2.2 Closed, Open & Short Circuits
A circuit needs a complete path
A basic electric circuit needs:
- a source that creates a potential difference
- a conductive path
- a load that uses or transforms electrical energy
- a path back to the source
A closed circuit has a complete path, so current can flow. [1, pp. 40-42]
Open circuit
An open circuit has a break in the path.
In the ideal model:
- current is 0 A
- resistance across the break is treated as extremely high
- voltage can still exist across the open gap
Grob uses a simple 10 V circuit to show that opening the path stops current everywhere in that series path. [1, p. 96]
Open circuits can be intentional or accidental:
- an open switch
- a disconnected cable
- a broken wire or PCB trace
- a failed component
- a blown fuse
No current does not automatically mean no voltage.
An open circuit can still have the applied voltage across the break.
Short circuit
A short circuit creates a path with very low resistance.
Ohm’s law predicts that, for a given voltage, lower resistance allows more current:
I = V / R
Real wires and sources never have exactly zero resistance, but a short can still allow enough current to overheat wiring or damage components. [1, pp. 96-97]
Fuses intentionally create an open
A fuse protects a circuit by opening the path if the current becomes too high.
Open Circuits shows cartridge fuses in which a metal element heats and melts when current exceeds the fuse rating. [2, pp. 54-58]
The sequence is:
excess current -> heating -> fuse element opens -> current stops
A blown fuse is usually evidence of another problem. Find the cause before simply replacing the fuse.
Switches are controlled opens and closes
A normally open pushbutton has a deliberate break in the circuit until it is pressed. Pressing the button moves a metal contact into place and completes the circuit. [2, p. 109]
This gives us a useful mental model:
- open switch -> path broken
- closed switch -> path complete
Measurement reminder
Measure resistance with the circuit power off. A voltage can remain across an open circuit even when no current is flowing. [1, pp. 95-96]
Quick check
For each situation, predict the current and explain why:
- a switch is open
- a switch is closed
- a wire directly connects the positive and negative sides of a low-voltage source
- a fuse has blown
- a USB cable has an internal broken conductor
Add a real troubleshooting example of an open or short circuit.