Electricity Tutorial 4 - Resistivity

 Contents

Resistivity

The resistance of a wire depends on three factors:

• the length; double the length, the resistance doubles.

• the area; double the area, the resistance halves.

• the material that the wire is made of.

Resistivity is a property of the material.  It is defined as the resistance of a wire of the material of unit area and unit length.

The formula for resistivity is: In physics code we write this as: We can rewrite this to give:  There are three bear traps The unit for resistivity is ohm metre (Wm), NOT ohms per metre.  Notice too that the physics code r (rho, a greek letter 'r') is the same as that for density.  Resistivity has NOTHING to do with density. The area is in square metres.  Real wires have areas in square millimetres; 1 mm2 = 1 x 10-6 m2

 Constantan has a resistivity of 47 ´ 10-8 W m.  How much of this wire is needed to make a 10 W resistor, if the diameter is 0.50 mm?  Give your answer to an appropriate number of significant figures.

Measuring Resistivity

This is a required practical.  You will have a sample of a resistance wire (usually constantan or nichrome).  You will need to measure the diameter of the wire with a micrometer screw gauge.  This should be done in three separate places to reduce the uncertainty.  You work out the area of the wire using: Then you will measure the resistance of lengths of the resistance wire, using a circuit like this: You measure lengths of 0.10 m, 0.20 m, etc.  You measure the voltage and current.  You should do repeat readings, and take averages.  Then you need to process the data to get the resistance.   (You know how to do this, don't you?)  Then the data are plotted on a graph like this: The gradient is measured.  To work out the resistivity, you need to multiply the gradient by the area of the wire.  This can be compared with the data book value of the resistivity.

The reciprocal or inverse of resistivity is conductivity.  It has the physics code s, (“sigma”, a Greek letter ‘s’), and units Siemens per metre (S m-1). Conductivity is given by the relationship: A super-conductor is a material that has zero resistance.  A current flows when there is no potential difference.  The piece of metal floating above the magnet shows that there must be a current flowing. Picture from Wikimedia Commons.

Authors: Julien Bobroff and Frederic Bouquet

For all metals the resistivity (hence resistance) decreases as they get colder.

For some metals like copper and silver, there is still a tiny bit of resistance left at very low temperatures.

Very low temperatures have to be maintained, which is expensive.  Room temperature superconductivity has not been seen.

Super-conductivity is seen in:

• Aluminium

• Tin;

• Some alloys;

• Some heavily-doped semi-conductors.

All superconductors have a critical temperature above which the phenomenon stops.  The graph below shows the idea: Above the boiling point of liquid nitrogen, 77 K (-196 oC), superconductivity can be observed in a few materials.  These are called high temperature superconductors.

Very large magnets such as those found in the large hadron collider have coils made of superconducting materials.  It is believed that the superconductivity will last 100 000 years, as long as the coils don’t go above their critical temperature.

The mechanism for super-conduction is complex, and cannot be explained in terms of electrons colliding with ions.  The Meisner Effect and flux trapping are not required on the syllabus.

The mechanism for super-conduction is complex, and cannot be explained in terms of electrons colliding with ions.

 Question 2 Explain what happens when a super-conducting metal reaches its critical temperature. Super-conduction is used in very powerful magnets used is MRI Scanners and machines used in high energy particle physics (e.g. cyclotrons, and accelerators).

It's quite simple - there is no conduction in a perfect vacuum.  There are no charge carriers to carry a current.   It is a perfect insulator.  High voltage switching is often carried out in a vacuum to prevent arcing between the poles of the switch.  In real life, there are no such thing as a perfect vacuum (even outer space has about 1 atom per cubic metre).  In the case of the switchgear, there may be a small spark as the poles of the switch open.  This is because the vacuum will be far from perfect.  It's better described as a low pressure gas, and there are plenty of molecules.

Ionic Conduction (Extension)

Electricity moves due to the movement of charge carriers.  If we think about an ionic solution, the positive ions are attracted to the negative terminal (the cathode), while the negative ion are attracted to the positive terminal (the anode). An inactive or inert electrode is made of an un-reactive metal, for example, platinum.  It just transfers electrons.

An active electrode participates in any chemical reaction.  For example, a copper anode (made of impure copper) in a copper sulphate solution takes up electrons from the solution.  The copper metal loses its electrons to become copper ions. At the cathode, which attracts the positive copper ions, the copper ions gain electrons to become (pure) copper metal.

The voltage current graph for conduction in an ionic solution looks like this: There is a range of negative to positive voltages between which no current flows.  This is because a certain amount of work has to be done to separate the positive ion from the negative ion.  Since voltage is work done per coulomb, the voltage represents the work that needs to be done.

