Electric Charge and Basic Circuits
5. Building and Drawing Circuits
Learning outcomes
- I can construct simple electrical circuits.
- I can draw schematic diagrams.
- I can identify open and closed circuits.
- I can safely build basic circuits.
- I can troubleshoot simple circuit faults.
From Circuit Diagrams to Real Circuits
An electrical circuit is a complete conducting pathway through which electric charge can move.
Learning about circuit components is only the beginning. We also need to be able to:
- connect components correctly
- represent circuits using standard symbols
- determine whether a circuit is complete
- operate circuits safely
- identify problems when a circuit does not work
A simple circuit might contain:
cell → switch → lamp → connecting wires → back to cell
The arrangement must provide a complete path from one terminal of the power source, through the components, and back to the other terminal.
Constructing a Simple Circuit
Consider a circuit containing:
- one cell
- one lamp
- connecting wires
One wire connects one terminal of the cell to one terminal of the lamp.
A second wire connects the other lamp terminal back to the other terminal of the cell.
The result is a complete conducting path:
cell → lamp → cell
If all the connections are secure and the components are working, the lamp should light.
This is one of the simplest electrical circuits we can construct.
A Circuit Must Form a Complete Loop
A common mistake is to connect only one side of a lamp to a cell.
For example:
cell → wire → lamp
This does not form a complete circuit.
There must also be a conducting path back to the other terminal of the cell.
A complete circuit is therefore a loop.
A useful question when examining any circuit is:
Can I trace one continuous conducting path from one terminal of the source, through the components, and back to the other terminal?
If yes, the circuit may be complete.
If not, the circuit is open.
Open and Closed Circuits
A closed circuit contains a complete conducting path.
Current can flow around the circuit.
A simple example is:
cell → closed switch → lamp → cell
The lamp can operate.
An open circuit contains a break somewhere in the conducting path.
For example:
cell → open switch → gap → lamp → cell
Because the path is incomplete, current cannot continuously flow around that path.
Open Does Not Mean "On"
The language can initially seem confusing.
An open switch means the contacts are separated.
This creates a gap.
Therefore:
open switch → open circuit → no continuous current through that path
A closed switch means the contacts are connected.
Therefore:
closed switch → closed circuit → current can flow
A useful memory aid is:
Closed = connected
Adding a Switch
A switch allows us to control a circuit without disconnecting wires.
Consider:
cell → switch → lamp → cell
Switch Open
The conducting pathway is broken.
Result: lamp off
Switch Closed
The conducting pathway is complete.
Result: lamp on
The switch does not provide the energy. The cell is the power source.
The switch simply controls the pathway.
Circuit Diagrams
Real circuits can become difficult to draw accurately.
Instead of drawing realistic pictures of cells, wires, lamps, and switches, scientists and engineers use schematic diagrams.
A schematic diagram uses standardized symbols to represent components.
This allows us to concentrate on:
- which components are present
- how they are connected
- whether the circuit is complete
rather than what each component physically looks like.
Common Circuit Symbols
You should be able to recognize and draw the symbols for common components.
Wire: a straight line
Cell: two parallel lines of unequal length
Battery: several cell symbols together
Lamp: a circle containing a cross
Resistor: commonly shown as a rectangle in IEC-style diagrams
Open switch: two contacts separated by a gap
Closed switch: contacts connected
Fuse: represented by the appropriate standardized fuse symbol
Circuit symbols should be drawn clearly using straight lines where appropriate.
Drawing a Simple Circuit
Suppose you are asked:
Draw a circuit containing one cell, one switch, and one lamp.
First identify the components:
- cell
- switch
- lamp
Then connect them into a complete loop.
In text, the arrangement can be represented as:
cell → switch → lamp → cell
In your actual schematic diagram, use the proper standard symbols rather than writing the component names.
When the switch is closed, the circuit should contain a continuous path through all three components.
Rules for Clear Circuit Diagrams
A good circuit diagram should be easy for another person to interpret.
When drawing:
- use standard circuit symbols
- use a ruler for straight connecting wires where practical
- make connections clear
- avoid unnecessary crossings
- make sure the circuit forms the intended pathway
- do not draw realistic pictures of components
- keep the diagram simple and organized
A circuit diagram is a model of the electrical connections, not a picture of the physical layout.
The real circuit might look messy while its schematic diagram remains simple.
Physical Position Does Not Have to Match the Diagram
Suppose a lamp is physically located to the left of a cell on your laboratory table.
In the circuit diagram, the lamp might be drawn on the right.
That is fine.
The important information is how the components are electrically connected, not where they physically sit on the table.
This is one of the major advantages of schematic diagrams.
