Physics 5054 · O Level · Electrical current

Electrical current — practice question

A student is examining how the current in a thermistor varies with temperature. She assembles the circuit shown in Fig. 1.1. She adds hot water to the beaker. She stirs the water, measures the temperature and checks the ammeter. She enters the temperature and current values in the table in Fig. 1.2. She then repeats the readings at $10\,^{\circ}\text{C}$ intervals while the water cools, until room temperature is reached. She puts all the results into the table, as shown in Fig. 1.2. Fig. 1.3 gives the ammeter reading when the water temperature is $40\,^{\circ}\text{C}$.
(a(i))[1]

Inspect the ammeter and fill in the missing current value in the table of Fig. 1.2.

(a(ii))[1]

State why the student stirs the water before taking a temperature reading.

(b)[4]

On Fig. 1.4, draw a graph of $I$ on the y-axis against $\theta$ on the x-axis. Begin both axes at the origin. Sketch the smooth best-fit curve.

(c(i))[1]

Extend your curve to predict the current at a temperature of $0\,^{\circ}\text{C}$. current at $0\,^{\circ}\text{C}$ = [BLANK] mA

(c(ii))[1]

Suggest how the student can change her investigation to test the prediction made in (c)(i).

(d(i))[1]

Use your graph to estimate the current $I$ when the water temperature is $75\,^{\circ}\text{C}$. $I$ = [BLANK] mA

(d(ii))[2]

The supply voltage $V$ is $5.0\,\text{V}$. Use the equation $R = \frac{V}{I}$ to find the resistance $R$ of the thermistor at $75\,^{\circ}\text{C}$. $R$ = [BLANK] $\Omega$

(e(i))[2]

Describe the relationship between current and temperature shown by your graph for the thermistor.

(e(ii))[1]

Deduce the relationship between resistance and temperature for the thermistor.

Worked solution & mark scheme

This 14-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Current reading = 0.85 mA” …

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