Resistors & I-V characteristics

Diodes are used in a variety of electrical circuits.

Sketch the current-potential difference graph for a diode.

A package containing a light emitting diode (LED) is marked

“20 mA 3.2 V to operate normally”.

A student uses a 5 V power supply of negligible internal resistance to operate the LED.

Calculate the resistance of a protective resistor required to operate the LED normally from this power supply.

Fuse wire is made of a metal that has a low melting point. The wire melts when the current exceeds a maximum value.

A student investigated how the cross‑sectional area affected the current at which a wire melted. She used wires of the same length and metal, but different diameters.

The student steadily increased the current through each wire and recorded the maximum current through the wire before it melted.

Describe how the student could determine an accurate value for the cross‑sectional area of a wire.

Explain why a current in a wire causes the resistance of the wire to change.

The length of a wire does not affect the value of current at which the wire melts.

Explain why. Assume the wire behaves as a black body.

A student carried out an experiment with a light dependent resistor (LDR). He connected the LDR in series with an ammeter and a power supply, as shown in the circuit diagram.

The student placed the LDR a known distance from a desk lamp, as shown.

The student noted the reading on the ammeter as he brought the LDR closer to the lamp.

The student planned to vary the intensity of light incident upon the LDR by adjusting the distance x between the LDR and the lamp.

He thought that the intensity of light on the LDR would increase uniformly if he decreased x by equal intervals. He therefore planned to take ammeter readings as he decreased x from 20.0 cm to 10.0 cm in equal intervals.

Criticise the student’s plan for data collection.

Explain one precaution that the student should take to ensure that results are accurate.

Explain why the ammeter reading increased as the LDR was brought closer to the lamp. Your answer should include reference to the charge carriers in the LDR.

A student connected a voltmeter to a potential divider, as shown in the circuit diagram.

The potential divider was adjusted by moving sliding contact S from position X to position Y.

Explain how the voltmeter reading V depends upon the position of S.

A filament bulb and ammeter were connected to the potential divider. The potential divider was used to vary V across the filament bulb. The ammeter gave the corresponding current I in the filament.

I varied with V, as shown in the graph below.

Explain the shape of the graph.

The temperature T of the filament varies with the potential difference V across the filament according to the expression

where a and b are constants.

Explain why a graph of logT against logV would give a straight line.

Data for T and V is shown in the table below.

Plot a graph of logT against logV on the grid opposite. Use the extra columns provided to show any processed data.

Determine a value for b using your graph.

The circuit shown provides an input to a transistor. A transistor is a type of electronic switch and in this circuit it can be assumed to have infinite resistance.

The resistance of the LDR varies with light level as shown below.

The transistor switches on when the potential difference across the input increases above 0.7 V. This should happen as the light level reaching the LDR increases above 30 lux.

Deduce whether this circuit responds as required.

Explain how the resistance of the LDR changes as the light level increases.

*5 The diagram shows a ‘shaker torch’. When the torch is shaken, a strong magnet moves forwards and backwards through a copper coil, powering a light-emitting diode (LED).

Each time the magnet moves through the coil a current pulse is generated. The coil is connected to a capacitor via a diode, as shown.

Once the torch has been shaken for a few minutes the switch is closed and the LED lights for a short while.

Explain how the shaker torch is able to light the LED.

The intensity of light incident on a light dependent resistor (LDR) can vary both its electrical resistance R and the number of charge carriers per unit volume n. The light intensity on an LDR is increased.

Which row of the table describes the effect on R and n?

The intensity of light incident on a light dependent resistor (LDR) can vary both its electrical resistance R and the number of charge carriers per unit volume n. The light intensity on an LDR is increased.

Which row of the table describes the effect on R and n?

A light dependent resistor is connected across a cell of negligible internal resistance. The light intensity is increased.

Which of the following statements about the current is correct?

Which of the following graphs shows how the current varies with potential difference for a filament lamp?

A student set up the circuit shown and measured the current I through the filament lamp for a range of values of potential difference (p.d.) V.

The student’s data is shown in the table.

Criticise the student’s recording of the data.

The student drew a graph of how current varies with p.d. She drew a straight line on the graph and claimed that the data demonstrates that the filament lamp obeys Ohm’s law because the graph is linear.

Assess the validity of the student’s statement.

Using the circuit shown the student was unable to obtain data for p.d.s less than 2.5 V.

Draw a diagram of a circuit the student could have used to enable a full range of p.d.s from 0 to 12 V to be investigated.

A student wanted to plot a graph of current against potential difference for a light emitting diode (LED). He used the circuit shown.

Add an ammeter and a voltmeter to the circuit diagram that would enable the data to be collected.

The graph of current against potential difference obtained by the student is shown.

The student wrote the following conclusion.

“The graph shows that in general the LED is not an ohmic conductor. However, for potential differences greater than +2 V, Ohm’s law is obeyed since the graph is linear in this region.”

Criticise the student’s conclusion.

The student used the LED with a 5.0 V power supply as shown in the circuit.

To be lit to normal brightness the current through the LED must be 18 mA.

Calculate the resistance of the resistor needed in the circuit.