Worked Example: From Description to Circuit
A student is told:
Build a circuit containing a cell, lamp, and switch so that the switch controls the lamp.
A suitable arrangement is:
cell → switch → lamp → cell
Before connecting anything, the student should:
- identify the components
- inspect the wires
- keep the switch open while assembling the circuit
- connect the components into a complete pathway
- check the circuit before closing the switch
Once the circuit has been checked, the switch can be closed.
If the circuit is working correctly, the lamp should light.
Building Circuits Safely
School circuits usually operate at low voltages, but safe procedures are still important.
When building a circuit:
- use only the power sources provided or approved by your teacher
- check that wires and components are not visibly damaged
- keep liquids away from electrical equipment
- avoid touching exposed conductors in circuits where your teacher has told you not to
- open the switch or disconnect the power before changing connections
- do not deliberately create a short circuit
- stop if a component becomes unusually hot
- follow the voltage and current limits of components
- follow your teacher's laboratory instructions
Never experiment with household mains electricity.
Short Circuits
A short circuit occurs when a very low-resistance pathway allows a large current to bypass the intended load.
For example, connecting the terminals of a cell directly together with a wire can create a short circuit.
This can cause:
- large current
- rapid heating
- damaged wires
- damaged cells or batteries
- fire risk in higher-power systems
Therefore:
Never intentionally connect the two terminals of a cell or battery directly together with a wire.
A load or suitable resistance should be included in the circuit.
Why a Lamp Might Not Light
Suppose you build a simple circuit but the lamp does not turn on.
Do not immediately assume the lamp is broken.
There are several possible faults.
The problem might be:
- an open switch
- a loose connection
- a disconnected wire
- a damaged wire
- a flat cell
- a faulty lamp
- incorrect connections
- a component not making electrical contact
Finding the problem systematically is called troubleshooting.
Troubleshooting Circuits
Troubleshooting means identifying and correcting faults in a circuit.
A good troubleshooter works systematically rather than randomly changing components.
Start with the simplest questions.
Is the circuit complete?
Trace the entire path.
source → components → back to source
Look for gaps.
Is the switch closed?
An open switch intentionally breaks the circuit.
Are the wires connected securely?
A wire may appear connected while failing to make good electrical contact.
Is the power source working?
A discharged cell may not provide sufficient potential difference.
Is the lamp working?
Try replacing it with a known working lamp if appropriate.
Are the components connected correctly?
Compare the physical circuit with the intended schematic diagram.
A Simple Troubleshooting Sequence
Imagine this circuit:
cell → switch → lamp → cell
The switch is closed, but the lamp does not light.
A sensible troubleshooting sequence might be:
Check the switch.
Is it actually closed?
↓
Check the connections.
Are all wires securely connected?
↓
Check the cell.
Does it work in another known working circuit?
↓
Check the lamp.
Does another lamp work in its place?
↓
Compare the circuit with the schematic.
Has everything been connected correctly?
This approach changes one thing at a time, making it easier to identify the fault.
Worked Example: The Loose Wire
A student builds a circuit containing:
- one cell
- one switch
- one lamp
The switch is closed, but the lamp is off.
The student notices that one wire is not fully connected to the lamp terminal.
What type of circuit is this?
Open circuit
Even though the switch is closed, the loose wire creates a break in the pathway.
After reconnecting the wire:
the pathway becomes complete → current can flow → lamp operates
This demonstrates an important idea:
A closed switch does not guarantee that the entire circuit is closed.
Every connection must form part of the complete pathway.
Worked Example: The Flat Cell
A circuit appears to be correctly connected.
The switch is closed.
The wires are secure.
The lamp still does not light.
The student replaces the cell with a known working cell.
The lamp now lights.
What was the likely fault?
The original cell was discharged or faulty.
Troubleshooting allowed the student to isolate the problem.
Worked Example: Finding a Fault
Consider a circuit containing:
cell → switch → resistor → lamp → cell
The lamp does not operate.
A student immediately removes every wire and rebuilds the entire circuit.
This may work, but it is not an efficient troubleshooting method because the student may never discover what caused the problem.
A better approach is to:
- inspect the circuit
- compare it with the diagram
- identify possible faults
- test one possibility at a time
Good troubleshooting is systematic.
From a Real Circuit to a Schematic
Sometimes you will be given a physical circuit and asked to draw its schematic.
First identify each component.
For example:
- one cell
- one switch
- two lamps
- connecting wires
Next, follow the wires to determine how the components are connected.
Then replace each physical component with its standard circuit symbol.
Do not try to copy the physical shape of the circuit.
Your diagram should represent the electrical connections.
Predicting Circuit Behaviour
Once you understand the connections, you can predict what will happen.
