Physics 9702 · AS & A Level · Kirchhoff's laws

Kirchhoff's laws — practice question

Fig. 5.1 depicts a circuit that contains a thermistor T with a negative temperature coefficient.
(a)[1]

State Kirchhoff’s first law for a junction.

(b(i))[2]

The thermistor has resistance $R_0$ at a temperature of $0^\circ\text{C}$. On Fig. 5.2, sketch one possible change in the resistance of the thermistor with temperature from $0^\circ\text{C}$ to $100^\circ\text{C}$.

(b(ii))[3]

With reference to the current in the cell, explain why the current in resistor R decreases as the thermistor temperature rises.

(c(i))[2]

The electromotive force (e.m.f.) $E$ of the cell in Fig. 5.1 is $1.50\,\text{V}$. The internal resistance $r$ of the cell is $0.12\,\Omega$. Resistor R has a resistance of $6.00\,\Omega$. At a particular temperature of the thermistor, the current in R is $0.200\,\text{A}$. For this temperature of the thermistor, determine the current in the cell.

(c(ii))[2]

The electromotive force (e.m.f.) $E$ of the cell in Fig. 5.1 is $1.50\,\text{V}$. The internal resistance $r$ of the cell is $0.12\,\Omega$. Resistor R has a resistance of $6.00\,\Omega$. At a particular temperature of the thermistor, the current in R is $0.200\,\text{A}$. For this temperature of the thermistor, determine the resistance of the thermistor.

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This 10-mark question has a full step-by-step worked solution and mark scheme. One marking point: sum of current(s) entering a junction = sum of current(s) leaving the junction

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