AND MR. SEREDA PRESENT….

Resisting the

movement of Charge

Resistance Electrons that travel through a light bulb

Filament experience Resistance Resistance is a property that hinders

motion AND converts electrical energy to other forms of energy

• E.g.: resistance of Tungsten wire is 400 times greater than copper

• When a current flow through the wire it is converted into ________________ and ________

Light

Heat

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Describing Resistance How much energy does it take to push a

50 kg crate across the floor? Depends on resistance Pushing across a carpet would be harder than

pushing across ice.

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Resistance How much energy does it take to push a

coulomb of charge across a filament? Defined as the ratio of POTENTIAL

DIFFERENCE across the LOAD POTENTIAL DIFFERENCE = VOLTAGE (V) LOAD = CURRENT (I)

V

R = ---

I

Voltage

Resistance = -------

Current

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Units of Resistance

Units of Resistance are called OHMS Named in honour of Georg Simon Ohm The symbol for Ohms is the greek letter

OMEGA (Ω)

Ohm was the first to publish results of various resistances on types of wires

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Georg Ohm’s Findings

Ohm took various types of conductors Applied a current across the wire No matter what the Potential Difference

(Voltage) --- the Resistance was the same Called Ohm’s Law

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Ohm’s Law

V = IR Voltage = Current x Resistance

• http://www.mhhe.com/physsci/physical/giambattista/circuits/circuits.html

• http://www.riedon.com/riedon_calculator.htm

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Model Problem

What is the resistance of the heating coil of an electric heater if a current of 12.5 A runs through it? I = 12.5 A V = 120 V

R = V / I R = 120 / 12.5 R = 9.6 V / A R = 9.6 Ω

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Wasting Energy

Do resistors have to convert energy to a different source? Can be used to control the current or Potential

difference at different points in the circuit.

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Measuring Resistance What is the Resistance? R = V / I

V = 6 I = 3

R = 6 / 3 = 2 Ω

3 Amps

Potential Difference = 6

Designing Circuit Materials Four factors Affect Resistance

Length Resistance INCREASES with Length The longer the wire…the more resistance

Width Resistance DECREASES with Width The wider the wire the less resistance

Temperature Resistance INCREASES with temperature The hotter the wire the more resistance

Material Some material allows electrons to move more freely

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Conducting wires that carry large currents have large diameters to minimize resistance. This is why stove cord is bigger than the cord

for a kettleCopper Core

Insulating Material

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• Do Questions 1 – 6 on Page 342 of the Textbook

• Worksheet:– http://www.kathimitchell.com/elect2.htm