Consider:
cell → switch → lamp → cell
Switch closed
The path is complete.
Prediction: lamp operates.
Switch open
The path is broken.
Prediction: lamp does not operate.
One wire disconnected
The path is broken.
Prediction: lamp does not operate.
Cell removed
There is no functioning power source and the path is incomplete.
Prediction: lamp does not operate.
Lamp removed from the simple loop
The path is broken.
Prediction: current cannot continuously flow around the circuit.
Circuit Diagrams as Models
A schematic circuit diagram is a type of scientific model.
It leaves out unnecessary details while preserving important information.
For example, a schematic does not normally show:
- the colour of the wires
- the physical size of the cell
- the shape of the laboratory table
- the appearance of the lamp
Instead, it shows:
- components
- connections
- pathways
- circuit structure
This makes circuit diagrams useful for designing, constructing, explaining, and troubleshooting circuits.
Practical Activity: Build, Draw, Break, Fix
This topic works especially well as a short practical investigation.
Build
Construct a simple circuit containing:
- one cell
- one switch
- one lamp
Test the circuit.
Draw
Draw a schematic diagram representing your circuit using standard symbols.
Break
With the power disconnected or switch open, one student introduces one safe fault, such as:
- loosening a wire
- opening the switch
- disconnecting one connection
Do not create a short circuit.
Fix
Another student must:
- inspect the circuit
- identify the fault
- explain why it prevents the circuit from operating
- correct the problem
Then swap roles.
This combines circuit construction, schematic diagrams, open and closed circuits, safety, and troubleshooting in one activity.
Common Mistakes
"If the switch is closed, the circuit must be closed."
Not necessarily.
A loose wire or another break elsewhere can still create an open circuit.
"Electricity only needs a path from the cell to the lamp."
Incorrect.
A simple circuit needs a complete conducting loop back to the power source.
"A circuit diagram should look like the real circuit."
Incorrect.
A schematic shows electrical connections using standardized symbols.
"If the circuit doesn't work, replace everything."
This is poor troubleshooting.
Test possible causes systematically, ideally changing one variable at a time.
"Connecting the battery terminals directly is a quick way to test the battery."
Do not do this.
It can create a short circuit and cause dangerous heating.
"Open switch means the circuit is switched on."
Incorrect.
An open switch creates a gap. A closed switch completes the pathway.
Check Your Understanding
1. What must a simple circuit have before current can continuously flow?
2. What is the difference between an open circuit and a closed circuit?
3. What happens when a switch is opened?
4. What happens when a switch is closed?
5. Why do we use schematic circuit diagrams instead of realistic drawings?
6. Name three rules for drawing a clear circuit diagram.
7. A switch is closed but the lamp does not light. Give three possible faults.
8. Why should you change only one thing at a time when troubleshooting?
9. Why should power normally be disconnected or the switch opened before changing a circuit?
10. What is a short circuit?
11. Why can a short circuit be dangerous?
12. A student has connected one side of a lamp to a cell but has not connected the other side. Is the circuit open or closed? Explain.
13. Why might a schematic diagram look very different from the physical arrangement of the components?
14. Describe a sensible sequence for troubleshooting a lamp that does not light.
Key Terms
- Circuit – a conducting pathway containing electrical components.
- Closed circuit – a circuit containing a complete conducting pathway.
- Open circuit – a circuit containing a break in the conducting pathway.
- Schematic diagram – a simplified diagram using standard symbols to represent circuit components and connections.
- Circuit symbol – a standardized symbol representing an electrical component.
- Switch – a component used to open or close a conducting pathway.
- Power source – a device that supplies energy to a circuit.
- Load – a component that transfers electrical energy into other forms.
- Short circuit – an unintended very low-resistance pathway that can allow dangerously large current.
- Fault – a problem that prevents a circuit from operating as intended.
- Troubleshooting – the systematic process of finding and correcting faults.
- Connection – the electrical link between components.
Key Takeaways
- A simple electrical circuit requires a complete conducting pathway.
- A closed circuit provides a complete path through which current can flow.
- An open circuit contains a break in the pathway.
- Standard circuit symbols are used to create clear schematic diagrams.
- A schematic shows electrical connections, not the physical appearance or position of components.
- Circuits should be constructed using appropriate low-voltage equipment and safe procedures.
- A direct low-resistance connection across a power source can create a dangerous short circuit.
- Circuit faults include loose wires, open switches, faulty components, incorrect connections, and discharged cells.
- Effective troubleshooting involves checking the circuit systematically and changing one thing at a time.
- Being able to build, draw, predict, and troubleshoot circuits are closely connected practical skills.