OR**

The resistance (Ro) of a metal at temperature θ is related to the temperature by the equation: Rθ = Ro (1 + αθ) (where α is temperature coefficient of the resistance of the metal at 0oC). For metals α is positive, hence, resistance increases with increase in temperature. **OR**

With increase in temperature of a metallic conductor, the length of the metal increases and since resistance is directly proportional to length, resistance increase.