Learning objectives Candidates should be able to a define resistivity of a material b select and use the equation c describe how the resistivities of metals and semiconductors are affected by temperature ID: 388511
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Slide1
ResistivitySlide2
Learning objectives
Candidates should be able to:
(a) define
resistivity
of a material;
(b) select and use the equation
(
c) describe how the
resistivities
of metals and
semiconductors are affected by temperature;
(d) describe how the resistance of a pure metal
wire and of a negative temperature coefficient
(NTC) thermistor is affected by temperatureSlide3
Resistivity or Resistance
They’re different things. Resistivity is a property of a material so copper will have a different resistivity to iron for exampleSlide4
definition
The resistivity
ρ
of a wire of length l, resistance R and area of cross-section A is given by
Ρ
=
RA
lSlide5
How do we know?
Resistance is proportional to length
Resistance is inversely proportional to cross-sectional areaSlide6
Can you work out the unitsSlide7
Units
Ω
mSlide8
The effect of temperature on resistivity
If we heat up a metal the internal energy of the atoms also increases.
This could be down to either potential energy or kinetic energySlide9
Potential or kinetic
When you heat up a metal it will expand (before it eventually melts) but only a comparatively small amount.
Therefore the atoms have gained very little extra space so it isn’t a gain in potential energy.
So it must be that heating a metal causes an increase in kinetic energySlide10
Kinetic energy
As we heat a metal the atoms within it start to vibrate more.
Conduction electrons moving through this are more likely to bang into one of these moving atoms.
This means that the increase in temperature increases the resistance.
In fact the resistance is directly proportional to the temperature in KelvinSlide11
How does this affect resistivity
Well expansion will slightly increase both length and cross-sectional area.
This means that with the minimal effect of temperature the resistivity must be also increasing (proportionally) with temperature.Slide12
Semiconductors - Thermistors
These are made from silicon to which a small impurity of other atoms.
This means that when temperature is increased it has a much greater affect on the resistance.Slide13
NTC
These impurity atoms help with conduction
The resistance actually reduces as temperature increases.
It is therefore said to have a negative temperature coefficient.
(This is because the increase in temperature releases more conduction electrons to transmit the electricity).
An NTC can have a resistance of 9000
Ω
at 0⁰C and of 240
Ω
at 100 ⁰CSlide14
Candidates should be able to:
(a) describe power as the rate of energy transfer;
(b) select and use power equations
P
=
VI
,
P
=I
2
R
and
V
2
=
P
R
(
c) explain how a fuse works as a safety device
(
d) determine the correct fuse for an
electrical device
;
(e) select and use the equation
W
=
IVt
;
(f) define the kilowatt-hour (kW h) as a unit
of energy
;
(g) calculate energy in kW h and the cost of
this energy
when
solving
problems.Slide15
Power
We have already seen that V=IR
And P=IV
Provided that we are applying this to one part of a circuit we can do some combining soSlide16
P=IV, V=IR
So P = I (IV) = I
2
R
Also I= V/R
So P = (V/R) V = V
2
/RSlide17
These are all work
outable
in an exam but why waste the time. LEARN THEMSlide18
We can also work out energy
By multiplying power by time
So
W =
Vit
W = I
2
Rt
W=tV
2
/RSlide19
Fuses
Made from thin copper wire.
Low resistivity
Because the wire is thin it’s resistance causes the wire to melt if the current passing through it is too high
This breaks the circuit if the current is too highSlide20
From GCSE
The fuse needs to be one which is rated just over the normal maximum current used by the device.
This maximum normal current is calculated by P=IVSlide21
Circuit breakers
Electromagnetic switches which are triggered when the system is overloaded
Cables deliver 60A at 230V. So what is the power?Slide22
The kilowatt hour
If we worked out how many joules of energy was used in a home the number would be massive.
A 60W bulb uses 60J every second.
Instead we use a bigger value of power (the kilowatt) and time (the hour)
Therefore Energy = Power x time
So kilowatt x hour = the kilowatt hourSlide23