4.3 Resistivity Set Practical AQA

Name:

Finding an “Unknown” Resistivity (ISA Prep Skills) 2 - hours

(I think/reply.....)

Having gained a working knowledge of resistivity and resistance in a previous lesson it is now time to apply your knowledge in a practical situation.

The Aim of this task is to...

1. construct an experiment yourself...

2. carry out the experiment accurately....

3. process your results in a meaningful way...

4. predict the type and SWG of a wire by finding out the resistivity....

A physics technician has constructed an experiment for GCSE students. The experiment consists of a piece of wire stretched between two points on a wooden board. The GCSE pupils are finding out the resistance of the wire. However, the technician is a couple of wires short and cannot remember which type of wire she used or the SWG (standard wire gauge)? With your new found knowledge can you find out the resistivity of the wire and tell her what metal it is?

Theory...

R = Resistance in ohms

= Resistivity in ohm metres m

A = Cross sectional area in metres squared m2

l = length in metres m

NB: is the value of the Resistivity at room temperature 20oC (will go up if it gets hotter)

Note all materials for this lesson and hyperlinks can be found on a specific ISA post here... http://www.animatedscience.co.uk/resources-for-physics-isa

Theory resources... http://www.animatedscience.co.uk/4-electric-current-as-unit-1

1 Equipment

You are only allowed..

1x wire mounted on a board

1x multimeter (set to ohms )

1 short wire, 1 long wire

1x digital vernier calliper or micrometer.

1 crocodile clip

Before you start and as you move through this task use the Personal Skills Checker to assess your ISA skill levels in this task … as a face or a tick, cross or – for not sure. Then show it to your teacher and get some help before you move on.

2 Personal Skills Checker......

Skill or Activity

Start

Middle

End

Setup a resistivity practical.

Taking precise readings of the length of a wire.

Confidently use a micrometre screw gauge or vernier calliper to measure diameters.

Work out the area of a wire precisely including recording measurement error.

Understanding the idea of a y = mx + c formulae from Maths GCSE.

Understand how to apply y = mx +c to the resistivity formulae.

Plot a graph and obtain a gradient from a graph.

Using a gradient from a graph to work out a formula such as grad x A = .

Finding a SWG from the diameter of a wire.

Estimation/Calculation of Errors in an AS Experiment.

3 Tabulation/Obtaining the Data

Diameter of wire =

4 Graphing

5 Gradient Calculation and Conclusion

y =

x =

Grad = y/x =

=

SWG =

Wire Type =

6) Overall Comments/Calculation on Errors ± in your value for

7) Mini Exam Revision Question. (Check your skills)

1) A metal wire of length 1.4 m has a uniform cross-sectional area = 7.8 × 10–7 m2.

a) Calculate the resistance, R, of the wire. Resistivity of the metal is = 1.7 × 10–8 m (Basic)

b) The wire is now stretched to twice its original length by a process that keeps its volume constant. If the resistivity of the metal of the wire remains constant, show that the resistance increases to 4R. (Harder)

Video Tutorials...(skill up!)

Using a traditional vernier calliper

http://www.youtube.com/watch?v=4hlNi0jdoeQ

Using a multimeter for resistance, current, voltage

http://www.youtube.com/watch?v=bF3OyQ3HwfU

Resistivity of a wire

http://www.youtube.com/watch?v=Tt-_7nfAJ5U

General Theory of Resistivity – and worked problem

http://www.youtube.com/watch?v=znEALzPQ32I

Extension – moving to A2 A* problem explained including density!

http://www.youtube.com/watch?v=lDmYt5MM_ZA

Extra Help Resources

Take your pick...

Tabulation

Diameter /m x 10-4 ±1 x 10-6 m

4.57

4.58

4.56

4.36

Average

4.52

Diameter of wire = 4.52 x 10-4 m ± 1 x 10-6 m

Length /m ±0.001m

Resistance / ±0.1

1

2

2

Ave

1.000

5.6

5.7

5.5

5.6

0.900

4.2

4.4

4.4

4.3

Extra Maths & Method Help...

Example Graph (Similar to what you might see if it were Gold!)

Example – single point calculation (using d not r in area!)

Example Reference Calculation (Reverse of what you need to do!)

Useful Data Tables

Material

Resistivity at 20°C

Ω·m

µΩ·cm

copper

1.7 × 10-8

1.7

gold

2.2 × 10-8

2.2

aluminium

2.7 × 10-8

2.7

magnesium

4.2 × 10-8

4.2

nickel

6.9 × 10-8

6.9

chromium

13.2 × 10-8

13.2

manganese

160 × 10-8

160

Manganin

44 × 10-8

44

Constantan (Eureka)

49 × 10-8

49

Nichrome

110 × 10-8

110

Extension Wider Reading for “Conduction Models” (extra depth to AQA textbook)

In metals

A metal consists of a lattice of atoms, each with an outer shell of electrons which freely dissociate from their parent atoms and travel through the lattice. This is also known as a positive ionic lattice.

This 'sea' of dissociable electrons allows the metal to conduct electric current. When an electrical potential difference (a voltage) is applied across the metal, the resulting electric field causes electrons to move from one end of the conductor to the other.

Near room temperatures, metals have resistance. The primary cause of this resistance is the thermal motion of ions. This acts to scatter electrons providing “resistance” but we also find that metals with impurities in the lattice also have a higher resistance. In pure metals this source is negligible

The larger the cross-sectional area of the conductor, the more electrons per unit length are available to carry the current. As a result, the resistance is lower in larger cross-section conductors. The number of scattering events encountered by an electron passing through a material is proportional to the length of the conductor. The longer the conductor, therefore, the higher the resistance. Different materials also affect the resistance

http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity

Answers to Mini Problem

Graph to show resistance against length

y = 18.253x

0.10.20.300000000000000040.40.50.600000000000000090.700000000000000070.80.910.180000000000000053.75.57.10000000000000059.2000000000000011111314.514.618

Length of wire in meters m

Resistance in milli ohms

4.3 Resistivity Set Practical AQA

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