Factors affecting the resistance of a metal wire.

Method

1. Set up the apparatus as shown:

2. Set the power pack voltage setting at 2 Volts and the variable resistor at its maximum resistance.
3. Connect 30cm of the wire in the gap XY. To reduce kinking of the wire stretch it out, then tape it to a half metre rule.
4. Close the switch and adjust the variable resistor and/or power pack until the current through the wire is 0.4 A.
5. Open the switch.
6. Close the switch and quickly record the readings on both the ammeter and voltmeter.
7. With the switch closed, adjust the variable resistor and/or power pack until the current through it reads 0.8 A.
8. Repeat steps 5. To 8. for this new current value.
9. Replace the 30cm wire with 40cm wire
10. Repeat steps 4. To 8.
11. Replace the 40cm wire with 50cm wire
12. Repeat steps 4. To 8.
13. Replace the 50cm wire with 60cm wire
14. Repeat steps 4. To 8.
15. Replace the 60cm wire with 70cm wire
16. Repeat steps 4. To 8.
17. Replace the 70cm wire with 80cm wire
18. Repeat steps 4. To 8.
19. Replace the 80cm wire with 90cm wire
20. Repeat steps 4. To 8.
21. Replace the 90cm wire with 100cm/ 1m wire
22. Repeat steps 4. To 8.

Dealing with Results

The resistance of the wire may be calculated knowing the current  I  through the wire and the p.d.  V  across it using the formula:

In symbols

                                V

                                       R =    I

Where                                                                                                          

V is p.d (potential difference or     Voltage) across the conductor (in volts)

 I is the current through the conductor (in amperes)

R the resistance of the conductor (in ohms)

Finding the average resistance

The average resistance can be calculated by averaging the 2 resistance values  from each sample.

Presenting results

The results will be tabulated below:

Strategy for Dealing with Results

To interpret my results I will be using a bar chart to show variation of average resistance with wire length. A graph of average resistance against material type will   be plotted.

Factors affecting the resistance of a metal wire

Background

In a metal some electrons are not tightly bound to their atoms and are said to be “free electrons.” At room temperature these free electrons are able to move randomly between the metals fixed positive ions and atoms.  These fixed positive ions and atoms are arranged in a regular lattice and vibrate about their fixed rest positions.

 When a cell or battery is connected across the ends of a metal wire a p.d. is set up across its ends. This applied p.d. sets up an electrical force which accelerates the free electrons and so they gain speed and kinetic energy. Therefore the free electrons are able to move in one direction thus creating what is known as an electrical current.

 As they move through the metal wire they collide with the vibrating lattice atoms or ions within the material. These collisions cause the moving electrons to slow down or decelerate, i.e. the current is reduced. This explains why the metal has resistance.

As the resistance of a material increases so too must the force required to drive the same amount of current. Resistance, in ohms(R) is equal to the potential difference, in volts (V) divided by the current, in amperes (I) - Ohm’s law.

As the length of the wire is increased the number of collisions that the current carrying charged particles makes with fixed particles also increases and therefore the resistance of the wire becomes higher. An everyday example would be the water system in your house.

                                                                                                                                                                                                                                                                                                                                                                                                                                Physics Coursework