Simple circuits

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Notes de leçon

Big idea: a loop that lets energy flow

Big idea. Key concept: Systems. A circuit is a system: a cell, wires and components that work together.
Related concept: Function. Every part has a job. The cell pushes, the wires carry, the lamp lights and the switch controls.
Global context: Scientific and technical innovation. Almost every machine you use, from torches to tablets, depends on circuits.
A circuit is a path that charge can travel round. The charge carries energy from the cell to a component such as a lamp, where it is transferred as light and heat.
A circuit must be a complete loop with no gaps, joined to both ends of the cell. If there is a gap, the charge cannot flow and the lamp stays off.
A switch is a deliberate gap you can open and close. Open means there is a gap (no current). Closed means the loop is complete (current flows).
A single cell is one unit. A battery is two or more cells joined together.

A complete circuit with a cell and a lamp

Drawing circuits with symbols

Scientists draw circuits using circuit symbols joined by straight lines for the wires. Anyone in the world can read the diagram.
A cell is drawn as one long line and one short line. The long line is the positive end. A battery is drawn with several pairs of lines.
A lamp is a circle with a cross inside. A switch is a gap in the line with a small lever.
An ammeter is a circle with the letter A. A voltmeter is a circle with the letter V.
Draw the wires as straight lines with square corners, and leave no gaps in the loop unless there is a switch.
Always check your diagram by tracing a finger from one end of the cell, round the loop, back to the other end. If you have to jump a gap, the circuit is not complete.

Circuit symbols

Current, voltage and resistance

Current is the flow of electric charge round a circuit. In a metal wire the charge is carried by tiny particles called electrons. Current is measured in amperes (A), often called amps.
By agreement, we draw conventional current flowing from the positive terminal of the cell round to the negative terminal. The electrons actually move the other way round.
The cell does not use up the current. The same current that leaves the cell comes back to it. What is transferred to the lamp is energy.
Voltage is the push the cell gives to the charge. It is measured in volts (V). A bigger voltage gives a bigger push, so more current flows. One cell is about 1.5 V. Two cells in series (the same way round) give 1.5 V + 1.5 V = 3.0 V.
Resistance tells us how hard it is for current to flow. It is measured in ohms (Ω). A longer or thinner wire has more resistance. For the same cell, more resistance means less current.
A simple rule links them: voltage = current × resistance. A 6 V cell across a 12 Ω lamp pushes a current of 6 ÷ 12 = 0.5 A.

Conductors, insulators and staying safe

A conductor lets current flow easily. Metals such as copper, iron and aluminium are good conductors, and so is carbon (in pencil lead).
An insulator does not let current flow easily. Plastic, rubber, glass and dry wood are insulators. Wires are made of copper (a conductor) covered in plastic (an insulator).
Switches, wires and components are joined so that the metal parts touch and the plastic keeps the current where it should be.
In school we use low-voltage cells and bulbs only. These are safe to handle.
Never touch a mains socket or plug, never put anything into a socket, and never mix water and electricity. Mains electricity can be deadly.
Do not join a wire straight across a cell with nothing else in the loop. This is a short circuit: a large current flows and the wire and cell can get hot.

Conductors and insulators

Series and parallel circuits

In a series circuit all the components are in one loop. There is only one path for the current.
In series, the current is the same everywhere in the loop. The cell's voltage is shared between the components, so each of two identical lamps is dimmer than a lamp on its own.
If one component in a series circuit breaks, or you unscrew one lamp, the loop is broken and everything goes off.
In a parallel circuit the current has more than one path: the circuit splits into branches.
In parallel, each branch has the same voltage as the cell, so each lamp is as bright as it would be on its own. The current from the cell is the sum of the branch currents. If 0.30 A flows in one branch and 0.20 A in the other, the cell supplies 0.50 A.
If one lamp in a parallel circuit goes out, the other branches still have a complete loop and stay on. This is why household lighting is wired in parallel.

A series circuit

Think like a scientist: what changes the brightness of a lamp?

Question: does the number of cells in series change how bright a lamp is?
The independent variable is the number of cells. The dependent variable is the current, measured with an ammeter in series (this is better than saying "bright" or "dim", which depends on opinion).
Control variables include the same lamp, the same wires and the same type of cell.
Measure the current for 1, 2, 3 and 4 cells. Repeat each one and work out a mean, because one reading could be a mistake.
To evaluate your results, ask what could be wrong. Did the cells run down as you worked? Were the connections loose? Did the lamp heat up and change its resistance?
Inquiry task: plan a fair test to find out whether the length of a wire changes the current. Use low-voltage cells only. List your independent, dependent and control variables, say how you will measure the current and explain how you will make your results reliable.

