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The resistance in a wire.

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The resistance in a wire is affected by a number of factors.  One of these factors is the cross section of a wire.  I will be investigating as to how this affects the resistance in a wire.  Another factor that I will also be investigating is the length of the wire.

Cross section of the wire

A wire conducting electricity is affected by a number of factors.  One of these factors is the Cross section of the wire.  The thickness of the wire affects the resistance of the wire.  The cross section a wire:

How does is the resistance of a wire affected by the cross section of the wire? In the flow of electricity electrons are free to move at random within the metal (wire).  So if the metal consists of ions rather than metals, it is easier to understand how the flow is affected.  When a voltage is applied, the electrons move through the wire.  This however is not a steady motion because ions are constantly colliding with each other, slowing them down.  When they collide they stop, and start again, and this continues.  This is what causes the resistance in a wire – the ions colliding with each other.  By increasing the cross section of the wire, the applied voltage will affect more electrons, effectively decreasing the resistance.

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R = w l/a

                                               Ra = wl

 R a/l = w

 w = R a/l

The resistivety of the wire (w) is measured in Ohms – (Ω).

This information was taken from the book ‘Physics’ by Tom Duncan

Ohm’s Law

Ohms Law is an equation that shows the relationship between Voltage, Current and Resistance in a circuit. This is Ohms law:

  V = I x R

  R = V / I

  I  = V / R


V = Voltage

I  = Current

R = Resistance

I have attempted below to explain how Ohms law works in an electrical circuit.  

The current in a circuit is the flow of electrons.  Below is a picture of a piece of metal:


All matter is made up of atoms and each atom is made up of a nucleus that has a certain number of protons and neutrons with an equal number of electrons in orbit.

For example, the diagram below shows a copper atom.  It has 29 protons and 29 neutrons in its nucleus, and 29 electrons orbiting the nucleus:

The electrons are arranged in orbits called shells. image01.png

In the copper atom (material of the wire), the outer shell (Valence Shell) has only one electron.  The electron on the valence shell (Valence electron) can break away from the atom, when it gains sufficient energy.  It then becomes a free electron.

The energy required for the electron to break away is very easy to provide.  The energy can come from heat light or electricity.  When the electron becomes free it leaves behind a positive charge.

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“If these sources of charges have a current path to flow through then the electrons will be attracted to the positive atoms to fill the holes. You need a complete circuit made of conductive material in order to have current flow and the current will flow from the negative source to the positive source.” - (http://www.technology.niagarac.on.ca/students/cmukendi/what_is_current.html)

Below is a battery shown as a voltage source.  The top half is all positive charges and the bottom half all negative:


This is how the battery can be used as a source to produce current:


Ohms law states that if you have one volt applied to a one ohm resistance the current will be one amp.

Knowing that, it is possible to calculate the voltage, resistance and current in a circuit if you know 2 of the values using the equations V=IxR, R=V/I and I=V/R

This is why Ohms law makes sense and is correct.

From all information I have presented in my introduction, it is now fair to conclude that:

RESISTANCE         =         K/AREA OF CROSS SECTION   (k = constant)

RESISTANCE        =         K x LENGTH OF WIRE

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This student written piece of work is one of many that can be found in our GCSE Electricity and Magnetism section.

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