Basic Circuit Diagrams and Open vs Closed Circuits
A circuit diagram uses standard symbols to show how the components of an electric circuit are connected, and a circuit must form a complete closed loop for current to flow: in a closed circuit current flows, and in an open circuit (a break, such as an open switch) it does not. In brief: a simple circuit has a source of energy (cell or battery), conducting wires, a load (lamp, resistor, motor) and usually a switch. An ammeter is connected in series and a voltmeter in parallel. A short circuit is an unintended low-resistance path that allows a large current.
To build, analyze or repair circuits, it is necessary to read and draw circuit diagrams. This guide describes the standard symbols and the rules for drawing diagrams, explains open, closed and short circuits, compares the circuit types in a table, works through a simple analysis and corrects common mistakes.
What are the standard symbols?
Circuit diagrams use agreed symbols, drawn with straight lines for wires and right angles at corners. The most common are:
- Cell and battery: a long thin line (positive terminal) and a short thick line (negative terminal). A battery is two or more cells.
- Switch: a gap in the line with a lever. Open: gap; closed: lever touches the contact.
- Lamp (filament): a circle with a cross.
- Resistor: a rectangle (or a zigzag line in some countries). Variable resistor (rheostat): a rectangle with an arrow.
- Ammeter: a circle with the letter A. Voltmeter: a circle with the letter V.
- Fuse: a rectangle with a line through it. Diode: a triangle pointing to a bar (current flows in the direction of the triangle). Thermistor, LDR, LED, motor (M in a circle) and earth also have symbols.
- Junction: a dot where wires are connected. Wires that cross without a dot are not connected.
What are open, closed and short circuits?
- Closed circuit: a complete conducting path from one terminal of the source to the other. Current flows, and the lamp lights.
- Open circuit: there is a break, for example an open switch, a broken wire or a blown fuse. No current flows. The p.d. of the source then appears across the gap.
- Short circuit: a path of very low resistance across a component or the source. The current in that path is very large (I = V ÷ R with R small), which can overheat wires and cause fires, so fuses and circuit breakers are used. The component shorted out receives little or no current.
How are meters connected?
An ammeter measures the current through a component and is connected in series with it, so that the same current flows through it. It has a very small resistance so as not to change the current. A voltmeter measures the potential difference across a component and is connected in parallel with it. It has a very high resistance, so that it draws almost no current. Reversing the connections of a voltmeter in series would make the current in the circuit nearly zero; an ammeter in parallel across a source would form a short circuit.
Circuit states compared
| Type | Path | Current | Result |
|---|---|---|---|
| Closed | Complete loop through the load | Normal | Load works |
| Open | Broken loop | Zero | Load off |
| Short | Low resistance path bypassing the load | Very large | Overheating, fuse blows |
Worked example: analyzing a simple circuit
A circuit consists of a 6.0 V battery, a switch, and a lamp of resistance 12 Ω in series. An ammeter is in the loop, and a voltmeter is across the lamp.
- Switch open: the circuit is open. The current is 0 A and the lamp is off. The ammeter reads 0. A voltmeter across the open switch would read 6.0 V (the battery p.d.), while the voltmeter across the lamp reads 0 V.
- Switch closed: I = V ÷ R = 6.0 ÷ 12 = 0.50 A. The ammeter reads 0.50 A (assuming negligible resistance), and the voltmeter across the lamp reads 6.0 V.
- Power of the lamp: P = VI = 6.0 × 0.50 = 3.0 W.
- Short circuit across the lamp (a thick wire connected in parallel with the lamp, with a resistance of 0.10 Ω): most of the current takes the wire: I ≈ 6.0 ÷ 0.10 = 60 A in the wire (limited only by the wire and the internal resistance of the battery in practice), and the lamp goes out, as the p.d. across it is almost zero. A fuse of 5 A would blow.
How did circuit notation develop?
The earliest circuit diagrams were pictures of the apparatus. Standard symbols developed in the late 19th century with telegraph and electrical engineering, and the International Electrotechnical Commission (IEC, founded in 1906) published international standards. Today, software such as SPICE and online simulators let students draw and test circuits.
Which mistakes are common?
Mistake 1: Leaving a gap in the circuit
Current flows only in a complete loop. A single gap stops the current everywhere in a series circuit.
Mistake 2: Placing the voltmeter in series
It must be in parallel with the component.
Mistake 3: Thinking that a short circuit has no current
A short circuit has a very large current, but the shorted component has little or none.
Mistake 4: Misreading the battery symbol
The long line is the positive terminal. Conventional current leaves it.