Resistance

After reading this section you will be able to do the following:

• Define resistance and how we measure it.
• Discuss the similarities between resistance in a wire and the resistance in a water hose.

Resistance is another important property that can be measured in electrical systems. Resistance is measured in units called ohms. Resistance is a term that describes the forces that oppose the flow of electrical current in a conductor. All materials naturally contain some resistance to the flow of electron current. We have not found a way to make conductors that do not have some resistance.

If we use our water analogy from the voltage lesson to help picture resistance, think of a hose that is partially plugged with sand. The sand will slow the flow of water in the hose. We can say that the plugged hose has more resistance to water flow than does an unplugged hose. If we want to get more water out of the hose, we would need to turn up the water pressure at the faucet. The same is true with electricity. Materials with low resistance let electricity flow easily. Materials with higher resistance require more voltage (EMF) to make the electricity flow.

The scientific definition of one ohm is the amount of electrical resistance that exists in an electrical circuit when one amp of current is flowing with one volt being applied to the circuit.

Resistivity (ρ) is an intrinsic property of a material relating to the materials resistance to the flow of electrical current. A high resistivity indicates that a material is not a good conductor of electricity. Resistivity is determined by measuring the electrical resistance over some length of material with a constant cross-section. The materials resistivity is the product of the resistance value and the cross-sectional area divided by the length as given in the following equation.

$\rho=\frac{RA}{l}$

where
ρ = resistivity (ohm meters)
R = electrical resistance of a uniform specimen of the material (ohms)
l = length of the specimen (measured in meters)
A = cross-sectional area of the specimen (meters 2)

Conductivity is calculated by simply taking the inverse of resistivity. A material with a resistivity of 1/58 ohm-meter will have a conductivity of 58 MegaSiemens per meter.