A student investigated how the current in a series circuit varied with the resistance of a variable resistor.
Figure 8 shows the circuit used.
Figure 9 shows the results.
i- The battery had a power output of 230 mW when the resistance of the variable box resistor was 36 Ω.
Determine the potential difference across the battery.
[4 marks]
SOLUTION
Using the electrical power formula from the equation sheet.
On subtituting currect, I in the above equation from I=V/R, the equation becomes
Don’t forget to convert, 230 mW to W.take the help of the following table
ii- The student concluded:
‘the current in the circuit was inversely proportional to the resistance of the variable resistor.’
Explain how Figure 9 shows that the student is correct.
[2 marks]
ANSWER
We will multiply current and resistance for two separate points, if their answers comes the same it can be concluded that there exist an inverse proportionality between current and resistance.
So, the two separate points from the graph are
(12, 0.24)
And (20, 0.14)
Now, since the product of current and resistance at any point on the graph is a constant value, the student’s consclusion stands correct.
iii- Figure 10 shows a circuit with a switch connected incorrectly.
Explain how closing the switch would affect the current in the variable resistor.
[2 marks]
ANSWER
Current will take path of least resistance and as the switch has zero resistance, all charges would flow through it. Therefore, the ammeter reading will drop to zero as there is no current through the variable resistor.
