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Physics · Electricity

Resistors and I–V characteristics

Explain the I–V characteristics of a resistor, filament lamp and diode, and the behaviour of thermistors and LDRs, including Required Practical 4.

  • 6 key terms
  • All boards
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Warm-up

Answer each one, then check.

  1. 1

    What is an ohmic conductor?

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    A conductor with constant resistance at constant temperature

  2. 2

    What does I–V stand for?

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    Current against potential difference

  3. 3

    What is a straight line graph through the origin?

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    Directly proportional

  4. 4

    What happens to a metal when its temperature rises?

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    Its resistance rises

  5. 5

    What does LDR stand for?

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    Light-dependent resistor

Learning Objectives

  1. 1Describe the I–V graphs of a resistor, filament lamp and diode.
  2. 2Explain the changes in resistance of a filament lamp, diode, thermistor and LDR.
  3. 3Describe how to investigate I–V characteristics (Required Practical 4).
  4. 4Use graphs to say whether a component is linear or non-linear.

I–V CHARACTERISTICS

The current through an ohmic conductor at constant temperature is directly proportional to the potential difference across it, so its resistance is constant.

The resistance of a filament lamp, diode, thermistor and LDR is not constant.

Component Behaviour

Learn each description.

  • Fixed resistor

    Resistance: Constant at constant temperature. Graph or behaviour: Straight line through the origin

  • Filament lamp

    Resistance: Increases as the filament gets hotter. Graph or behaviour: Curve that flattens; gradient decreases

  • Diode

    Resistance: Very high in the reverse direction. Graph or behaviour: Current flows in one direction only

  • Thermistor

    Resistance: Decreases as temperature increases. Graph or behaviour: Used in thermostats and temperature sensors

  • LDR

    Resistance: Decreases as light intensity increases. Graph or behaviour: Used in automatic lights

Required Practical 4: Method

Find the I–V characteristic of a component.

  1. 1 Set up

    Connect the component in series with the ammeter and variable resistor, and the voltmeter across the component.
  2. 2 Vary

    Change the variable resistor to get different pds.
  3. 3 Record

    For each setting record V and I.
  4. 4 Reverse

    Swap the connections and repeat to get negative values (not for a diode with a protective resistor).
  5. 5 Plot

    Draw current against pd and draw a smooth line.
  6. 6 Repeat

    For the filament lamp and the diode.

Resistance of a Filament Lamp

The current through a filament lamp is 0.34 A at 2.0 V and 0.62 A at 6.0 V. Calculate the resistance at each pd and explain the difference.

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  1. 1 At 2.0 V \(R = 2.0 \div 0.34 = 5.9\ \Omega\)
  2. 2 At 6.0 V \(R = 6.0 \div 0.62 = 9.7\ \Omega\)
  3. 3 Explain At the higher pd the current is greater, so the filament is hotter and its resistance is higher

Answer5.9 Ω and 9.7 Ω; the resistance increases as the filament gets hotter.

Uses in Circuits

Applications you need to know.

  • Thermistor

    In a thermostat: as the temperature rises the resistance falls, changing the pd across it to switch heating off.

  • LDR

    In automatic lights: as it gets dark the resistance rises, which switches the light on.

  • Diode

    Allows current in one direction only, protecting circuits from being connected the wrong way round.

Match the Graph

Match each component to its I–V graph description: resistor, filament lamp, diode. (1) Curve that flattens as pd increases. (2) Straight line through the origin. (3) Almost no current one way, then a rapid increase the other way.

1. Look at the shape.

2. Explain the resistance change.

A good answer shows: Resistor: (2). Filament lamp: (1). Diode: (3).

Can I...?

  1. 1Draw and describe the I–V graph of a resistor.
  2. 2Describe the I–V graph of a filament lamp.
  3. 3Describe the I–V graph of a diode.
  4. 4Explain why a filament lamp's resistance increases.
  5. 5Describe how a thermistor behaves.
  6. 6Describe how an LDR behaves.
  7. 7Describe Required Practical 4.
  8. 8Give one use each of a thermistor and an LDR.

Summary & Exam Focus

  • Resistor: constant resistance, straight line through the origin.
  • Filament lamp: resistance increases with temperature.
  • Diode: current in one direction only.
  • Thermistor and LDR resistances decrease as temperature or light increases.

Exam focus

Explain why the resistance of a filament lamp increases as the current increases. (3 marks) (3 marks)

Say the filament gets hotter and link it to resistance.

Key terms

The vocabulary this lesson expects you to use. Each one is linked from the first place it appears above.

