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Ohm's Law Investigation

Extracts from this document...


Adam Burclaff

Ohm’s Law

Aim: How does the length of a piece of wire (which current is passing through) affect on its resistance?

Introduction: An electric current (D.C. – direct current) is a flow of charge from the positive terminal of a cell to the negative terminal. The charge is carried by conduction electrons, which are not fixed to a particular ion – they are called free electrons. When p.d. (potential difference) is applied across the ends of a wire (by a battery etc.) these electrons will move towards the positive terminal. Some of the electrons in metals are not bound to particular atoms, they are shared between them. These ‘delocalised’ electrons are free to move around the metal and are what actually carries the current. As the delocalised electrons are charged and can move, they can carry a current. The most important property which controls how well a certain metal conducts electricity is the number of delocalised electrons there are per atom. The more of these electrons there are in the metal the better it conducts electricity.

Basically, a copper wire consists of millions and millions of copper atoms. Each copper atom has one or two electrons which are not tight to the atom but are loosely held – and seeing as electrons have negative charges, once an atom loses an electron it becomes positively charged and is now called an ion. These ions vibrate as atoms always do, with these “free” electrons moving randomly from one ion to the next – this is true of all metals.

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First I decided upon the type of wire I was going to use (copper), I made sure it was the same thickness each time I took readings because as aforementioned, if the cross sectional area is not kept constant, it will definitely affect the resistance and thus make my results inaccurate. Then I measured it to 100cm by laying it across a metre ruler, because this way I can be accurate to the millimetre.

I then hooked the 2 cells up to the copper wire stretched across the metre ruler using my wires and crocodile clips. I connected the two terminals on the metre ruler 40cm apart. For this experiment I will take readings for p.d (potential difference) and the resistance using an ammeter and a voltmeter at distances ranging from 40cm to 100cm – I will take measurements in 5cm intervals (i.e. take measurements from 40cm, 45 cm and so on so forth). I made sure that the voltmeter and ammeter were set up in PARALLEL not series as this would damage them. Below is a diagram of my apparatus and how I set it up:


And the (simple) schematic of the circuit:


In this experiment I will keep all things constant (apart from the length of wire). I will keep do all my measurements in one day in a short space of time in the same room away from the windows (out of the sun) so the temperature does not change noticeably while I am carrying out the experiment, as this would affect my results and make them inaccurate.

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In my experiment, I could also have investigated a number of other things, such as the effect of cross sectional area or temperature on the resistance. If I had looked at the effect that the cross sectional area had on resistance I would probably discover that as the wire doubled in cross sectional area the resistance would halve. This would be due to there being twice as many electrons - the current would travel a lot quicker and thus decrease the resistance. If I looked at how temperature affected resistance I would probably find that as the temperature of the wire increases, the particles within begin to vibrate much more because they have some extra energy, therefore it is much harder for the electrons to move through and thus the resistance will rise. So instead of just investigating how length affected the resistance of a piece of wire I could also have investigated the affect of temperature or cross sectional area on the piece of wire.

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