Electric Circuits, Symbols, Series and Parallel

British & IGCSE › Checkpoint 6 › Checkpoint 6 Science — Complete Course

1. What makes a circuit work?

An electric circuit is a path that electricity flows around. A circuit only works when it is a complete loop with no gaps: from one end of the cell, through the wires and components, and back to the other end of the cell. The flow of electricity around the circuit is called the current.

• A cell pushes the current around the circuit. Two or more cells joined together make a battery, which gives a bigger push.
• Wires are made of a metal such as copper, which is a good electrical conductor. They are covered in plastic, an insulator, to keep us safe.
• Components such as lamps and buzzers use the electricity: a lamp gives out light, a buzzer makes a sound.
• A switch opens or closes a gap in the circuit. Closed switch = complete circuit = current flows. Open switch = gap = no current.

2. Circuit symbols

Scientists all over the world draw circuits with the same simple symbols instead of pictures. Learn these eight — you must be able to draw them and recognise them.

Rules for drawing circuit diagrams

1. Use a ruler and a pencil. Draw wires as straight lines with square corners.
2. Use the correct symbol for every component — never a picture.
3. Leave no gaps in the wires (except an open switch, which is meant to be a gap).
4. Do not draw a wire through the middle of a symbol.
5. Put a dot at every junction where wires join.
6. Label components if the question names them (lamp A, switch S1…).

3. Series circuits

In a series circuit, all the components are joined one after another in a single loop. There is only one path for the current, so the same current flows through every component.

• Adding more lamps in series makes every lamp dimmer. The push from the cell has to be shared between the lamps.
• If one lamp breaks or is unscrewed, there is a gap in the only path, so all the lamps go out.
• One switch anywhere in the loop turns everything on and off together.
• Adding more cells (in the same direction) gives a bigger push, so lamps get brighter and buzzers get louder. Too many cells can make a lamp blow (its thin wire breaks).

4. Parallel circuits

In a parallel circuit, the components are on separate branches. At each junction the current splits up and then joins again. There is more than one path for the current.

• Each lamp in parallel gets the full push of the cell, so each lamp is as bright as one lamp on its own with the same cell.
• If one lamp breaks, the other branch is still a complete loop, so the other lamp stays on (at the same brightness).
• A switch on a branch controls only the lamp on that branch. A switch on the main part of the circuit (next to the cell) controls everything.
• The lights in your home are wired in parallel: you can switch each light on and off on its own, and one broken bulb does not turn off the whole house.

5. Comparing brightness

6. Series or parallel? Summary

Why might a circuit not work? Check: Is a switch open? Is there a gap or a loose connection? Is a lamp broken (blown)? Is the cell used up (flat)? Has an insulator (plastic, wood, rubber) been placed in the circuit? Are two cells in a battery facing opposite ways so their pushes cancel?

7. Worked examples

Worked example 1 — Look at the circuit below. (a) How many cells are there? [1] (b) Name the two components on the bottom wire. [1] (c) Is the switch open or closed? [1]

Answer: (a) 3 cells (count the pairs of long and short lines). (b) a lamp and a buzzer. (c) closed — the line joins both terminals, so there is no gap. It is a series circuit, so the lamp lights and the buzzer sounds.

Worked example 2 — Draw a circuit diagram for: one cell, an open switch, a lamp and a buzzer, all in series. [2]

1. Draw a rectangle loop with a ruler.
2. Put the cell symbol on the top side, the open switch on one side, and the lamp and buzzer on the bottom.
3. Check: one single loop, correct symbols, no extra gaps, no wire through a symbol.

Mark scheme style: 1 mark for all four correct symbols; 1 mark for everything in one complete loop (series). Common mistake: drawing the cell as two equal lines, or drawing a picture of a bulb.

Worked example 3 — Circuits A, B and C in section 5 all use identical cells and lamps. (a) In which circuit(s) are the lamps dimmest? [1] (b) Explain why. [1]

Answer: (a) B. (b) The two lamps are in series, so the push of one cell is shared between two lamps. (In C, each lamp in parallel gets the full push of the cell, so each is as bright as the lamp in A.)

Worked example 4 — Mariam's circuit has two lamps in parallel (like the parallel circuit in section 4). Lamp A breaks. What happens to lamp B? Explain. [2]

Answer: Lamp B stays on at the same brightness (1 mark), because it is on a separate branch that is still a complete loop with the cell (1 mark). Common mistake: saying lamp B gets brighter — in a parallel circuit it stays the same.

Worked example 5 — Look at the circuit with switches S1, S2 and S3. Which switches must be closed to light lamp P but not lamp Q? [2]

Answer: close S3 (the main switch, so current can leave the cell) and S1 (to complete lamp P's branch); keep S2 open so lamp Q stays off.

Worked example 6 (investigation) — Ali investigates how the number of cells affects the loudness of a buzzer. (a) Name the independent variable. [1] (b) Give one control variable. [1] (c) Predict the result. [1]

Answer: (a) the number of cells. (b) the same buzzer / the same wires / the same distance from the buzzer to the ear or sound meter. (c) The more cells, the louder the buzzer, because more cells give a bigger push to the current.

Practice

All pages in this course

Start here

• Checkpoint 6 Science: The Test and How to Prepare
• Thinking and Working Scientifically: Investigations, Variables, Tables and Graphs

Biology

• The Heart and Circulatory System
• The Lungs and Respiratory System
• Human Reproductive System and Puberty
• Diseases, Hygiene and the Body's Defences
• Food Chains, Food Webs, Energy and Toxins

Chemistry

• Properties of Substances: Melting and Boiling Points, Gases, Conductors
• Reversible Changes and Dissolving
• Boiling and Evaporation
• Chemical Reactions: Reactants, Products and Evidence

Physics

• Mass, Weight and Gravity
• Force Diagrams and Forces on Moving Objects
• Floating and Sinking
• Light: Reflection and Refraction
• Electric Circuits, Symbols, Series and Parallel

Earth and space

• Rocks and Fossils
• Types of Soil
• The Rock Cycle
• The Solar System and the Phases of the Moon

Practice tests

• Checkpoint 6 Science — Practice Test 1
• Checkpoint 6 Science — Practice Test 2
• Checkpoint 6 Science — Practice Test 3