A similar situation is found in gases at low pressure.  At atmospheric pressure, gases are poor conductors (good insulators).  However if a high enough voltage is applied across a gas at low pressure, the gas becomes ionised, and will conduct electricity.  You will see the gas glow.  If the voltage is high enough, all the electrons are removed from the nucleus.  The gas is a plasma.

Metallic Conduction (Extension)

In a metallic conductor (wire), the simplest model of conduction is to consider the metal as a lattice of metal ions in a sea of free electrons.  The electrons move about randomly. When a voltage is applied across the ends of the wire, the electrons  continue to move randomly, but there is an overall drift to the positive end of the wire.  So you will (rightly) think that electrons go from negative to positive.  The protons don't move.  So this idea is opposite to what you have been told.  The explanation is that the earliest physicists got it wrong.  They didn't know about electrons in the Eighteenth Century.  So instead of rewriting all the rules of electricity, people talked about conventional current going from positive to negative.  All currents are regarded as conventional.

The voltage-current graph for a metallic conductor shows direct proportionality (as long as the temperature and other physical conditions remain the same). Resistivity is the resistance of a wire of 1 metre length and of 1 m2 cross sectional area.  According to this model, resistance arises due to the vibration of the metal ions, and the probability of the ions colliding with the free electrons.  Resistance is a representation of the probability of collisions. Such collisions involve a change of energy from kinetic energy to internal energy.  The increase in internal energy makes the temperature in the ions rise, hence increases the probability of a collision.  Therefore the resistance increases as the wire gets hotter.

How fast do electrons move?

When a voltage is applied across the ends of the wire, the electrons  continue to move randomly, but there is an overall drift to the positive end of the wire.  So you will (rightly) think that electrons go from negative to positive.  The protons don't move.  So this idea is opposite to what you have been told.  The explanation is that the earliest physicists got it wrong.  They didn't know about electrons in the Eighteenth Century.  So instead of rewriting all the rules of electricity, people talked about conventional current going from positive to negative.

We can write an equation for the conduction of a current in a wire.  The current depends on:

• The speed of the charge carriers (v (m s-1));

• The area of the wire (A (m2));

• The charge on the electrons (e = 1.6 × 10-19 C);

• The number of charge carriers per unit volume (n (m-3)).

The symbols in italics are the physics codes for the various quantities, and the units are given as well.

The formula is:

I = nAve

The number of charge carriers per unit volume is probably the hardest quantity to get your head round.  It means the number of electrons in a cubic metre of the material.  For example, copper has 8 × 1028electrons in each cubic metre of material.  You can look up the number of electrons per cubic metre in a data book.

Derivation

Consider a piece of metal that is l m long and has an area of A m2.  It is made of a metal that has N free electrons m-3.  Let's suppose that a current of I A is flowing.  Each electron has a charge of e C. We know that current is the flow of charge every second. The total charge is the number of electrons multiplied by the volume of the metal and the charge on each electron: So we can substitute for Q: The term l/t is a distance over time which is speed, v m s-1.  So we write:

I = nAve

 Worked Example A wire is carrying a current of 200 A. Its cross sectional area is 7.85 × 10-5 m2. If copper has 8.0 × 1028 electrons per cubic metre, what is the speed of the electron drift?  Give your answer to an appropriate number of significant figures. Answer Rearrange the equation: v = 200 A ÷ (8.0 × 1028 m-3 × 7.85 × 10-5 m2 × 1.6 × 10-19 C) = 2.0 × 10-4 m s-1 The answer is given to 2 significant figures as the data are given to two significant figures.

The result from this calculation shows that the speed of electron drift is slow, 1 mm every 5 seconds. The propagation of the message "that the current is flowing" is very fast, not far off the speed of light. But the drift speed of electrons is not at all fast. We can show this by putting a small crystal of potassium permanganate (a dark purple ionic compound) onto a filter paper which has been soaked in sodium chloride solution. When a current flows, the purple permanganate ions move towards the positive terminal, but it takes some time.

In questions on this, you may be given a diameter of a wire. To work out the area, you must use the formula: You must convert millimetres to metres when using the equation.

Now try this example:

 A copper wire of diameter 1.4 mm connects to the tungsten filament of a light bulb of which the diameter is 0.020 mm.  The current in both materials is 0.52 A. Find the speed of an electron in each of the two materials. Copper has 8 × 1028 electrons per cubic metre.  Tungsten has 3.4 × 1028 m-3.

The tungsten filament has a higher resistance than the copper wire. For the same current to pass through, the electrons have to go faster, because:

• The number of charge carriers per cubic metre is smaller;

• The area is smaller.

This is opposite to the generally perceived idea that electrons are slowed down by high resistances.  There is a greater chance of a collision, so the wire will hotter.

Models of semiconduction are to be found in Tutorial 6.