Magnetism and Electricity

Diagram Preliminary results. Voltage Length of wire Current Resistance? Preliminary Conclusion. From these results it is evident that two 1. 5 Volt batteries should be used because they produce the desired range of current. However, when the length of wire is 10cm the current is too high because the temperature of the wire increased instead of staying constant. Therefore the range of the lengths in the investigation is 20cm to 100cm. These results conclude that to produce the desired range of current values the voltage of two 1.

5V batteries should be used on the lengths of wire 20cm - 100cm. From the preliminary work it has also come to the attention that the current should be on at a minimum, i. e. short periods of time, to keep the temperature of the wire constant. Prediction I predict that as the length of the wire increases the resistance will increase. I also predict that the length of the wire will be directly proportional to the resistance: R ? L Scientific reasoning In the introduction it has already been stated that as the length of a resister is increased the resistance increases.

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This is because the ions in the resistor are the particles that oppose or resist the flowing of the electrons because the outer-shell of the ions are negative electrons they repel other particles of the same charge i. e. the negative valence electrons. Thus if the length of the resistor increases then the number of ions increase. The only channels for the valence electrons to flow though are the gaps between the ions.

The shorter the resistor the less times the electrons is knocked off course, which lowers the resistance because the electrons pass through the resistor quicker.

The longer the resistor the more times the electrons are likely to be knocked off course which increases the time the electrons flow through the resistor and thus increases the resistance. We can also put into context that if you double the amount of ions, by doubling the length of the wire, then you double the resistance because the number of obstacles (ions) is doubled. If you go to the next extreme, if you triple the length of wire then the resistance is tripled. From this theory we can derive: Resistance is directly proportional to Length. R ? L Apparatus > 1. 5V Batteries x 2 > Electronic Ammeter x 2 > Electronic Voltmeter x 2

> 1m ruler x 1 > 1m Constantan wire (0. 02 diameter) > Sellotape > Electrical leads x 6 > Crocodile clips x 2 Diagram Circuit Diagram Method 1. Set-up apparatus as shown above in the diagrams. There must be a gap in the circuit so it can be turned on and off. 2. The Constantan wire show be sellotaped taut again the ruler and that the wire spans the whole length of the ruler. 3. Connect one crocodile clip on the wire at the 0cm mark on the ruler. 4. Connect the second crocodile clip at wire on the 100cm mark. 5. Connect the wires to the 2 x batteries so that a full circuit i. e. a current flow is flowing. 6.

Record the current using the ammeter. 7. Record the potential difference across the wire using the voltmeter. 8. Disconnect the wires from the batteries as quickly as possible so that the circuit is broken. 9. Then repeat points 4-8 but this time change the length of the wire by moving the second crocodile clip (at point 4) to 90cm and then decrease the length of the wire until the length of the wire is 20cmm in 10cm intervals. 10. Repeat point 4 -9 once. 11. The table of results should then have nine lengths of wire with an average voltage and current for each length. 12. Calculate the resistance by using Ohm's Law.

(The figure used in calculating the resistance are the average) Ohm's Law states: Ohm's law states that: "the current flowing through a metal wire is directly proportional to the potential difference across it, providing the temperature remains constant". We can derive the equation: V = IR. This equation can then be re-arranged to find the resistance of the wire. Rearranging the equation we get: R = V/I. Fair Testing In order to make this experiment a fair test the only variable that will be changed is the length of the wire. This means that the other variables that can change the resistance of a wire must be kept constant.

These other variable s are: > Cross-sectional area of the wire (0. 02mm) > Material (Constantan 36 SWG) > Temperature (The voltage does not need to be kept constant but to be able to calculate the resistance of the wire it must be measured at each length because Ohm's Law is true providing the temperature is constant. ) Safety > Ensure that all connections are secure before the power is switched on. > Do not touch the wire being used as a resistor until after the power has been turned off. It is recommendable not to touch the wire for a short while after the power has been switched off because the wire may be hot.

> Ensure that the work area is dry because water is a very good conductor and could cause electrocution. > Take care when using scissors to cut the wire/sellotape. > All accident must be reported to the teacher immediately. Potential Difference (Volts) Current (Amps) Resistance (? ) Length of Wire (cm) 1 2 Average 1 2 3 Average.

Conclusion and Graph It was previously predicted that the greater the length of wire the greater the resistance. I also predicted that the resistance of the wire was directly proportional to the wire's length. The graph produced from the averages of the resistances obtained in the experiment proves the prediction because in the graph a straight-line through the origin was produced.

The graph plotted was Resistance versus wire length graph (R against L) therefore the resistance is directly proportional to the length of the wire (R ? L), as predicted in the prediction. The prediction predicted that the greater the length of wire the greater the resistance. The resistance will be greater because, in longer wires, electrons have the pass more ions (opposition) when travelling through the wire. This means that there is a greater probability that an electron will collide with an ion. The more collisions the more times an electron is knocked off course and thus more time to travel through the material.