Circuit Terminology

Circuit

A circuit is a group of components connected together in a loop. At a minimum it will feature a voltage or current source and a resistance.

Note if a voltage or current source is connected only by wires then a resistance still exists in practice as wires have a very small but finite resistance (mΩ) which we can model as a series resistor. This is why every practical circuit has finite resistance.

Open Circuit

An open circuit (open or o/c) is a circuit which features a break or gap which current cannot flow through.

Note we will learn later that current can in effect flow through an open but will either exist only momentarily or vary with time.

Short Circuit

A short circuit (short or s/c) is a circuit which features a very low resistance path which results in a large current flowing through the circuit.

In the circuits shown below the resistance is shorted out by the parallel connection which directly connects back to the battery which most of the current will flow through.

Note a short circuit is a faulty and undesirable circuit as the circuit in the worst case (no equivalent series resistance) will only have a resistance equal to the wires and the resistance of the battery which are both very low leading to a high current which will most likely burn out the wires and/or destroy the power source for the case of some batteries!

Load

A load is a component which draws (consumes) electrical energy from a power source.

A load could be as simple as a resistor which converts the electrical energy into heat or as complex as a processor which performs a specific function such as a CPU in a computer.

Node

A node is the connection between two or more components and has a voltage associated with it known as the node voltage.

Nodes are effectively a single point as all connections in a schematic have a resistance of zero ohm's (ideally) hence the voltage is the same at all points between components along the connections.

In the schematic shown below all 4 resistors connect to node A. The red connections, are all at the same voltage.

Branch

A branch is a path containing a two-terminal component between two nodes. We can say that components have branch currents flowing through them.

In the circuit shown below there are three branches as there are three components between nodes with branch currents Is, I1 and I2 flowing through them. The nodes a, b and c are highlighted in different colours.

Reference Node

Voltages are always measured relative to a reference, a reference node (or ground) is a node we choose to measure voltages relative to in a schematic.

In the above circuit for branches, node c would typically be taken as the reference node.

Junction

A junction is a point in a circuit where at least three components meet.

In the circuit shown under the branch section, there are two junctions labelled b and c as they each connect to three components.

Loop

A loop represents any closed path in a circuit.

A loop begins by starting at a node and passing through nodes until returning back to the first node where each node is only passed through once.

Mesh

A mesh is a loop which does not enclose other loops. We can consider it as a loop which has no other loops inside it.

The circuit below has 3 loops but only the two inner loops in red are meshes as the outer loop encloses the other two.

Questions:

1. a) How many nodes are in the circuit below?

Answer

4

b) How many branches are in the circuit?

Answer

4

c) How many meshes are in the circuit?

Answer

3

2. a) How many nodes are in the circuit below?

Answer

3

b) How many branches are in the circuit?

Answer

4

c) How many meshes are in the circuit?

Answer

3

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