Resistance, temperature and light

The temperature of a metal wire increases while the potential difference across it is kept constant.
Which statement explains why the resistance of the wire increases?

The intensity of the light falling on a light-dependent resistor (LDR) increases.
Which row describes the changes in the number of charge carriers per unit volume in the LDR and in its resistance?

An NTC thermistor is connected in series with an ammeter and a cell of constant e.m.f. and negligible internal resistance. The thermistor is warmed gently with a hair dryer.
Which statement is correct?

A resistance thermometer uses a coil of fine platinum wire as its sensing element. The coil is placed in a liquid whose temperature is to be measured.

An LDR and a 10 kΩ fixed resistor are connected in series across a 6.0 V battery of negligible internal resistance. The output p.d. is measured across the fixed resistor.

Figure 1 shows four sketch graphs, A to D, of resistance R against Celsius temperature θ.
Which graph shows how the resistance of a copper wire varies between 0 °C and 100 °C?

In the circuit shown in Figure 1, the battery has negligible internal resistance and the voltmeter has a very high resistance. In dim light the resistance of the LDR is 30 kΩ.
What is the reading on the voltmeter?

Figure 1 shows a simple model of the same electrical component at 15 °C and at 75 °C.
Which row identifies the component and describes how its resistance changes between 15 °C and 75 °C?

Figure 1 shows a circuit that switches on a street lamp automatically. The battery has negligible internal resistance and the switch unit draws negligible current. The switch unit turns the lamp on when the output p.d. V_out reaches 4.0 V.

Figure 1 shows a simple model of the material of an LDR at the same temperature, first in the dark and then in bright light.

A small filament lamp takes a current of 0.50 A when the potential difference across it is 6.0 V, and a current of 0.30 A when the potential difference is 2.0 V.
What is the ratio (resistance at 6.0 V) ÷ (resistance at 2.0 V)?

A length of metal wire is connected to a supply of constant potential difference. As the wire warms up, the current in it falls from 2.40 A to 1.80 A. The number of conduction electrons per unit volume and the dimensions of the wire do not change.
What is the ratio (final drift velocity of the electrons) ÷ (initial drift velocity)?

A light-dependent resistor and a fixed resistor of resistance 2.2 kΩ are connected in series across a 6.0 V supply of negligible internal resistance. In daylight the resistance of the LDR is 3.3 kΩ.
What is the potential difference across the LDR in daylight?

An NTC thermistor and a fixed resistor are connected in series across a battery of constant e.m.f. and negligible internal resistance. A voltmeter is connected across the fixed resistor. The temperature of the thermistor increases.
Which row describes the changes in the current in the circuit and in the voltmeter reading?

The coil of an electromagnet is wound from copper wire of cross-sectional area 1.5 mm². It is connected to a supply of constant potential difference. When the coil is first switched on, at room temperature, the current is 12.0 A. Copper has 8.5 × 10²⁸ conduction electrons per m³. e = 1.60 × 10⁻¹⁹ C.

A student increases the potential difference across an NTC thermistor in steps from zero and records the current. For small potential differences the current is proportional to the p.d. At larger potential differences the graph of current against p.d. curves upwards: its gradient increases. The thermistor is in air at room temperature throughout.

A student investigates how the resistance of an NTC thermistor depends on temperature, using a water bath, a thermometer with a resolution of 1 °C and a digital ohmmeter. Her results are:
20 °C: 5.74 kΩ; 40 °C: 2.68 kΩ; 60 °C: 1.37 kΩ; 80 °C: 0.755 kΩ.

A student measures the current in a 6.0 V filament lamp for a range of potential differences. Her results are:
p.d. / V: 0.50, 1.0, 2.0, 4.0, 6.0 — current / A: 0.212, 0.324, 0.418, 0.478, 0.500.

Figure 1 shows how the resistance R of a filament lamp varies with the potential difference V across it.
What is the power dissipated in the lamp when the p.d. across it is 6.0 V?

Figure 1 shows how the resistance of a coil of metal wire varies with temperature. The coil is connected to a supply that keeps the potential difference across it constant at 12.0 V.
By how much does the current in the coil decrease when the coil is heated from 0 °C to 100 °C?

