S4.12.4 For the adult
Different components
A resistor gives a straight line on a graph of current against p.d. Does a lamp do the same? What about a diode?
What this chapter teaches
Only an ohmic conductor at a steady temperature gives a straight line on a graph of current against p.d. A filament lamp curves because its resistance rises as it heats. A diode conducts one way only. Thermistors and LDRs fall in resistance with heat and light, which makes them useful in sensors.
Three questions to ask
There is no one right wording. Listen for the idea.
- Why is the current against p.d. graph of a lamp a curve and not a straight line?
- Which way round must a diode be joined for the lamp to light, and how can you tell?
- What happens to a thermistor and to an LDR when each is warmed or lit, and how could that be used?
After the chapter, the learner can
- Compare current and voltage graphs for a resistor, a lamp and a diode
- Explain how thermistors and light dependent resistors work in sensors
What they copy into their notebook
Resistor: straight I-V line. Lamp: curve, resistance rises as it heats. Diode: one way only, above 0.6 V. Thermistor: R falls as hotter. LDR: R falls as brighter.