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Figure 9 shows a practical circuit in which a variable resistor is used to control the brightness of a lamp - AQA - A-Level Physics - Question 7 - 2019 - Paper 1
Question 7
Figure 9 shows a practical circuit in which a variable resistor is used to control the brightness of a lamp. The voltmeter reading is monitored as the variable resis... show full transcript
Worked Solution & Example Answer:Figure 9 shows a practical circuit in which a variable resistor is used to control the brightness of a lamp - AQA - A-Level Physics - Question 7 - 2019 - Paper 1
Step 1
Explain why the reading on the voltmeter decreases as the brightness of the lamp increases.
Answer
As the brightness of the lamp increases, more current flows through the circuit. This increased current leads to a larger voltage drop across the internal resistance of the power source (cell). Since the total voltage supplied by the cell remains constant, the voltage available for the lamp decreases, resulting in a lower reading on the voltmeter.
Step 2
Explain why V2 is greater than V1.
Answer
When a second identical cell is connected in parallel, the combined internal resistances are less than that of the single cell, which lowers the equivalent internal resistance of the circuit. As a result, the effective voltage drop across the internal resistance decreases, permitting more voltage across the load (lamp). Therefore, the reading V2 will be greater than V1 due to this decreased voltage drop across the internal resistance.