Series and parallel circuits GCSE physics lesson: current, p.d. and total resistance

Series and parallel: the virtual circuits

Virtual experiments: the circuits follow a model, and they are not the required practical. Record your readings first, then answer the questions about them.

1. Virtual practical, series. R1 and R2 are in one loop with three ammeters, A1, A2 and A3; V1 is across R1 and V2 across R2. Change the cell's p.d. (B1) and R2, and record at least 3 readings, each at different settings.They set up a virtual circuit and record its meters' readings, each checked.
2. Series: compare A1, A2 and A3 in each of your readings. What do they show?
   1. A1 is the biggest and A3 the smallest: each resistor uses some current up
   2. All three read the same: the current is the same all the way round
   3. A2 is the biggest: the current builds up between the resistors
   4. They add up to the current from the cell
3. Set the cell to 12 V and R2 to 20 Ω. What does V2 read, in volts?
4. Series: in each of your readings, what is V1 + V2?
   1. Less than the cell's p.d.: some is used up in the wires
   2. Twice the cell's p.d.: each resistor gets the cell's p.d.
   3. The cell's p.d.: the supply's p.d. is shared between the resistors
   4. It depends on R2: sometimes more than the cell's p.d.
5. At 12 V with R2 = 20 Ω, the current is 0.40 A. Use total resistance = p.d. of the cell ÷ current. What is the total resistance, in ohms?
6. Virtual practical, parallel. R1 and R2 are in two branches; A1 measures the current from the cell, A2 the current in R1's branch and A3 in R2's. Change the cell's p.d. (B1) and click the switch S1 to open and close R2's branch. Record at least 3 readings, each at different settings, with S1 open in at least one.They set up a virtual circuit and record its meters' readings, each checked.
7. Parallel, with S1 closed: how does A1 compare with A2 and A3?
   1. A1 = A2 + A3: the branch currents add up to the current from the cell
   2. A1, A2 and A3 all read the same, as in series
   3. A2 and A3 are each half of A1: the current splits equally
   4. A1 is less than A2 + A3: some current is used up in the branches
8. Parallel: set the cell to 6 V with S1 closed. What does A1 read, in amps?
9. Use that reading: total resistance = 6 V ÷ the current from the cell. What is the total resistance of the two branches, in ohms?
10. Closing S1 adds R2 in parallel with R1. From your readings, what happens to the current from the cell (A1), and so to the total resistance?
    1. A1 goes up, and the total resistance goes up too
    2. A1 goes down, so the total resistance goes up: more resistors, more resistance
    3. A1 stays the same; A2 goes down to make room for A3
    4. A1 goes up, so the total resistance goes down

Series and parallel: practice

Show your working on paper: say first whether it is series or parallel, then which rule you use. The stretch is optional.

Diagnostic question: choose and explain

1. A 6 V cell has two branches in parallel: a 10 Ω resistor and a 30 Ω resistor. The current in the 10 Ω branch is 0.6 A. What is the current in the 30 Ω branch?
   1. 1.8 A
   2. 0.6 A
   3. 0.2 A
   4. 0.15 A

Practice questions

1. A 5 Ω resistor and a 15 Ω resistor are in series. What is their total resistance, in ohms?
2. A 9.0 V battery is connected to two resistors in series. The p.d. across one is 3.5 V. What is the p.d. across the other, in volts?
3. In a parallel circuit, the current from the cell is 0.9 A. One branch carries 0.5 A. What current is in the other branch, in amps?
4. A 12 V supply is connected to an 8 Ω resistor and a 16 Ω resistor in series. What is the current, in amps?
5. In question 4, what is the p.d. across the 16 Ω resistor, in volts?
6. A 20 Ω resistor and a 30 Ω resistor are connected in parallel. Their total resistance is:
   1. 50 Ω
   2. less than 20 Ω
   3. between 20 Ω and 30 Ω
   4. 25 Ω, the average
7. Sort each statement: is it always true of series circuits, of parallel circuits, or of both?They sort 7 items into Series and Parallel and Both.
8. A 6.0 V cell is connected in series with a 10 Ω resistor and a resistor R. The current is 0.20 A. What is the resistance of R, in ohms?
9. Stretch: a 12 V cell has a 20 Ω resistor and a resistor R in parallel. The current from the cell is 1.0 A. What is the resistance of R, in ohms?

Support: For question 3: the branch currents add up to the current from the cell. Take the branch you know away from the total.

Series and parallel: exit

One calculation, then two or three sentences.

1. A 6.0 V cell is connected to a 4 Ω resistor and an 8 Ω resistor in series. What is the current, in amps?
2. Explain, using what happens to the current from the cell, why adding a resistor in parallel makes the total resistance of a circuit smaller.

Exit check

A series calculation and an explanation of why parallel lowers the resistance.

A series calculation (total resistance, then current), then why adding a resistor in parallel makes the total resistance smaller, using current.

Series and parallel circuits: the rules in use

Say first whether each circuit is series or parallel, and which rule you use. Show the equation, the numbers and the answer with its unit.