TEST OF PRACTICAL KNOWLEDGE QUESTION You are provided with cells, a potentiometer, an ammeter, a voltmeter, a bulb, a key, a jockey, and other necessary mat...

Answer Details

Practical: potentiometer, voltmeter and ammeter with a bulb

For each length \( x \) (from \( PJ = x = 10\,\text{cm} \) up to 60 cm) you record the voltmeter reading \( V \) and the ammeter reading \( I \), then evaluate \( \log V \) and \( \log I \).

Graph: plotting \( \log I \) (vertical) against \( \log V \) (horizontal) gives a straight line since \( I \propto V^{s} \) implies \( \log I = s\log V + c \). The slope s is \( \dfrac{\Delta(\log I)}{\Delta(\log V)} \) and the intercept c is where the line meets the vertical axis.

Two precautions

• Make firm, quick contacts with the jockey on the potentiometer wire to avoid heating and sparking, and open the key when readings are not being taken.
• Avoid parallax when reading the voltmeter and ammeter, and check for zero errors before starting.

(b)(i) Brightness of the bulb as x increases

As \( x \) increases, the potential difference tapped off the potentiometer wire increases, so both \( V \) and \( I \) through the bulb increase and the bulb becomes brighter.

(b)(ii) Two electrical devices that do not obey Ohm's law

• A semiconductor diode.
• A filament lamp (its resistance rises as it heats). A thermistor is also acceptable.