Physics 9702/52 — February/March 2019

A student is investigating the behaviour of a capacitor-resistor circuit as shown in Fig. 1.1.

A neon lamp flashes on and off when it is connected across the capacitor with a potential difference $V_F$ across the lamp of approximately $90\text{ V}$.

The student has a number of unmarked resistors.

It is suggested that the period $T$ of the flashes of the lamp is related to the resistance $R$ of the resistor by the expression

$$T = RCK$$

where $C$ is the capacitance of the capacitor and $K$ is a constant.

The constant $K$ is given by

$$K = \ln \left( \frac{V_i - V_L}{V_i - V_F} \right)$$

where $V_i$ is the potential difference across the input, $V_F$ is the potential difference required to make the lamp flash and $V_L$ is a constant.

Design a laboratory experiment to test the relationship between $T$ and $R$.
Explain how your results could be used to determine a value for $K$ and $V_L$.

You should draw a diagram, on page 3, showing the arrangement of your equipment. In your account you should pay particular attention to:

• the procedure to be followed
• the measurements to be taken
• the control of variables
• the analysis of the data
• any safety precautions to be taken.