Resistivity Quiz
Try a few calculations based on Resistivity. For these you just need to use the information on the "Resistivity" and "Resistance in conductors" pages. Hopefully it'll be a breeze. Because you may be using more than one formula for any problem its important to remember is to use the correct formula at the right time.
Before you start, think about these few tips; they will make the problems easier if you follow them carefully.
1. Work out the answers using pencil and paper; if you don't write out the problem you WILL get mixed up half way through and end up with the wrong answer.
2. Of course the answer is not just a number, it will be a certain number of Ohms or metres; don't forget to show the correct unit (e.g. Ω) or your answer is meaninless.
3. Convert all sub units such as mm to metres when you put them into the appropriate formula. If you slip up here you'll get really stupid answers, thousands of times too big or too small.
To help you on the right track why not should download our "Maths Tips" booklet, which shows you how to use your calculator with exponents and engineering notation to deal with those sub-units and get the right answer every time.
Not got a scientific calculator? The "Maths Tips" booklet explains what you need (and what you don't need so you don't spend your money unnecessarily). If you don't want to buy a scientific calculator, you can always pick up a free one on the net. PC users can try Calc98 from www.calculator.org/download.html. Whichever calculator you choose remember that you should read the instructions to become familiar with the working methods you should use as these do vary from calcuator to calculator.
OK so now you have read these instructions, you are ready to start. Here is a way to set out a typical problem on paper so you (with practice) don't get confused.
Firstly write down what we know from the question, and what we don't know: For example we may want to know the resistance of a cable when we know its dimensions and the material it is made from.
R = ?
ρ (of copper) = 1.7 x 10-8 Ωm (1.7 E-8 or 500 EXP-8 when entering it into your calculator, depending on which model you use)
L (Length of cable) = 7m
d (Diameter of cable) = 0.5mm
Note that if we are given the diameter (or the radius) of a cable we would first need to work out its cross sectional area before we can use the information in our formula relating R, to ρ, length and cross sectional area). Check this info from the "Resistivity" page for help in working out the cross sectional area.
Note: If you are using Calc98 for your calculations you need to set the View>Option>Display menu to Engineering (under the "Decimal" choices) and it would be a good idea whilst you are in this menu to select 2 from the Decimals drop down box to set the number of digits after the decimal place. This will round your answer down to two decimal places which is sufficiently accurate for most uses and stops you getting silly answers such as 4.66666666667mm, which would be far too accurate for any practical scale of measurement we would be likely to use.
Resistivity Calculations Practice
(Calculate your answers with pencil, paper and calculator, then check your answers below.)
Previous Page Top of Page Home Page