Open and Closed Circuit Diagram: How Current Flows Explained
This is a free printable open and closed circuit diagram: download the diagram as SVG or open it and print to paper or PDF.
A closed circuit is a complete, uninterrupted conducting path from the positive terminal of a source, through the load, and back to the negative terminal, allowing current to flow continuously. An open circuit has a break somewhere in the conducting path — the gap may be an open switch, a broken wire, or a blown fuse — so no current flows and the load does not operate. Understanding open and closed circuit diagrams is the most fundamental concept in basic electronics and is covered in primary and Class 10 physics curricula worldwide.
Every electric circuit requires three things to be a closed (working) circuit: an energy source (battery or power supply), a conducting path (wires), and a load (bulb, resistor, motor). When the path is complete with no gaps, current flows from the negative terminal (conventional current from positive) through the load and returns to the source. This is a closed circuit.
Symbol representation: In circuit diagrams, a closed switch is drawn as two contacts touching (a complete line), while an open switch is drawn with a gap between the contacts (a line with a break). A battery is represented by long/short alternating lines (long=positive plate, short=negative plate). A lamp is shown as a circle with an X inside or a resistor symbol.
Closed circuit behaviour: With the switch closed in a simple series circuit containing a battery (say 6 V) and a 100 Ω resistor, current flows: I = V/R = 6/100 = 0.06 A = 60 mA. The lamp or LED lights up. The voltmeter across the load reads 6 V (ignoring internal resistance). Ammeters in series read 60 mA throughout the loop.
Open circuit behaviour: Opening the switch breaks the conducting path. Current immediately drops to zero (I = 0 A). The voltage across the open switch equals the full supply voltage (6 V) because all the EMF appears across the largest impedance — the gap. The voltmeter across the load reads 0 V. The lamp goes out.
Open circuit voltage (Voc): In battery testing, the open-circuit voltage is measured with no load connected (I=0). For a fresh AA alkaline cell, Voc is approximately 1.5 V. Under load, the terminal voltage drops due to the battery's internal resistance r: V_terminal = EMF − I×r.
Practical examples of open circuits: blown fuse, tripped circuit breaker, broken filament in an incandescent bulb, disconnected wire, open switch, corroded connector. Practical examples of closed circuits: a functioning torch, a doorbell circuit, a car starter motor circuit when the ignition key is turned.
Series vs parallel open/closed: In a series circuit, an open circuit anywhere in the chain breaks current flow for the entire string. In a parallel circuit, opening one branch only stops current through that branch; the other branches remain functional. This is why homes use parallel wiring — each appliance can be switched independently.
Testing with a multimeter: To distinguish open from closed, use a multimeter in continuity mode (beep function) or resistance mode. A closed path reads low resistance (near 0 Ω for a wire) and the meter beeps. An open path reads overrange (OL or ∞) and no beep occurs.
Build and label an open and closed circuit diagram side by side in the free editor at circuitdiagrammaker.com using switch symbols, battery symbols, and bulb symbols to visualise current flow.
How to wire open and closed circuit diagram
- Draw the energy source Place a battery symbol (long line = +, short line = −) or a power supply symbol as the starting point of the circuit diagram.
- Add a switch symbol Place an SPST switch symbol in series with the battery. In the closed position the contacts touch; draw the open version with a gap for the open circuit diagram.
- Connect the load Place a lamp (circle with X) or resistor symbol in series after the switch, completing the series path from battery through switch to load and back.
- Add a return wire Draw a wire from the far side of the load back to the negative terminal of the battery, completing the loop. In the closed circuit, this forms the unbroken conducting path.
- Label current direction Draw an arrow on the wire indicating conventional current flow from the positive terminal through the external circuit back to the negative terminal.
- Draw the open circuit version Copy the diagram but draw the switch with a gap, and add an annotation showing I=0 A through the load and the full supply voltage (e.g., 6 V) appearing across the switch gap.
- Simulate both states In circuitdiagrammaker.com, toggle the switch to show the lamp lighting (closed) and going dark (open), confirming your understanding of both circuit states.
