S4.12.6 For the adult
Parallel circuits
Ana joins two lamps side by side across one battery. Both glow as brightly as one lamp, and she can unscrew one while the other stays lit. How is this loop different?
What this chapter teaches
In a parallel circuit each branch has the full p.d. of the supply, and the branch currents add up to the current from the supply. The total resistance is smaller than the smallest branch, because each branch is another path. For two resistors it is the product over the sum.
Three questions to ask
There is no one right wording. Listen for the idea.
- Two lamps are in parallel on a battery. What is the p.d. across each, and why?
- How do the currents in the branches compare with the current from the battery?
- Why does adding a resistor in parallel make the total resistance smaller and not bigger?
After the chapter, the learner can
- Explain how current splits between parallel branches
- Explain why adding resistors in parallel lowers the total resistance
What they copy into their notebook
Parallel: branches across the supply. Same p.d. in each. Branch currents add. Smaller R takes more current. Total R is less than the smallest. Two resistors: R = R₁R₂ ÷ (R₁ + R₂).