3.5.1.3 - Resistivity Flashcards
What is the resistivity of a material defined as?
The resistance of 1m length with 1m^2 cross sectional area. It is measured in ohm-metres.
Resistivity =
Resistance x cross sectional area / length
What 3 things determine resistance?
Length, area and resistivity.
How does length affect resistance?
The longer the wire, the more difficult it is to make a current flow.
How does area affect resistance?
The wider the wire, the easier it will be for the electrons to pass along it.
What is the effect of temperature on most materials?
Resistance increases due to the increases kinetic energy of the atoms.
How are thermistors affected by temperature?
They are negative temperature coefficients, so an increase in temperature results in a decrease in resistance.
Why are thermistors affected by temperature like this?
Because thermistors are made from silicon, a semiconductor containing a small number of impurities. The impurities significantly help conduction.
Below what do materials such as mercury become superconductors?
Below a transition temperature.
What resistance do superconductors have?
0
What is good about superconductors?
Without any resistance, none of the electrical energy is turned into heat, so none of it is wasted, this means you can start a current flowing in a circuit using a magnet field, take away the magnet and the current would carry on flowing forever.
What is the down side of superconductors?
Most normal conductors have transition temperatures below 10 kelvin. Getting things that cold is hard and very expensive.
What are the uses of superconductors?
- Power cables that transmit electricity without any loss of power.
- Really strong electromagnets that don’t need a constant power source.
- Electronic circuits that work really fast, because there’s no resistance to slow them down.
To determine the resistivity of a wire, what do you need to know first?
The cross sectional area of the wire. Find this using a micrometer to measure the diameter.
Describe the circuit for the experiment?
The test wire should be clamped to a ruler with the circuit attached to the wire where the ruler reads 0. Attach the flying lead to the wire. Also add an ammeter in series and a voltmeter in parallel.
What is the purpose of the flying lead?
It allows connection to any point along the wire.
What values do you need to record?
Record the length of wire connected in the circuit along with the voltmeter reading and the ammeter reading.
What is the independent variable?
The length of wire - repeat the experiment varying the wire length to get a variety of values for resistance.
What graph do I need to plot?
Resistance against length.
What do I use the graph for?
To find the resistivity. Times the gradient by the cross sectional area to get the resistivity.
Why should you keep the temperature of the wire constant?
As a temperature increase can lead to random errors and invalid results.
How can you keep the wire temperature constant?
By only having small currents flow through the wire.
