S4.12.3 For the adult
Ohm's law
Priya joins a resistor to a 3 V battery and measures the current. Then she doubles the p.d. to 6 V. What happens to the current?
What this chapter teaches
Resistance is R = V ÷ I, in ohms. For an ohmic conductor at a steady temperature, current is proportional to p.d., so the graph is a straight line through the origin and V = IR holds with R constant. On that graph a steeper line means a smaller resistance.
Three questions to ask
There is no one right wording. Listen for the idea.
- What does the graph of current against p.d. look like for a resistor, and what does it tell you?
- A p.d. of 12 V drives 0.5 A through a resistor. How do you find its resistance, and what is it?
- Why does the steeper line on a current against p.d. graph mean a smaller resistance?
After the chapter, the learner can
- Use V equals I times R in calculations
- Plot a graph of current against voltage for a resistor
What they copy into their notebook
R = V ÷ I, in ohms. V = IR, I = V ÷ R. An ohmic conductor at a steady temperature gives a straight I-V line through the origin. Steeper means smaller R.