Ohmic conductor
A conductor whose resistance is constant at constant temperature.
Filament lamp
A lamp with a thin wire that gets hot and glows.
Diode
A component that lets current flow in one direction only.
Thermistor
A resistor whose resistance decreases when the temperature increases.
LDR
A resistor whose resistance decreases when the light intensity increases.
Linear
Following a straight line graph.

Practice questions

Have a go at each one before you open its answer.

  1. Question 1 Complete 3 marks

    Name the component whose I–V characteristic shows current flowing in one direction only, and the component whose resistance decreases as the temperature increases. Then state what happens to the resistance of an LDR in bright light.

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    Model answer

    A diode; a thermistor; the resistance of an LDR decreases in bright light.

    Mark scheme

    • Diode — 1 mark
    • Thermistor — 1 mark
    • LDR resistance decreases — 1 mark
  2. Question 2 Use the graph 4 marks

    The graph shows the I–V characteristic of a filament lamp. (a) Calculate the resistance of the lamp when the potential difference is 6.0 V. (b) The resistance at 2.0 V is 5.9 Ω. Explain why the resistance at 6.0 V is different.

    A current against potential difference graph for a filament lamp, curving and flattening as the pd increases.
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    Model answer

    (a) At 6.0 V the current is 0.62 A. \(R = 6.0 \div 0.62 = 9.7\ \Omega\). (b) At 6.0 V the current is greater so the filament is hotter, and the resistance of the filament increases with temperature.

    Mark scheme

    • Reads 0.62 A — 1 mark
    • 9.7 Ω — 1 mark
    • Higher current means higher temperature — 1 mark
    • Resistance increases with temperature — 1 mark
  3. Question 3 Explain 3 marks

    A thermistor is used in a circuit to control a heating system. Explain how the resistance of the thermistor changes as the room warms up.

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    Model answer

    As the temperature increases the resistance of the thermistor decreases, so the current through it increases and the potential difference across it changes; this is used to switch the heating off.

    Mark scheme

    • Resistance decreases as temperature increases — 1 mark
    • Current or potential difference changes in the circuit — 1 mark
    • Used to switch the heating off — 1 mark
  4. Question 4 Describe 6 marks

    Describe how you would investigate the relationship between the current through a filament lamp and the potential difference across it. Include a circuit diagram description, the readings to take and how you would present and use the results.

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    Model answer

    See levels-of-response scheme.

    Mark scheme

    • Level 3 (5 to 6 marks): a correct circuit (ammeter in series, voltmeter across the lamp, variable resistor or supply), a range of pd values and matching currents, a graph of I against V and a description of its shape or the changing gradient — 5 to 6 marks
    • Level 2 (3 to 4 marks): a method with the main apparatus and some readings; the graph is mentioned — 3 to 4 marks
    • Level 1 (1 to 2 marks): simple statements about measuring current and pd — 1 to 2 marks
  5. Question 5 Explain 2 marks

    A student measures the resistance of a diode. Explain why the resistance is very high when the diode is connected the other way round.

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    Model answer

    A diode has a very high resistance in the reverse direction, so almost no current flows.

    Mark scheme

    • Very high resistance in reverse — 1 mark
    • Almost no current flows — 1 mark
  6. Question 6 Suggest 3 marks

    An automatic outdoor light contains an LDR. Explain how the light switches on when it gets dark.

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    Model answer

    When it gets dark the resistance of the LDR increases. The pd across the LDR increases (or current decreases), which triggers the circuit that switches the light on.

    Mark scheme

    • Resistance of LDR increases in the dark — 1 mark
    • Change in pd or current in the circuit — 1 mark
    • Circuit switches the light on — 1 mark

Quick check

  1. An ohmic conductor has an I–V graph that is...

    1. Aa curve
    2. Bhorizontal
    3. Cvertical
    4. Da straight line through the origin
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    D: a straight line through the origin

    Constant resistance gives a straight line.

  2. The resistance of a filament lamp increases because the filament...

    1. Agets hotter
    2. Bgets longer
    3. Cgets colder
    4. Dchanges colour
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    A: gets hotter

    Higher temperature means higher resistance.

  3. A diode allows current in...

    1. Aboth directions equally
    2. Bone direction only
    3. Cno direction
    4. DAC only
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    B: one direction only

    It has very high resistance in the reverse direction.

  4. As light intensity increases, the resistance of an LDR...

    1. Aincreases
    2. Bstays the same
    3. Cdecreases
    4. Dbecomes infinite
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    C: decreases

    LDR resistance falls in bright light.

  5. As temperature increases, the resistance of a thermistor...

    1. Aincreases
    2. Bstays the same
    3. Cdoubles then halves
    4. Ddecreases
    Show answerHide answer

    D: decreases

    Thermistor resistance falls as it warms.

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