Download Lesson 5 - Resistance and Conductance

Resistance
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A resistor is a component in a circuit that
transforms electrical energy into other types
of energy.
Some resistors like the heating element in a
toaster transform electrical energy into
thermal energy.
In a light bulb, the filament
transforms the electrical
energy into light energy
The electric kettle
A resistor is a component designed to reduce the current.
 A variable resistor has a resistance that can be changed.
 A fixed resistor has a resistance that remains the same.
Many domestic appliances use
resistance to transfer electrical
energy to heat and light energy.
The heating element in this
kettle is a coil of metal that
resists electricity. As current is
passed through the metal,
resistance causes the metal to
get hot and so boils the water.
Resistance
Resistance: The ability of a material to hinder
the flow of electric current.
Symbol: R
SI unit: Ohm (Ω) –or 1 V / 1 A
Circuit diagram:
V
R
I
Resistance lowers the energy level supplied on
the charges, as this energy is transformed
Name the Parts of the circuit
Voltmeter
Resistor
Resistor
Power
Ammeter
Light bulb
Switch
Ohm’s Law
V  IR
V
R
I
V
I
R
Potential
difference
measured in
Volts (V)
Current
intensity (I)
measured in
Amperes (A)
Resistance
measured in
ohm’s (Ω)
Example
What is the resistance of a circuit component if
the current going through the wire is 0.50A and
the electric potential around the component is
3.5V ?
V
R
I
3.5V
R
0.50 A
R  7 .0 
Resistors (don’t write!)
• Resistors are often placed into
a circuit to limit the flow of
electrons.
• Some resistors have a
ceramic coating with colour
coded bands to indicate the
resistance
Determining the resistance using colour codes.
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The 1st and 2nd bands give you the first two digits of the resistance.
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The 3rd band is the exponent on ten by which the first two digits must be
multiplied.
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The 4th band indicates the tolerance of the resistor (+/- %). This tells you
how close the actual resistance will be to the value indicated by the bands.
Number
Tolerance
±1%
±2%
±5%
±10%
Colour
brown
red
gold
silver
Colour
0
black
1
brown
2
red
3
orange
4
yellow
5
green
6
blue
7
violet
8
grey
9
white
What is the Resistance on the following
resistor?
Factors that effect conductivity
Not all metals conduct electricity equally. The
conductivity of wires depends upon four factors:
1.
2.
3.
4.
nature of the wire  copper is an excellent
conductor; nichrome is a poor conductor
wire diameter  thicker wires are more conductive
(up to a point)
wire length  shorter wires are more conductive
wire temperature  colder wires are more
conductive