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4.3 Resistance
Electricity in wires is a flow of electrons along the wire. As the electrons
move along the wire they collide with the metal atoms. These collision
make the atoms vibrate more…which makes the metal hotter.
Resistance is a measure of how much a material tries to stop
electricity passing through it.
Electrical resistance is the opposition to the flow of electrons.
It is caused by the repeated collisions between charge carriers and
fixed positive ions in a metal conductor
Resistance (R) of a component =
R (ohm)
=
p.d across component
the current through it
V
I
(V)
(I)
(volt)
(amp)
4.3 Resistance
Ohm’s law:
The pd across a metallic conductor is proportional to the current through it,
provided the physical conditions do not change
Q. For the resistor opposite calculate:
a) the resistance at this current
b) the new pd when the current
is 50 micro Amp
R ohm
2mA
12V
a) R = V
I
=
4.3 Resistance
Ohm’s law:
The pd across a metallic conductor is proportional to the current through it,
provided the physical conditions do not change
Q. For the resistor opposite calculate:
a) the resistance at this current
b) the new pd when the current
is 50 micro Amp
R ohm
2mA
12V
a) R = V
I
=
12
2.0 x 10-3
= 6000 Ohm
4.3 Resistance
Ohm’s law:
The pd across a metallic conductor is proportional to the current through it,
provided the physical conditions do not change
Q. For the resistor opposite calculate:
a) the resistance at this current
b) the new pd when the current
is 50 micro Amp
R ohm
2mA
12V
a) R = V
I
=
b) V = I R =
12
2.0 x 10-3
= 6000 Ohm
50x10-6 x 6000
= 0.3 V
http://phet.colorado.edu/simulations/index.php?cat=Physics
http://www.batesville.k12.in.us/physics/PhyNet/e&m/current/ECurrent_Notes.htm
4.3 Resistance
Measurement of resistance:
r
Record the pd across R for
increasing values of current.
( change r to change the
circuit current ()
R
A
V
4.3 Resistance
Measurement of resistance:
r
Record the pd across R for
increasing values of current.
( change r to change the
circuit current ()
R
A
V
The ammeter has a
very low resistance ( 0.2 ohm)
The voltmeter has a
very high resistance ( 20,000 ohm)
Why?
4.3 Resistance
Measurement of resistance:
r
Record the pd across R for
increasing values of current.
R
A
( change r to change the
circuit current ()
V
The ammeter has a
very low resistance ( 0.2 ohm)
The voltmeter has a
very high resistance ( 20,000 ohm)
Pd / V
Gradient =
V
I
Gradient = resistance
Why?
I/A
4.1 Resistance
Resistivity relates to the material rather than a specific component
L
R
L
R
1
A
Metal rod
A = Area of cross section (m2)
4.1 Resistance
Resistivity relates to the material rather than a specific component
L
R
L
R
1
A
R =
Metal rod
L
A
A = Area of cross section (m2)
4.1 Resistance
Resistivity relates to the material rather than a specific component
L
R
L
R
1
A
R =
Metal rod
A = Area of cross section (m2)
L
A
R/
ohm
L /m
4.1 Resistance
Resistivity relates to the material rather than a specific component
L
R
L
R
1
A
R =
Metal rod
A = Area of cross section (m2)
L
A
R/
ohm
Gradient =
A
L /m
4.1 Resistance
Resistivity relates to the material rather than a specific component
L
R
L
R
1
A
R =
Metal rod
A = Area of cross section (m2)
L
A
R/
ohm
= R A
L
Gradient =
A
Units:
L /m
Ohm m2 1 = ohm metre
m
4.1 Resistance
Superconductivity
Tc
This is the property of a material which is at or below a critical temperature
Tc where it has zero resistivity .
Implications:
* Zero resistance
* no pd exists across a superconductor with a current flowing
* the current has no heating effects
temp
4.1 Resistance
Superconductivity
Tc
This is the property of a material which is at or below a critical temperature
Tc where it has zero resistivity .
Implications:
* Zero resistance
* no pd exists across a superconductor with a current flowing
* the current has no heating effects
Applications:
* high power electromagnets with strong magnetic fields
* power cables can transfer energy without wasting energy
temp
Superconductivity
Tc
Properties of a superconductor:
* material losses the effect above the critical temperature Tc.
temp
Superconductivity
Tc
Properties of a superconductor:
* material losses the effect above the critical temperature Tc.
* If Tc is above 77K ( -196 C)
it’s a high temperature superconductor
temp
Superconductivity
Tc
Properties of a superconductor:
* material losses the effect above the critical temperature Tc.
* If Tc is above 77K ( -196 C)
current Tc max = 150 K - 123C
it’s a high temperature superconductor
temp