To investigate the factors affecting current in a wire.

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To Investigate The Factors Affecting Current In A Wire

Introduction

The aim of this investigation is to find out the factors that affect current in a wire.

Electric current is defined as the rate of flow of electric charge.

Variables that may affect the resistance of the wire

Every appliance has a resistance. If the resistance is high, the current will be low. If the resistance is low, the current will be high.

The variables that can affect the resistance in a wire are:

- Cross-sectional area of the wire

- Length of the wire

- Temperature

- The material (resistivity)

- Voltage

Cross-Sectional Area

The thicker the wire is, the lower the resistance, therefore the higher the current (given that all other factors are constant).

When a wire is thick, there are more electrons available to carry the current. Free electrons are available in metal (wire), if the metal has a larger cross-sectional area - more electrons will be available. If the wire is thinner, fewer electrons will be available to carry the current. A thick wire has more free electrons and more space. A thin wire has less free electrons and is more 'squashed' together. This means that the electrons are likely to collide more with the metal ions in the wire. Collisions in a metal wire will result in an increase in resistance because the ions from the metal wire will be getting in the way of the flow of electrons. And if there are more collisions, the flow of electrons will be disrupted more. Therefore there'll be less current.

R ? 1

a

Length of a Wire

As the length of the wire increases, the resistance increases. Length is proportional to resistance. And as the resistance increases, the current decreases. As the length of the wire decreases, the resistance decreases hence the current increases (provided that all other factors are kept constant - especially voltage).

In order to explain why this is, one must consider the positive and negative terminals in a wire. All electrons are negatively charged hence they're all attracted towards the positive terminal. But as the length increases, more electrons will become available, but the electrons that are closer to the positive terminal will form a 'barrier'. This 'barrier' makes it more difficult for the electrons on the negative terminal to flow across to the positive terminal. And also the attraction from the positive terminal decreases if there are lots of electrons rushing towards the positive terminal.

Another reason why a longer wire has a greater resistance is because the electrons have a greater distance to travel.

This factor causes more resistance than the fact that a barrier is formed which blocks the way for electrons to flow more freely.

Resistance ? Length

Temperature

When the temperature increases, the resistance increases and so does the length to a certain extent (because when something is heated it expands a little).

When there is more heat in a wire, the positive ions collide more. Since current is the flow of electrons, when the electrons try to flow passed these ions, they will be at risk to more collisions. Hence the electrons get slowed down.

So a higher temperature in a wire will result in a higher resistance therefore a lower current.

The Material (Resistivity)

Different metals have different resistances per 1 metre of length.

E.g. 32 SWG (Standard Wire Gauge) Copper wire and Nickel Chrome wire of the

same length will have different resistances.

The resistivity of a metal is the resistance between the ends of a specimen 1m long and 1m^2 in cross-section.

So at 20 C:

Copper has a resistivity of 1.78 * 10-8 ?

Nickel Chrome has a resistivity of 110 * 10-8 ?

Silver has a resistivity of 1.66 * 10-8 ?

Voltage

The potential difference or voltage is the energy that is given to each charge as it passes through the power source.

The potential difference across a circuit component measures the amount of energy that is transferred to that component as current flows through it.

When the voltage is increased, the current increases too.

Voltage ? Current

V ? I

In my investigation, I am going to keep every controllable variable constant except for length. Length is a non-linear variable because I don't expect it to be directly proportional to current. I previously considered investigating the cross-sectional area of the wire because this factor is probably the most effective one that has the greatest influence on the current in a wire. But instead I chose to investigate the length because it is easier to do practically and is therefore more accurate and reliable.

However, there is a formula to work out the resistance of a wire and it includes the cross-sectional area, length and resistivity of the wire:

Fair Test

In order to see if the length of the wire really does affect the current in the wire, all other factors must be kept constant. This experiment must be conducted under safe and fair test conditions.

All factors (cross-sectional area of wire; temperature; voltage; and material) will be kept constant except the length of the wire. Safety precautions can also be considered as fair test conditions. For example, I am going to make sure that no foreign objects or water come into contact with the experimental apparatus.

To keep the temperature constant, I will have to make sure that a large amount of heat doesn't build up in the wire (otherwise it will melt or affect the readings). In order to do this, the power pack will be switched off for a minute or so in between taking readings. I must also keep the surrounding room temperature the same or the particles in the wire will move faster (if the temperature is increased) and this will therefore have an effect on the resistance.
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To keep the cross-sectional area of the wire constant, I will note down the SWG of the wire I'm using. And make sure that I'm using the same material.

To keep the voltage constant, I will just make sure the reading off the digital voltmeter is always the same. So if the potential difference across the wire is below the constant value, the rheostat can be adjusted to maintain the p.d across the wire.

The equipment used must be the same when doing repeats etc.

Prediction

Increasing the length will increase the ...

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