Draw switches and trace complete circuit paths
Interpret open and closed contacts and show junctions without mistaking every crossing for a connection.
Section 4.3.1 · Circuit diagrams and circuit components
Core and Extended; Physics 0625, 2026–2028. Use the stated source, operating conditions and meter ranges. Written practice is self-checked.
Before you begin
- Identify source terminals.
- Distinguish a path from an unconnected crossing.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which condition does a simple lamp circuit need?
What you will learn
- Draw open and closed switch states.
- Interpret joined wires and interruptions in a circuit.
Use the contact state
An open switch has separated contacts with a lever that does not meet the far contact. It interrupts that conducting path. A closed switch joins the contacts and permits current if the rest of the circuit provides a complete path and a suitable source. Closing a switch does not guarantee light if another break remains.
A switch in the sole source path controls every load beyond it. A switch within one parallel branch can control that branch while another branch remains complete. Always trace the actual wires rather than relying on whether a switch is drawn near a particular lamp.
Show connections unambiguously
Use a filled junction dot where connected wires meet. A crossing with a drawn bridge shows no connection. Where an unmarked crossing could be ambiguous, redraw it or add explicit connection notation. Moving a symbol on the page does not change topology if its terminal connections remain the same.
Written construction exercises begin with the source off, a planned isolated low-voltage diagram and a teacher check before energising. They practise reasoning about connections, not practical competence. Do not draw a plain wire directly across the source terminals: that bypasses the intended load and can produce excessive current.
Pause and explain
A switch in one parallel branch opens. What determines the other lamp state?
Worked example
Two parallel lamp branches share a source. A switch is in only the upper branch. Describe what opening it changes.
Show the worked solution
- Opening separates the upper branch contacts.
- The upper lamp no longer has a complete path.
- The lower branch still has its own complete path across the source.
Answer The upper lamp is off; the lower can stay on.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw a cell, lamp and open switch in one loop; redraw the switch closed.
Give me a hint
Keep the component connections fixed while changing the contacts.
Compare my reasoning
- Draw source and lamp symbols joined around a return path.
- Leave a gap between switch contacts for the open state.
- Join the contacts for the closed state.
The open contact interrupts the loop; the closed contact completes it if no other break exists.
Look for these in your work
- Show both states.
- Retain the return wire.
A diagram has a closed switch but an unconnected wire at the lamp terminal. Will the lamp conduct? Explain.
Give me a hint
Trace the entire loop rather than inspecting the switch alone.
Compare my reasoning
- The switch contacts are joined.
- The lamp terminal still has a gap.
- The whole loop remains incomplete, so no ideal loop current flows.
No; the separate unconnected terminal interrupts the complete path.
Look for these in your work
- Identify the remaining break.
- Do not equate closed switch with guaranteed light.
Two wires cross and only the intended branch junction is dotted. Redraw the crossing with a bridge and explain its purpose.
Give me a hint
Distinguish a connection from lines that merely cross on paper.
Compare my reasoning
- Retain the dot at the true branch connection.
- Draw one crossing wire with a bridge over the other.
- Explain that the bridge means the crossing wires are not joined.
The dot marks the true junction; the bridge makes the separate crossing unambiguous.
Look for these in your work
- Preserve topology.
- Label ambiguous connections.
Common mistakes
- Trace wires instead of physical proximity.
- A closed switch does not repair a separate break.
- Show junctions and non-connected crossings clearly.
What can you explain now?
What does an open switch do to its path?
Compare with the explanation
Interrupts the conducting path.
The separated contacts prevent current through that path in the ideal circuit model.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Follow complete paths, switch contacts and explicit junctions rather than the page layout.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.