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1.A lamp is joined to a cell by two wires, but one wire is not touching the cell. What happens?
Easy
- AThe lamp lights dimly because some current still gets through
- BThe lamp lights for a short time and then goes out
- CThe lamp stays off because the loop is not complete
- DThe lamp lights because the cell is still pushing the charge
2.In a circuit diagram, a cell is drawn as…
Easy
- Aone long line and one short line
- Ba circle with a cross inside it
- Ca circle with the letter A inside it
- Da small rectangle on the wire
3.What does a circle with a cross inside it show in a circuit diagram?
Easy
- AA switch
- BA cell
- CA voltmeter
- DA lamp
4.What is the job of a switch in a circuit?
Easy
- ATo push the charge round the loop with much more force
- BTo open or close the loop so current stops or flows
- CTo store up energy until the lamp needs to use it
- DTo measure how much current is flowing through the loop
5.Which unit is used to measure electric current?
Easy
- AVolt (V)
- BAmpere (A)
- COhm (Ω)
- DNewton (N)
6.Which meter measures electric current?
Easy
- AA voltmeter
- BA thermometer
- CA newton meter
- DAn ammeter
7.How should an ammeter be connected to measure the current through a lamp?
Medium
- AIn series, so the current flows through the meter
- BIn parallel, across the two ends of the lamp itself
- CNext to the lamp but not joined to any of the wires
- DAcross the two ends of the cell and nowhere else
8.How should a voltmeter be connected to measure the voltage across a lamp?
Medium
- AIn series, as part of the main loop of the circuit
- BIn series, placed between the cell and the switch
- CIn parallel, across the two ends of the lamp
- DAnywhere at all, as long as it touches a wire

