# Measuring The Resistivity Of A Pencil Lead.

PHYSICS COURSEWORK 16TH JANUARY 2003

Measuring The Resistivity Of A Pencil Lead

Hypothesis

The resistance of two objects is not always the same due to "resistivity". The resistance of a uniform conductor depends on :

* Its length (L)

* Its cross-sectional area (A)

* The material of which it is made

Therefore, the relationship between these three variables and resistance is as follows:

Where : - R = Resistance

P = Resistivity

L = Length

A = Cross-sectional Area

Resistivity is defined as a materials opposition to the flow of electric current of a cubic metre of a material.

To measure the resistivity of pencil lead I will need the following apparatus.......

* 1 x Power Pack (Ranging from 2V - 12V)

* 1 x Voltmeter (Reading up to 20V, giving readings to 0.00 with a ?3% error reading)

* 1 x Rheostat

* 1 x Micrometer (with an error reading of ? 0.005mm)

* 1 x Metre rule (with mm graduations, with an error reading of ? 0.5mm)

* 2 x Crocodile Clips

* 2 x Identical HB Pencils

* 6 x Electrical Wires

The above apparatus will be set-up as below :

Method

Once the apparatus has been set-up as above I will first carry out some preliminary experimentations to help me carry out my main investigation. From the preliminary experiments I aim to find the following :

* What settings the power pack should be set at i.e. 2V, 3V, 4V, 6V......etc

* What the current should be throughout the circuit

* What lengths I should take readings from and at what intervals

Preliminary Experiment

I set up the apparatus as shown above and started by setting the power pack to 4V and moving the rheostat until the ammeter read 0.1amps. These were my first settings for my preliminary work.

I sharpened the pencil at both ends so that the crocodile clips would be in full contact with the pencil lead and not the wood, as this would affect my results by increasing the resistance in the circuit. I carried out my test five times at one-minute intervals at the two extremes of my pencil length so I could see if the resistance would fluctuating too much. In my preliminary work I was also looking to see if the pencil would heat up, I would be able to tell if the pencil was heating up by a fluctuating resistance. If the resistance did fluctuate I would need to change the current, as the rise in temperature of the pencil would affect resistance and thus my results. My results from these settings are as follows :

Pencil Length

Voltage

Current

Resistance

71mm

.03V

0.1A

7.923?

.03V

0.1A

7.923?

.03V

0.1A

7.923?

.03V

0.1A

7.923?

.03V

0.1A

7.923?

50mm

0.39V

0.1A

3?

0.39V

0.1A

3?

0.39V

0.1A

3?

0.39V

0.1A

3?

0.39V

0.1A

3?

As you can see from my preliminary results my resistance did not fluctuate at a current of 0.1A at each extremity of the pencils ...