Transcript Resisting the movement of Charge

AND MR. SEREDA PRESENT….
Resisting
the
movement
of Charge
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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
Light
converted into ________________
and
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/gia
mbattista/circuits/circuits.html
• http://www.riedon.com/riedon_calculator.ht
m
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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.
Measuring Resistance
What is the Resistance?
R = V / I
3 Amps
V = 6
I = 3
R = 6 / 3 = 2 Ω
Potential
Difference
=6
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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 kettle
Copper
Core
Insulating
Material
• Do Questions 1 – 6 on Page 342 of the
Textbook
• Worksheet:
– http://www.kathimitchell.com/elect2.htm