Resistors and I-V characteristics

Thermistors are circuit components whose resistance varies with temperature.

There are two major types; negative temperature coefficient (NTC) thermistors, whose resistance decreases with increasing temperature and positive temperature coefficient (PTC) thermistors, whose resistance increases with increasing temperature.

A student is investigating how the resistance of a thermistor varies with temperature by measuring current and voltage. The thermistor is placed in a water bath and the temperature of the water measured using a thermometer.

The diagram below shows how the student set up the experiment (water bath not shown). The circuit has been set up incorrectly.

Describe how the student should change the circuit.

The circuit was corrected and then used to collect data.

The table shows data collected from the investigation.

Temperature / C	Current / mA	Voltage / V
30	0.75	3.00
40	1.10	3.00
50	1.51	3.00
60	2.10	3.00
70	2.80	3.00
80	3.66	3.00
90	4.76	3.00

The axes below show a plot of current against temperature. The first four points from the table have been plotted. Plot the remaining points.

Draw a suitable line of best fit through the data points.

Describe, using the graph and calculations using data from the table, how the resistance of the thermistor varies for increasing temperature. Hence determine whether the thermistor the student used was an NTC or a PTC thermistor.

The thermistor is used in a temperature‑sensing circuit for a heating system to warm milk for a baby.

The student considers two possible designs for the circuit which are shown below.

In each circuit, the voltage V_out across the thermistor is connected to the heating system for warming the milk.

Discuss which circuit may be suitable for the heating system by considering the response of the circuit to changes in temperature.

Explain what is meant by

internal resistance of a cell

e.m.f. of a cell.

A wire in a circuit obeys Ohm’s law.

Which statement about the wire is linked to this law?

A The current in the wire is directly proportional to the potential difference across it.

B The current in the wire is inversely proportional to its resistance.

C The resistance of the wire is directly proportional to its length.

D The resistance of the wire is inversely proportional to its cross-sectional area.

Your answer

A circuit with four light-emitting diodes (LEDs) P, Q, R and S is shown below.

Two LEDs are lit in this circuit. Which two LEDs are lit?

A P and Q

B P and R

C Q and R

D Q and S

Your answer

Calculate the frequency f of the transmitted wave.

The electromagnetic wave is caused by electrons oscillating in the transmitter aerial. Each electron oscillates with simple harmonic motion.

Calculate the maximum acceleration a_max of an electron which oscillates with an amplitude of 4.0 × 10^–6 m.

=........ m s^–2 a_max

Suggest why the diode in Fig. 5.1 is necessary for an ammeter to detect a signal at the receiver aerial.

Use Fig. 19.2 to show that the p.d. across the 50 resistor is 0.50 V.

Calculate the e.m.f. E of the power supply.

The LED emits blue light of wavelength 4.7 × 10^–7 m.

Estimate the number of blue light photons emitted from the LED per second.

number of photons per second =........ s^–1

The light from the LED is incident on a metal of work function 2.3 eV.

Explain, with the help of a calculation, whether or not photoelectrons will be emitted from the surface of the metal.

17* A metal circular plate is rotated at a constant frequency by an electric motor. The plate has a small hole close to its rim. Fig. 17.1 shows an arrangement used by a student to determine the frequency of the rotating

A light-dependent resistor (LDR) and a fixed resistor of resistance 1.2 k are connected in series to a battery. The battery has e.m.f. 4.5 V and has negligible internal resistance. The potential difference V across the resistor is monitored using a data-logger.

Fig. 17.2 shows the variation of V with time t.

Use your knowledge and understanding of potential divider circuits to explain the shape of the graph shown in Fig. 17.2. Include in your answer the maximum and minimum values of the resistance of the LDR. Describe how the student can determine the frequency of the rotating plate.