Specifications
| Closed circuit current | I = V / R (Ohm's Law, non-zero) |
|---|---|
| Open circuit current | I = 0 A (no current flows) |
| Voltage across open switch | Equal to full supply EMF |
| Voltage across load (open circuit) | 0 V (no current, no voltage drop) |
| Open-circuit voltage (battery) | V_oc = EMF (no internal voltage drop) |
| Loaded terminal voltage | V_T = EMF − I × r_internal |
| Switch symbol (closed) | Two contacts touching, complete line |
| Switch symbol (open) | Line with a gap between contacts |
| Continuity test (closed path) | ~0 Ω (multimeter beeps) |
| Continuity test (open path) | OL / ∞ Ω (no beep) |
| Series open circuit effect | Entire series chain stops; all loads off |
| Parallel open branch effect | Only the open branch stops; other branches unaffected |
| Typical cause of open circuit | Blown fuse, open switch, broken wire, bad connector |
Safety warnings
- Never short-circuit a battery by connecting its terminals together with a wire — the resulting high current causes rapid heating, potential battery rupture, and chemical burns from leaked electrolyte.
- When testing mains-connected circuits for open circuits, always isolate the circuit from the mains supply before probing; use a non-contact voltage tester to confirm the circuit is de-energised.
Tools needed
- Battery (1.5 V AA or 6 V lantern) or variable DC supply
- SPST switch (toggle, push-button, or knife switch)
- Lamp or LED with 330 Ω resistor
- Connecting wires and breadboard or crocodile-clip leads
- Digital multimeter (continuity/resistance/voltage modes)
- circuitdiagrammaker.com for drawing labeled circuit diagrams
Common mistakes
- Drawing the switch in an open state (gap) but labelling the diagram as a closed circuit, confusing students about which state allows current flow.
- Placing the switch in parallel with the load instead of in series, which short-circuits the load when the switch closes rather than enabling current flow through it.
- Forgetting the return wire from the load back to the battery, creating an incomplete circuit that students mistakenly label as a closed circuit.
- Measuring voltage across the load instead of across the open switch to find the supply voltage during an open-circuit test.
- Confusing an open circuit (I=0) with a short circuit (I=maximum), which are opposite failure modes.
Troubleshooting
- Lamp does not light in a simple closed circuit
- Cause: The circuit is actually open — a wire is disconnected, the switch is faulty, or the bulb has failed. Fix: Use a multimeter in continuity mode to probe each series segment systematically from the battery terminals outward to find the break.
- Voltmeter reads full supply voltage across the load in a simple circuit with no lamp glow
- Cause: The lamp is open (burned-out filament), so all supply voltage appears across the lamp's open terminals. Fix: Replace the lamp; confirm by testing the lamp in continuity mode (no beep = open filament).
- Switch does not break the circuit when opened
- Cause: The switch contacts are welded closed (stuck closed) due to past arcing, or there is an alternative parallel current path bypassing the switch. Fix: Replace the switch; inspect the circuit for unintended parallel wire connections that bypass the switch.
Frequently asked questions
What is an open and closed circuit diagram?
An open circuit diagram shows a break in the conducting path (such as an open switch) where no current can flow; a closed circuit diagram shows a complete, uninterrupted path so current flows and loads operate.
Why does voltage appear across an open switch?
With no current flowing, there is no voltage drop across the resistances in the series path (V=IR=0), so the full EMF of the supply appears across the only remaining gap — the open switch.
What are common causes of an open circuit?
Blown fuse, tripped circuit breaker, open or broken switch, corroded terminal, snapped wire, and a burned-out lamp filament are the most common causes of an open circuit.
How do you test for an open circuit with a multimeter?
Set the multimeter to continuity mode and probe both ends of the suspected section. A beep and near-zero resistance indicate a closed (good) path; OL (over-limit) with no beep confirms an open circuit.
What happens to the other lamps in a parallel circuit if one branch opens?
In a parallel circuit, each branch is independent. If one lamp's filament breaks (open circuit in that branch), the other lamps remain lit because the supply voltage is still applied to their branches.
What is the difference between open circuit and short circuit?
An open circuit has an infinite resistance path where no current flows. A short circuit has near-zero resistance creating excessive current, which can cause overheating, fire, or blown fuses.
What is open circuit voltage and why is it important for batteries?
Open circuit voltage (Voc) is measured with no load and equals the battery's EMF. Comparing Voc to the rated voltage indicates battery health; a significantly lower Voc suggests the battery is partially discharged or has high internal resistance.