# Difference between load resistance and Thevenin/Norton resistance?

In the Norton/Thevenin theorem, what is the definite difference between the load resistance and Thevenin/Norton resistance?

I cannot find any sources defining the difference, all of the sources use them as if I already were to know the difference.

Take this example.

Here I know the load resistance is the 40 ohm resistor. But not really why. Is it because of the direct connection to ground? If the circuit was more complex, I might not have been able to see it as clear as in this picture, because I don't really know what qualifies 'the load resistor' to differ from the rest of the circuit.

• Do any of your sources give a circuit schematic example that uses both a Thévenin resistance and a load resistance? May 24 '17 at 11:53
• @ChrisM. Answering by editing origianl question. May 24 '17 at 12:52
• The Thevenin/Norton resistance is the resistor that comprises the resistance of the Thevenin/Norton. The load resistance is what connects to the two terminals of the equivalent circuit (i.e. directly between nodes "A" and "B"). May 24 '17 at 13:04
• So the load resistor, part of or not of the Th/N eq., is determined by where the terminals are placed? May 24 '17 at 13:12

In the example you've posted, we see two Thévenin circuits that share a 40-ohm load resistor. But the question is what differentiates between the "load" resistor and the "Thévenin" resistor.

I've redrawn the circuit below:

simulate this circuit – Schematic created using CircuitLab

The Thévenin equivalent circuits are enclosed in boxes as shown.

Alone both Thévenin and Norton circuits have open-ended terminals. Whatever impedance is found across those terminals is deemed the "load" that the equivalent circuit is driving.

For a Norton equivalent, we would see the following circuit:

simulate this circuit

Difference between load resistance and Thevenin/Norton resistance?

A load resistance is simply a resistor which is being used as a load for an electrical network.

Generally, the load is the something that is connected to the output of an electrical circuit. It can be anything like a resistor, capacitor, inductor, LED or anything.

A Thevenin/Norton resistance is the equivalent resistance of the network when looking from the output terminals by shorting voltage sources and opening the current sources.

Here I know the load resistance is the 40 ohm resistor. But not really why. Is it because of the direct connection to ground?

It's not because of the direct connection to ground but the given terminals $a$ and $b$. Basically, in Thevenin/Norton theorem analysis, we assume the resistor connected in between the terminals say $a$ and $b$ is the load resistor.

Thevenin & Norton Resistances are the same.

The difference is whether you model the circuit with a Thevenin-voltage-source or a Norton-current-source.