Plus de questions

9.Which of these is a good electrical conductor?
Easy
- AA rubber band
- BA glass rod
- CA plastic ruler
- DA copper wire
10.In a series circuit with two lamps, one lamp is unscrewed. What happens to the other lamp?
Medium
- AIt gets brighter because it gets all the current
- BIt goes out because the loop is now broken
- CIt stays on, exactly as bright as before
- DIt stays on, but only dimly
11.In a parallel circuit with two lamps on separate branches, one lamp is unscrewed. What happens to the other lamp?
Medium
- AIt goes out because the whole circuit is now broken too
- BIt goes out because the cell loses all of its voltage
- CIt stays on because its own branch is still a complete loop
- DIt gets much dimmer because there is less current left
12.Two identical lamps are joined in series to one cell. How does each lamp compare with a single lamp on the same cell?
Medium
- AEach is dimmer, because the current is smaller
- BEach is brighter, because there is more energy to share
- CEach is exactly as bright as a single lamp would be
- DOnly the lamp next to the cell lights up at all
13.Two identical lamps are joined in parallel to one cell. Why is each lamp as bright as a single lamp on the cell?
Medium
- AThe cell gives out twice as much voltage when asked
- BThe lamps share the current equally between them
- CEach branch has the full voltage of the cell
- DParallel wiring makes the lamps use less energy
14.Why are the lights in a house wired in parallel rather than in series?
Medium
- AEach light works on its own, and one failing does not switch the rest off
- BParallel wiring uses much less wire than a series loop would
- CSeries circuits are not able to carry any current at all
- DParallel lights are always brighter than any series lights could be
15.What is an electric current?
Easy
- AA store of energy kept inside the cell
- BA kind of light given out by hot wires
- CThe push that the cell gives to the charge
- DA flow of electric charge round a circuit
16.What carries the charge in a metal wire?
Medium
- AAtoms of metal travelling round the loop
- BElectrons
- CBeams of light from the cell
- DDrops of liquid inside the wire
17.What does the voltage of a cell tell us?
Medium
- AHow much charge is stored up inside the cell itself
- BHow big a push it gives the charge in the circuit
- CHow quickly the wires are able to conduct electricity
- DHow many lamps the cell is able to light up at once
18.In circuit diagrams, which way do we draw the conventional current?
Medium
- AFrom the negative terminal round to the positive terminal
- BBoth ways at once, so the arrows meet at the lamp
- CAway from the cell and never back to it again
- DFrom the positive terminal round to the negative terminal
19.Which change gives a piece of wire a higher resistance?
Medium
- AMaking the wire longer
- BMaking the wire shorter
- CMaking the wire thicker
- DJoining it to a stronger cell
20.Which is a safe way to investigate circuits in a school lab?
Easy
- APlug the circuit into a mains socket to get a bigger current
- BHold the bare ends of wires with wet hands
- CUse low-voltage cells and keep water away from the equipment
- DPush a wire into a socket to see if it works
21.A thick wire is joined straight across a cell with nothing else in the loop. Why is this dangerous?
Hard
- ANo current can flow at all, so the cell goes flat very quickly
- BThe voltage of the cell rises sharply, so the cell can explode
- CThe wire becomes an insulator and the cell slowly cools down
- DA very large current flows, which heats the wire and the cell
22.In a series circuit, an ammeter before a lamp reads 0.40 A. What does a second ammeter, placed after the lamp, read?
Hard
- A0.30 A, because the lamp uses up some of the current flowing
- B0.40 A, because the current is the same all round a series loop
- C0.00 A, because the lamp stops all of the current flowing
- D0.80 A, because the lamp doubles the current that enters it
23.A student tests whether the length of a resistance wire changes the current. What is the independent variable?
Hard
- AThe reading on the ammeter
- BThe voltage of the cell used
- CThe length of the wire
- DThe thickness of the wire used
24.A student tests how the number of cells affects how bright a lamp is. Which is the best way to measure the dependent variable?
Hard
- ARead the current with an ammeter each time
- BLook at the lamp and call it bright or dim
- CCount how many wires are in the circuit
- DAsk friends which lamp looks the best
25.
A student records the current in a circuit as cells are added in series.
Number of cells Current (A)
1 0.10
2 0.20
3 0.30
Which conclusion is best supported by these results?
Hard
- ACurrent is always the same as the number of cells in amperes
- BAdding cells made no difference to the current
- CIn this test, more cells in series gave a larger current
- DCurrent went down every time a cell was added
26.Which of these materials are good conductors of electricity? (select all that apply)
Easy
- ACopper wire
- BGraphite pencil lead
- CRubber band
- DIron nail
- EPlastic ruler
27.Which statements are true about a parallel circuit? (select all that apply)
Medium
- ATaking out one lamp turns all the lamps off
- BEach branch has the same voltage as the cell
- CThere is more than one path for the current
- DIf one branch is broken, the others can still work
- EThe current is the same in every branch
28.Which of these are sensible safety rules? (select all that apply)
Medium
- AKeep water away from electrical equipment
- BNever touch a mains socket or plug
- CUse low-voltage cells for experiments in school
- DPull a plug out of a socket by tugging the cable while your hands are wet
- EPlug into a socket if you need a bigger current
29.Which changes would make the lamp in a simple one-lamp circuit brighter? (select all that apply)
Hard
- AReplace a flat cell with a fresh one
- BReplace the connecting wire with a much longer thin wire
- CUse a shorter piece of the same connecting wire
- DAdd a second cell in series, the same way round
- EAdd a second lamp in series with the first
30.A lamp will light if the switch in its circuit is open.
Easy
True or false?
31.A battery is made of two or more cells joined together.
Easy
True or false?
32.The cell uses up the current, so less current comes back to the cell than leaves it.
Medium
True or false?
33.An ammeter must be connected in parallel with the component being measured.
Easy
True or false?
34.In a parallel circuit, every branch has the same voltage as the cell.
Medium
True or false?
35.In a metal wire the electrons move from the negative terminal round to the positive terminal, opposite to the conventional current.
Hard
True or false?
36.Match each component to how it is drawn in a circuit diagram.
Easy
- Cell
- Lamp
- Ammeter
- Voltmeter
- A circle with the letter V
- A circle with a cross
- A circle with the letter A
- One long and one short line
37.Match each quantity to its unit.
Easy
- Current
- Voltage
- Resistance
- Time
- Volt (V)
- Ohm (Ω)
- Ampere (A)
- Second (s)
38.Match each part of a circuit to its job.
Easy
- Cell
- Switch
- Lamp
- Ammeter
- Measures the current
- Gives the charge a push
- Opens or closes the loop
- Transfers energy as light and heat
39.Put these steps for building a working torch circuit in order.
Easy
- Collect a cell, a lamp, a switch and some wires
- Check that the lamp lights
- Close the switch
- Join the parts in one loop with no gaps
40.Put these combinations of 1.5 V cells in series, the same way round, in order of total voltage from smallest to largest.
Medium
- One cell (1.5 V)
- Three cells (4.5 V)
- Four cells (6.0 V)
- Two cells (3.0 V)
41.Put the stages of a scientific inquiry into a sensible order.
Hard
- Plan a fair test and choose the variables
- Collect repeated measurements
- Draw a conclusion from the results
- Evaluate how reliable the method was
- State a question to investigate
42.Complete the sentence about circuits.
Easy
For current to flow, a circuit must be a complete ____.
43.Complete the sentence about units.
Easy
Electric current is measured in ____.
44.Complete the sentence about series circuits.
Easy
In a ____ circuit there is only one path for the current.
45.Complete the sentence about materials.
Easy
Materials that do not let current flow easily are called ____.
46.Complete the sentence about parallel circuits.
Hard
In a parallel circuit the currents in the branches ____ up to give the current from the cell.
47.Describe two differences between a series circuit and a parallel circuit.
Medium
48.Explain why a lamp goes out when the switch in its circuit is opened.
Medium
49.Plan a fair test to find out whether the length of a resistance wire changes the current in a circuit. Use low-voltage cells only.
Hard
50.Two 1.5 V cells are joined in series, the same way round. Calculate the total voltage in volts.
Medium
51.In a parallel circuit, one branch carries a current of 0.30 A and the other branch carries 0.25 A. Calculate the current supplied by the cell in amperes.
Medium
52.A 6 V cell is connected across a lamp of resistance 12 Ω. Use voltage = current × resistance to calculate the current in amperes.
Hard
53.Two lamps are joined in series to a 6.0 V battery. The voltage across the first lamp is 2.5 V. Calculate the voltage across the second lamp in volts.
Medium

