Resistivity Surveying OhmMapper Capacitively Coupled Resistivity System (CCR)
Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material;...
-
Upload
brenda-casey -
Category
Documents
-
view
223 -
download
0
Transcript of Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material;...
![Page 1: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/1.jpg)
Resistivity
![Page 2: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/2.jpg)
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 temperature
![Page 3: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/3.jpg)
Resistivity or Resistance
• They’re different things. Resistivity is a property of a material so copper will have a different resistivity to iron for example
![Page 4: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/4.jpg)
definition
• The resistivity ρ of a wire of length l, resistance R and area of cross-section A is given by
• Ρ=RA l
![Page 5: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/5.jpg)
How do we know?
• Resistance is proportional to length
• Resistance is inversely proportional to cross-sectional area
![Page 6: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/6.jpg)
Can you work out the units
![Page 7: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/7.jpg)
Units
• Ωm
![Page 8: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/8.jpg)
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 energy
![Page 9: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/9.jpg)
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 energy
![Page 10: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/10.jpg)
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 Kelvin
![Page 11: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/11.jpg)
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.
![Page 12: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/12.jpg)
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.
![Page 13: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/13.jpg)
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 9000Ω at 0⁰C and of 240 Ω at 100 ⁰C
![Page 14: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/14.jpg)
• Candidates should be able to:• (a) describe power as the rate of energy transfer;• (b) select and use power equations P = VI,• P=I 2R 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.
![Page 15: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/15.jpg)
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 so
![Page 16: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/16.jpg)
P=IV, V=IR
• So P = I (IV) = I2R
• Also I= V/R
• So P = (V/R) V = V2/R
![Page 17: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/17.jpg)
• These are all work outable in an exam but why waste the time. LEARN THEM
![Page 18: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/18.jpg)
We can also work out energy
• By multiplying power by time• So• W = Vit• W = I2Rt• W=tV2/R
![Page 19: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/19.jpg)
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 high
![Page 20: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/20.jpg)
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=IV
![Page 21: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/21.jpg)
Circuit breakers
• Electromagnetic switches which are triggered when the system is overloaded
• Cables deliver 60A at 230V. So what is the power?
![Page 22: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/22.jpg)
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 hour
![Page 23: Resistivity. Learning objectives Candidates should be able to: (a) define resistivity of a material; (b) select and use the equation (c) describe how.](https://reader035.fdocuments.net/reader035/viewer/2022062516/56649da75503460f94a93e85/html5/thumbnails/23.jpg)