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Resistance

Ohm’s Law

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Georg Ohm(1789 – 1854)

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Resistance

• The “loss” of energy in the form of heat creates inefficiencies in light bulbs, household wires and computer circuits to name a few.

• However in stovetop heating elements, for example, the heat is wanted.

• This “loss” of energy is called Resistance and is very important to understand when dealing electricity.

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What is resistance?For current in a conductor there is heat produced. Resistance is the

heat produced from using voltage to enable current.

Georg Simon Ohm (1789 - 1854) was a high school teacher who made his own equipment and discovered that amount of current that could be in a wire was dependent on the diameter of the wire and the length of the wire.

From this he developed Ohm’s Law, which relates resistance, voltage and current. In his honour the unit for measuring resistance was named after him.

1 Ohm = A decrease of 1Volt is required for 1Ampere to flow

The symbol for Ohm is the Greek letter omega:

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Factors of a material that affect resistance include: • Thickness: Thinner increases resistance• Length: Longer increases resistance• Temperature: Hotter increases resistance• Material: Impurities and defects in molecular structure

Materials can be of act as one of four types:1. Superconductor: No resistance (below 20 degrees K)2. Conductor: Low resistance (copper, aluminum …)3. Semiconductor: Controlled resistance (doped silicon …)4. Insulator: High resistance (plastic, rubber …)

Factors of Resistance

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Voltage, Current and Resistance

A common analogy to help you understand voltage, current and resistance is to think of a waterfall that is diverted into a pipe down a hillside.

Voltage• The height of the source of the water determines how much potential

energy the water has before it enters the pipe. This also determines the pressure of the water.

• This is like the amount of attraction there is between the charges.Current• The amount of water flowing past a point is like the amount of charges

flowing past a point in the circuit.Resistance• The diameter of the pipe will significantly restrict or allow the water to flow.

Changing the diameter of the pipe changes the amount of current that can flow from the pressure.

• If the voltage is constant then resistors change the amount of current that can flow in the circuit.

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QuestionWith what you now know about resistance, why is beneficial that

computers are kept cool when they are being used? The circuits in computers use incredibly thin “wires” which create heat

from their resistance. This heat then increases the resistance even more. As the resistance increases the current is restricted and the computer starts to slow down or even ironically “freeze”. People who “over-clock” their CPU’s often use elaborate systems to cool their computers. Some even used liquid cooled systems!