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**Resistance of a Wire Investigation**

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The resistance of a conductor is a measure of how easy or hard it is for a current to flow through it. A wire will warm up when a current flows through it. An increase in temperature will increase the resistance of the wire. So measurements have to be taken quickly before the wire has a chance to warm up.

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**Resistance ( Ω) = Voltage (V)**

In this investigation you measured the current in a circuit containing a length of wire. You also measured the voltage across the wire when a current was flowing through it. From these measurements you calculated the resistance of the wire. The formula for calculating resistance is Resistance ( Ω) = Voltage (V) Current (A)

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**What was the Independent variable in this investigation (The variable that we deliberately changed)?**

What is your dependent variable?

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**Here are some sample results from a similar experiment**

Length of wire /cm Test 1 / Ω Test 2 / Ω Test 3 / Ω Mean Resistance / Ω 20 3.1 3.4 3.2 40 5.9 6.0 6.2 60 9.3 9.2 9.0 80 12.4 12.2 12.3 100 15.7 15.4 19.2 15.6

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**What was the range of this variable?**

20cm to 100cm Was this a suitable range to choose? Yes Why was this? This spread was big enough for us to see if there was a pattern to the results.

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**Suggest one thing that could affect the accuracy of your results?**

How could you prevent this happening?

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**Controlled Variables:**

In this experiment we had to keep some variables constant. Why do you think we did this? Which variables do we need to keep constant? Controlled Variables:

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**Here are the results again. Look at the result in red**

Length of wire /cm Test 1 / Ω Test 2 / Ω Test 3 / Ω Mean Resistance / Ω 20 3.1 3.4 3.2 40 5.9 6.0 6.2 60 9.3 9.2 9.0 80 12.4 12.2 12.3 100 15.7 15.4 19.2 15.6

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**What is wrong with the result in red?**

What should you do if you get an anomalous result in an investigation?

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**Here is a graph of results from the investigation.**

What is the link between the Independent and Dependent variable?.

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How do you know this?.

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**Why did we use a line graph to display our results?**

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**Here are the results again.**

Length of wire /cm Test 1 / Ω Test 2 / Ω Test 3 / Ω Mean Resistance / Ω 20 3.1 3.4 3.2 40 5.9 6.0 6.2 60 9.3 9.2 9.0 80 12.4 12.2 12.3 100 15.7 15.4 19.2 15.6

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**In this graph we have plotted the results from the individual tests instead of the mean result.**

Anomalous result The results for each length of wire show a spread or scatter Can you spot the anomalous result on this sort of graph?

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**What can cause errors in our investigations?**

1. Systematic errors. These are caused because either the equipment has been set up incorrectly or is not being used properly. This means the error will be the same each time 2. Zero Errors. These are caused because a piece of measuring equipment is giving a reading that is too high or too low because it has not been set at zero properly. This means the error will be the same each time 3. Random errors. These are caused by things not under our control such as the temperature of the room changing while we carry out the experiment. These errors can change each time and can make our results slightly different each time.

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To draw a line of best fit on a graph like this you try to take the line through the centre of each scatter of results. You can ignore anomalous results.

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Instead of length of wire imagine you wanted to find out if the thickness of a wire affects the resistance. What would be the Independent variable in this investigation? What would need to stay the same about the wires you use?

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**What else would you need to keep the same in the investigation?**

Apart from the thickness of the wire what other measurements would you have to take and what equipment would you use to make them?

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**How many times would you test each thickness of wire and why?**

How many different thicknesses of wire should you use and why?

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Summary of last weeks work on Variables Predict that there is a relationship between two variables An independent variable is one that is changed or selected.

Summary of last weeks work on Variables Predict that there is a relationship between two variables An independent variable is one that is changed or selected.

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