Ohms Law Essay, Research Paper

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 can be obtained with the single factor altered: a graph of V

against I can be drawn and an average resistance can be produced in this case,

rendering it a fairer test. Since each test does not take long, the tests can

also be repeated to obtain a more accurate mean. DC supply is used so that the

voltage and current are not fluctuating while a steady reading is needed to be

obtained.

To make it a fair test, only one factor should be altered at a time. When the

material of the wire is being tested, the length and the diameter of the wires

should be the same at 1mm wide and 25cm long. When the length and the diameter

are being tested, the wire used should be made of copper, with the diameter and

length remaining at 1mm and 25cm where possible.

Prediction

* An atom consists of a nucleus and orbiting electrons. These electrons can

create a flow of current, so the more free electrons there are, the more

conducting capability that material has; thus copper is more conductive that

iron. Alloys tend to have less free electrons so they will be less conductive.

The order of resistance is consequentially copper, iron, constantan then

nichrome increasingly.

* 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.

Methods

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

* Make sure that the circuit is properly connected before turning the power

supply on, and do not touch the apparatus, especially the tested, naked wires

until the power is switched off

* The changing of the tested wires should only occur when the power is off

* Do not carry out the experiment in wet areas, as water is a very good

conductor.

* Do not switch on the power pack when there is no resistant wire and do not

turn the power supply up too high because normal laboratory wires may melt

Background knowledge

Using a circuit such as this one on the left, an important general relationship

can be seen. The variable resistor is used to control the current in the

circuit and the voltmeter measures how the potential difference (voltage)

across the resistor varies. Provided that the temperature does not change

significantly, the results give a graph looking like this. This means that the

current is proportional to the p.d. The relationship is called Ohm’s law. Ohm’s

law only applies if the temperature is constant, and does not apply to all

electrical components.

We can write Ohm’s law in symbols:

V ??I

Or

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.