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Ohms Law.

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Aims and hypotheses
This investigation is designed to look into the resistance of different material in the form of wires and their conducting capability in different shapes. In order to do so, the materials are to be tested for their resistance in the shape of wires, and the hypotheses are such that different thickness and length of the wire and the material that makes up the wire itself will affect the electric conduction capability. Hence, the factors are:

* The thickness of the wires: 1, 2 , 3and 4 mm in diameter
* The length of the wires: 25, 50 , 75and 100 cm long
* The material of the wires: copper, iron, constantan and nichrome
* There will be 16 tests to be carried out

The experiment will require both the readings of voltage and current in order to produce the value of resistance according to the formula:

R = V/I ??
(Resistance is measured in Ohms)

The power supply, however, is varied between 0-12V with 1V interval so that a series of results

...read more.


* Wires with wider diameter have more free electrons because the cross-section surface area is larger in proportion to the length, so the wider the wires are, the less reactive they would be. Resistance is proportional to the cross-section are of the wire given that the length and the material should be the same.
* Longer wires will cause an increase in resistance because the electrons have to travel past more atoms and collisions between the electrons and the atoms are more likely then in shorter wires. Resistance should also be proportional to the length of the wires.


The equipment needed consists of:

* A variable DC power pack
* Ordinary wires
* An ammeter
* A voltmeter
* 2 crocodile clips
* Assorted wires for tests

Then a circuit is set up in the same way as the illustrated diagram below.

* Connect the wire to the circuit by the crocodile clips
* Take the voltage and current readings from the meters
* Increase/decrease the supply from the power pack and take the readings again
* Repeat the experiment with different pieces of wire

Safety precautions

...read more.


We can write Ohm's law in symbols:

V ??I
V = IR

And R is the resistance of the resistor. It can be rearranged so that R is the subject, hence:

R = V/I

The larger the resistance, the greater the gradient will be. Gradient of the graph gives the value of resistance.
Ohm's law does not always apply. A light bulb in place of the resistor in the circuit gives a different pattern for the current and voltage relationship, as shown in the graph. Here the current and voltage are not proportional. The bulb obviously gets hotter and hotter. Since "resistance" is measured by the gradient of the graph, we have here an example where the resistance is increasing.
A heat-dependent resistor or thermistor gives the opposite pattern. Its resistance decreases as the temperature rises

But obviously we are dealing with "normal" resistors in this investigation, so the gradient of the graphs obtained should be the same throughout - in a linear fashion - and the resistance should remain constant as the voltage/current is altered

...read more.

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