Series and parallel circuitsIB MYP Physics: Revision notes

In a series circuit the components are connected one after another in a single loop.

• The current is the same at every point.
• The potential differences add up to equal the supply p.d.
• The total resistance is the sum: R total = R₁ + R₂

If one component breaks, the loop is broken and every component stops working.

Worked example: a 6.0 V cell is connected to a 4.0 Ω and an 8.0 Ω resistor in series. R total = 12 Ω. I = 6.0 ÷ 12 = 0.50 A. The p.d. across the 8.0 Ω resistor is 0.50 × 8.0 = 4.0 V.

In a parallel circuit the components are connected on separate branches.

• The p.d. across each branch is the same as the supply p.d.
• The currents in the branches add up to equal the current from the supply.
• The total resistance is less than the smallest resistor, because adding another branch gives the current more paths.

If one branch breaks, the other branches keep working.

Worked example: a 6.0 Ω and a 12 Ω lamp are connected in parallel to 12 V. The currents are 12 ÷ 6.0 = 2.0 A and 12 ÷ 12 = 1.0 A, so the total current is 3.0 A. The total resistance is 12 ÷ 3.0 = 4.0 Ω, which is less than 6.0 Ω.

To find missing values in a simple circuit:

1. Find the total resistance (add the resistances in series; in parallel it is the supply p.d. ÷ the total current).
2. Use I = V ÷ R to find the current.
3. Apply the series or parallel rules for p.d. and current: p.d.s add in series; currents add in parallel.
4. Use V = I × R for each component.

Write each step with the equation, the numbers and the unit.

When cells are connected in series, end to end in the same direction (positive to negative), their potential differences add up. Two 1.5 V cells in series give 3.0 V.

If one cell is turned round the wrong way, it opposes the other and the total p.d. is reduced (1.5 V − 1.5 V = 0 V for two identical cells).

Lights and sockets in homes are wired in parallel.

• Every appliance gets the full mains p.d., so it works at its full power.
• Each appliance has its own switch and can be turned on and off independently.
• If one appliance or lamp fails, the others keep working.
• Adding more appliances does not reduce the p.d. across the others.

The disadvantage is that the currents add, so too many appliances on one socket can draw a dangerously large current.