Resistance, Ohm's Law and Conductance

As charged particles try to make their way around a circuit they encounter resistance to their flow - for example, electrons collide with atoms in a metal.

The more resistance there is the more energy that is needed to push the same number of electrons through part of the circuit.

Resistance is measured in ohms, Ω, and the resistance of a component can be found using an ohmmeter.

The ohm is defined by:

"If it takes 1 volt (1 joule per coulomb) to drive a current of 1 amp through a resistor, it has a resistance of 1 ohm."

Resistance can be calculated using the equation:

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Where:

R = resistance (ohms, Ω)

V = potential difference (volts, V)

I = current (amps, A)

For certain components, such as metal resistors at constant tempertaure, the resistance, R, doesn't change. These components obey Ohm's Law.

Ohm's Law states that the current through a metallic conductor is proportional to the potential difference across it if the temperature remains constant.

So, if you plot a graph of current against voltage you will get:

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Note: The gradient of a current against voltage graph is equal to 1/resistance of the component.

Any resistor that obeys Ohm's Law is called an ohmic resistor. Any resistor that doesn't do this is cleverly called a non-ohmic resistor.

Conductance, G, is the opposite of resistance, and tells us how easy it is for a current to flow through something. Conductance is measured in siemens, S.

1 S = 1 ohm -1

Conductance can be calculated using the equation:

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Where:

G = conductance (siemens, S)

I = current (amps, A)

V = potential difference (volts, V)

R = resistance (ohms, Ω)

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