Resistance Investigation

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Aim:

Resistance is a measurement describing the difficulty of electron flow through a conductor. The OHM (W) is its measurement.

In this investigation, I am going to be finding out and investigating one of the four factors, which affects resistance in a wire: Length

Prior Knowledge/Research:

There are many variables that will be relevant to this investigation. Although I am only going to be investigation one of these factors in this experiment, all of the others have a certain degree of relevance making them part of the investigation automatically. These factors are:

· Length

· Thickness

· Temperature

· Material

.Temperature: If the wire is heated up the atoms in the wire will start to vibrate because of their increase in energy. This causes more collisions between the electrons and the atoms as the atoms are moving into the path of the electrons. This increase in collisions means that there will be an increase in resistance.

2.Material: The type of material will affect the amount of free electrons, which are able to flow through the wire. The number of electrons depends on the amount of electrons in the outer energy shell of the atoms, so if there are more or larger atoms then there must be more electrons available. If the material has a high number of atoms there will be high number of electrons causing a lower resistance because of the increase in the number of electrons. Also if the atoms in the material are closely packed then the electrons will have more frequent collisions and the resistance will increase.

Materials that are good conductors, such as copper, allow a large amount of electron flow and so they have very little electrical resistance. Poor conductors, like nichrome, have very high resistance and as such do allow such a current flow even with the same voltage across it.

3.Wire length: If the length of the wire is increased then the resistance will also increase as the electrons will have a longer distance to travel and so more collisions will occur. Due to this the length increase should be proportional to the resistance increase.

4.Wire width: If the wires width is increased the resistance will decrease. This is because of the increase in the space for the electrons to travel through rather than increasing the size of the atoms themselves. Due to this increased space between the atoms there should be less collisions, as the flow of electrons is not interfered by the atoms.

I know that resistance is affected by length and cross sectional area. If the length is increase, so is the number of atoms that collide with free electrons and thus increase resistance. If the cross sectional area is increased so is the space between the atoms rather than the atoms themselves. This means that there are fewer collisions between the electrons and atoms and greater flow of electrons thus decreasing resistance. Therefore the resistance of any material is inversely proportional to its cross sectional area (A):

R=1/A

However, when talking of length and cross sectional area, it is more common to define the resistance of a material instead of a certain aspect. This is known as the Resistivity; P

Resistivity =resistance x area

Length

P = R X A

L

We measure resistivity in the ohm metre (Wm)

The Ohm

In and around 1825, German scientist George Ohm discovered and made a law on resistance that was: The current flowing through a metal wire is proportional to the potential difference (p.d) across it providing that the temperature remains constant. Thus the unit of resistance is now referred to as the Ohm (W).

Resistance (R) = p.d across the wire (v)

Current through the wire (I)

R=V/I V=p.d in volts

I=Current in amps

R=Resistance in a unit ohm

The formula is commonly rearranged though and can sometimes look like this when a different aspect is trying to be calculated:

I=V/R V=IxR

Investigating length on resistance:

Metals conduct electricity because the atoms in them do not hold on to their electrons very well, and so creating free electrons, carrying a negative charge to jump along the line of atoms in a wire. Resistance is caused when these electrons flowing towards the positive terminal have to 'jump' atoms. So if we double the length of a wire, the number of atoms in the wire doubles, so the number of jumps double, so twice the amount of energy is required: There are twice as many jumps if the wire is twice as long. A diagram in my prediction explains these jumps and the collisions between the electrons and atoms.
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Prediction:

I predict that if the length increases then the resistance will also increase in proportion to the length. I think this because the longer the wire the more atoms are present and so the more likely the electrons are going to collide with the atoms. So if the length is doubled the resistance should also double. This is because if the length is doubled the number of atoms will also double resulting in twice the number of collisions slowing the electrons down and increasing the resistance. My graph should show that the length is proportional to ...

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