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5.1.11Chemical Energetics

Investigating V-I Characteristics — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 4 of 12 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5Circuit and Method

5.1.11 Investigating V-I Characteristics
1
Set up a circuit with a power supply, a variable resistor, an ammeter in series with the component and a voltmeter in parallel across it.
2
Adjust the variable resistor to set a voltage, then record the current from the ammeter and the potential difference from the voltmeter.
3
Repeat for 8 to 10 voltages, increasing the voltage by about 0.5 V each time, and take three current readings at each voltage.
4
Switch the circuit off between readings, and reverse the supply to take negative values for the diode and the lamp.

Part 2 of 5Shapes of the Graphs

Fixed resistor
  • Gives a straight line through the origin, which shows a constant resistance at constant temperature.
Filament lamp
  • Gives a curve with a decreasing gradient, because the resistance rises as the filament heats up.
Diode
  • Gives almost no current in reverse bias and a sharp rise in current in forward bias.
Variables
  • Voltage is the independent variable and current is the dependent variable, with the temperature of the components kept constant.

Part 3 of 5Evaluating the Practical

Evaluating the Practical
Meter resistance
  • Real ammeters and voltmeters have some resistance, so their readings are slightly inaccurate.
Temperature
  • Heating changes the resistance of the components, so switch off between readings to keep them cool.
Zero error
  • Start the meters from zero to avoid a zero error, which would shift every reading by the same amount.
Safety
  • A thin wire carrying a high current becomes hot, so never touch it while the circuit is switched on.

Part 4 of 5Worked Example

Question Explain how an I–V graph shows that a component is a fixed resistor, and calculate its resistance from one reading of 2.0 V and 0.25 A.

  1. Step 1: A straight line through the origin shows a constant resistance, so the component is a fixed resistor.
  2. Step 2: Calculate the resistance from one point using R = V ÷ I = 2.0 V ÷ 0.25 A.
  3. Step 3: Check that the gradient of the current against voltage graph equals 1 ÷ R, giving the same value.

Answer The straight line through the origin shows a fixed resistor, and R = 8.0 Ω.

Part 5 of 5Exam Checklist & Tips

5.1.11 – Key Points for the Exam

  • V–I characteristic A graph of current against potential difference for a component.
  • Fixed resistor A component whose resistance stays constant at constant temperature.
  • Filament lamp A lamp whose resistance increases as its filament heats up.
  • Diode A component that conducts in one direction only.
  • Zero error A reading error caused by a meter that does not read zero when it should.

Common Mistakes

  • Plotting current on the horizontal axis;
    • for an I–V graph current goes on the vertical axis and voltage on the horizontal axis.
  • Claiming that a filament lamp obeys Ohm's law at all temperatures; its resistance changes as it heats up.
  • Leaving the power supply on between readings, which heats the components and changes their resistance.
  • Taking only one reading at each voltage, so that anomalous readings cannot be spotted.

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