In an experiment to investigate the internal resistance of a DC power supply, a student records the current III and the terminal potential difference VVV across the power supply as the external resistance is varied.
The recorded data is shown in the table below:
Current I/ATerminal Potential Difference V/V0.152.650.302.350.452.050.601.750.751.45 \begin{array}{|c|c|} \hline \text{Current } I / \text{A} & \text{Terminal Potential Difference } V / \text{V} \\ \hline 0.15 & 2.65 \\ \hline 0.30 & 2.35 \\ \hline 0.45 & 2.05 \\ \hline 0.60 & 1.75 \\ \hline 0.75 & 1.45 \\ \hline \end{array} Current I/A0.150.300.450.600.75Terminal Potential Difference V/V2.652.352.051.751.45Use the data to calculate the gradient MMM of the graph of VVV (yyy-axis) against III (xxx-axis). Show your working.
The internal resistance rrr is equal to the magnitude of the gradient MMM. Write down the value of rrr to a suitable number of significant figures and include the appropriate unit.
Suggest one practical reason why the student should not connect the circuit using extremely short lengths of the external resistance wire (which corresponds to very low external resistance).
Instead of varying the length of a resistance wire, a variable resistor can be used. Describe the standard circuit symbol for a variable resistor.
452 exam-style questions on CIE IGCSE Physics 4.2 Electrical quantities, covering 4.2.1a Positive and negative charge, 4.2.1b Charging by friction and detecting charge, 4.2.1c Electrical conductors and insulators, 4.2.1d Electric fields and electric field patterns, 4.2.2a Electric current, charge and measuring current, 4.2.2b Conduction in metals and the direction of current, 4.2.2c Direct current and alternating current, 4.2.3a Electromotive force, 4.2.3b Potential difference and voltmeters, 4.2.4a Resistance, 4.2.4b Resistance of a metallic wire, 4.2.4c Current–voltage graphs, and 4.2.5 Electrical energy and electrical power. Each one has a worked solution and a mark scheme showing where the marks go.