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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:**

**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:

* 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:**

**or **

**Where:**

** **

**G** = conductance (siemens, S)

**I** = current (amps, A)

**V** = potential difference (volts, V)

**R** = resistance (ohms, Ω)