I-V Characteristics & Sensors

IGCSE Edexcel Physics
2.9–2.12 I-V graphs, diodes, LDRs and thermistors
Key Concepts: An I-V graph shows how current varies with voltage for a component. A resistor gives a straight line (ohmic). A filament lamp curves (resistance increases with temperature). A diode only allows current in one direction. LDR resistance decreases with increasing light; thermistor (NTC) resistance decreases with increasing temperature.

Section A — I-V Characteristics

1. Sketch and describe the I-V graph for a fixed resistor (ohmic conductor). Explain what the shape tells you about resistance. [3]
2. Sketch and describe the I-V graph for a filament lamp. Explain why the graph has this shape. [3]
3. Describe the I-V graph for a diode. State the term for the minimum voltage needed for the diode to conduct. [3]
4. Using the I-V graph for a filament lamp, explain how you would calculate the resistance at a particular voltage. [2]

Section B — Sensors

5. State what LDR stands for and describe how its resistance changes with light intensity. [2]
6. State the type of thermistor used in IGCSE circuits (NTC) and describe how its resistance changes with temperature. [2]
7. An LDR is connected in series with a fixed resistor and a battery. Explain what happens to the voltage across the fixed resistor as the light intensity increases. [3]
8. Suggest a practical application of: [2]

a) An LDR

b) A thermistor

Section C — Practical Skills

9. Describe how you would use an ammeter and voltmeter to obtain an I-V graph for a resistor. Include how the circuit should be connected. [4]

Total marks: 24

Mark Scheme

1. Straight line through the origin [1]; constant gradient [1]; resistance is constant (ohmic behaviour — current proportional to voltage) [1] [3]
2. Curve that becomes less steep at higher voltages [1]; as current increases, the filament gets hotter [1]; resistance increases with temperature, so current increases less for each additional volt [1] [3]
3. Very little (no) current in reverse direction [1]; current only flows above a threshold voltage in the forward direction [1]; threshold voltage (also called forward voltage or turn-on voltage, ~0.6–0.7 V for silicon diode) [1] [3]
4. Read off the current ($I$) at that voltage ($V$) from the graph [1]; calculate $R = V/I$ [1] [2]
5. Light Dependent Resistor [1]; resistance decreases as light intensity increases [1] [2]
6. NTC = Negative Temperature Coefficient [1]; resistance decreases as temperature increases [1] [2]
7. As light increases, LDR resistance decreases [1]; total circuit resistance decreases, so total current increases [1]; greater current through fixed resistor means greater voltage across it [1] [3]
8. a) LDR: automatic street lights / security lights / light meters [1]; b) Thermistor: thermostat / temperature alarm / fire detector [1] [2]
9. Connect ammeter in series with the component [1]; connect voltmeter in parallel across the component [1]; vary the voltage using a variable resistor/rheostat [1]; record current and voltage at multiple settings to plot the graph [1] [4]