Notes de révision

## Circuits - A **circuit** is a complete loop. A gap, such as an open **switch**, stops the current. - Use **circuit symbols** for the cell, lamp, switch, ammeter and voltmeter. ## Current, voltage and resistance - **Current** (A) is a flow of charge, measured by an **ammeter in series**. It is not used up. - **Voltage** (V) is the push from the cell, measured by a **voltmeter in parallel**. - **Resistance** (Ω) is how hard it is for current to flow. **Voltage = current × resistance.** ## Series and parallel - **Series:** one path, same current everywhere, voltage shared, one break stops everything. - **Parallel:** branches, each with the cell's voltage, branch currents add, other branches stay on. ## Materials and safety - **Conductors** (metals, carbon) and **insulators** (plastic, rubber, glass). - Use low-voltage cells only. Never touch mains electricity or mix water with it. ## Inquiry - Name the independent, dependent and control variables, repeat readings and evaluate the method.

Circuit: A complete loop that electric charge can flow round.
Cell: A part of a circuit that gives the charge a push. Two or more cells make a battery.
Switch: A part that opens or closes a gap in the loop.
Current: The flow of electric charge, measured in amperes (A) with an ammeter in series.
Voltage: The push the cell gives the charge, measured in volts (V) with a voltmeter in parallel.
Resistance: How hard it is for current to flow, measured in ohms (Ω).
Conventional current: Current drawn from the positive terminal round to the negative terminal.
Series circuit: A circuit with one loop and only one path for the current.
Parallel circuit: A circuit with branches, so the current has more than one path.
Conductor: A material that lets current flow easily, such as copper.
Insulator: A material that does not let current flow easily, such as plastic.
Short circuit: A path with almost no resistance across a cell, which lets a large current flow and heat the wires.
Ammeter: A meter that measures current. It is connected in series.
Voltmeter: A meter that measures voltage. It is connected in parallel.

Cartes mémo

What is a circuit?

A complete loop that electric charge can flow round.

What does an open switch do?

It makes a gap in the loop, so no current flows.

How is a cell drawn in a circuit diagram?

One long line (positive) and one short line (negative).

How is a lamp drawn?

A circle with a cross inside.

What is electric current?

A flow of electric charge, measured in amperes (A).

Which meter measures current, and how is it connected?

An ammeter, connected in series.

What is voltage?

The push the cell gives the charge, measured in volts (V).

Which meter measures voltage, and how is it connected?

A voltmeter, connected in parallel across the component.

What is resistance, and what is its unit?

How hard it is for current to flow. The unit is the ohm (Ω).

What is the equation linking voltage, current and resistance?

Voltage = current × resistance.

What is the total voltage of two 1.5 V cells in series?

3.0 V.

What is true about the current in a series circuit?

It is the same everywhere in the loop.

What happens if one lamp is removed in a series circuit?

All the lamps go out.

What happens to the current in a parallel circuit?

It splits into branches, and the branch currents add up to the current from the cell.

What happens if one lamp is removed in a parallel circuit?

The other lamps stay on.

Name two conductors and two insulators.

Conductors: copper and iron. Insulators: plastic and rubber.

What is a short circuit?

A path with almost no resistance across a cell. A large current flows and the wires heat up.

Which voltage is safe for school experiments?

Low voltage from cells. Never the mains.

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Number of cells	Current (A)
1	0.10
2	0.20
3	0.30