Figure 1 shows how the resistance R of an LDR varies with the illuminance E of the light falling on it. The LDR is moved from a place where the illuminance is 25 lux to a place where it is 400 lux. Its temperature does not change, so the drift velocity of the charge carriers for a given p.d. is unchanged.
By what factor does the number of charge carriers per unit volume in the LDR increase?

A filament lamp is connected to a 6.0 V supply of negligible internal resistance at time t = 0. Figure 1 shows how the current in the lamp varies with time.
What is the increase in the resistance of the filament between t = 0 and the time when the current has become steady?

A coil of metal wire is placed first in melting ice at 0 °C and then in boiling water at 100 °C. At each temperature the current in the coil is measured for several values of potential difference. The current is kept small, so the coil stays at the temperature of its surroundings. Figure 1 shows the two I–V graphs, labelled P and Q.

Figure 1 shows the I–V characteristic of a 12 V filament lamp for positive potential differences.

A student investigates how the resistance R of an LDR depends on the illuminance E. She places a lamp at different distances from the LDR, measures E with a light meter placed next to the LDR and measures R with an ohmmeter. She works in a darkened room, and the temperature of the LDR stays constant. Figure 1 shows her values of 1/R plotted against E, with a line of best fit.

Figure 1 shows how the resistance of an NTC thermistor varies with temperature, and the circuit of a temperature warning device for a greenhouse that uses it. The battery has negligible internal resistance and the voltmeter has a very high resistance.

A semiconductor component is connected to a supply of constant potential difference. When it is heated from 20 °C to 50 °C, the number of charge carriers per unit volume increases by a factor of 6.0 and, because of increased lattice vibrations, the drift velocity of the carriers falls by 20%. The dimensions of the component do not change.
By what factor does the resistance of the component change?

A filament lamp is connected in series with a 10 Ω resistor and a supply of negligible internal resistance. Measurements on the lamp alone show that the p.d. across it is 1.0 V when the current is 0.20 A, and 2.5 V when the current is 0.30 A. In the series circuit the current is 0.30 A.
What is the e.m.f. of the supply?

An LDR is connected in series with a 1.0 kΩ resistor across a 5.0 V supply of negligible internal resistance. Over the range used, the resistance of the LDR is inversely proportional to the light intensity. At a certain light intensity the LDR's resistance is 4.0 kΩ. The light intensity is then doubled.
What is the new p.d. across the 1.0 kΩ resistor?

A constant potential difference of 5.0 V is applied across an LDR. In dim light the current is 0.45 mA. When a lamp is switched on nearby, the current rises to 2.5 mA. The temperature of the LDR does not change significantly.

A heat alarm for a kitchen uses an NTC thermistor in series with a fixed resistor across a 5.0 V supply of negligible internal resistance. The alarm sounds when the p.d. across its input reaches 3.0 V. The resistance of the thermistor is 10.0 kΩ at 25 °C and 1.76 kΩ at 70 °C. The alarm input draws negligible current.

Figure 1 shows the I–V graphs for an LDR in dim light and in bright light. In bright light, the LDR is connected in series with a 2.0 kΩ resistor and a 6.0 V supply of negligible internal resistance.
What is the current in the circuit?

Figure 1 shows the I–V graphs for an NTC thermistor in air and for a resistor. The thermistor and the resistor are connected in parallel across a supply of negligible internal resistance. The current from the supply is 30 mA.
What is the p.d. of the supply?

Figure 1 shows how the resistance of an NTC thermistor varies with temperature, and a temperature-sensing circuit that uses it. The battery has negligible internal resistance and the voltmeter has a very high resistance. The voltmeter reading must be 3.0 V when the thermistor is at 40 °C.
What value of R is needed?

Figure 1 shows how the ratio R/R₂₀ varies with temperature θ for a metal wire and for an NTC thermistor, where R₂₀ is each component's resistance at 20 °C. Both components have a resistance of 500 Ω at 20 °C.

Figure 1 shows how the number of charge carriers per unit volume n in the material of a thermistor varies with temperature, and the dimensions of a thermistor rod made from this material. A constant potential difference is applied between the ends of the rod. At 20 °C the current is 3.6 mA. Each charge carrier has a charge of 1.60 × 10⁻¹⁹ C.