AIM: TO INVESTIGATE HOW THE ELECTRICAL RESISTANCE OF A WIRE CHANGES IN RELATION TO ITS LENGTH

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Gabriella Blake

AIM TO INVESTIGATE HOW THE ELECTRICAL RESISTANCE OF A WIRE CHANGES IN RELATION TO ITS LENGTH

PLAN

Introduction and background research

·Resistance can be found using the equation

Resistance = voltage

current

·It is measured in Ohms ( )

·Ohms Law states that the current (I) through a conductor is directly proportional to the voltage (V) across the conductor and inversely proportional to its resistance.

·The resistance of a wire is effected by -

ØLength

ØDiameter

ØTemperature

ØType of metal

ØCross-sectional area

·In a conductor, some electrons break away from the atoms. These electrons move freely inside the conductor. When there is a voltage across a conductor, all of the free electrons arrange themselves in lines moving in the same direction. This forms an electrical current. Resistance is come across when the charged particles that make up the current collide with other fixed particles in the material. As the length of wire is increased, the number of collisions the current carrying charged particles makes increases and, therefore, the value for the resistance of the wire becomes higher.

·In 186, Georg Ohm discovered that the current flowing through metal wire is proportional to the potential difference across it (providing the temperature remains constant.) Georg Simon Ohm 1787-1854

·An electric current is the flow of electrons (tiny negative particles), which flow from the negative end of the battery, they travel around the wire and arrive back at the positive end.

·Potential difference is measured in volts (V), current is measured in amps (A).

·Resistance is a measurement describing the difficulty of electric current flowing a conductor.

·In metals, the outermost electrons are held only very weakly to the atom and often wander away from it and go to the nearby atom or one a bit further away. These wandering electrons are called conduction electrons and the more of these there are, for a given volume of metal, the better the metal will be as a conductor of electricity. When you connect a battery across a wire, one end becomes positive and attracts the conduction electrons, which drift towards that end of the wire. But the electrons have obstacles to face because the metal atoms are moving about because of their thermal energy and so the electrons collide with them and are knocked everywhere. It's this difficulty that the electrons have in moving along the wire that we call resistance.

·The higher the resistance, the lower the current. If there is high resistance, to get the same current a higher voltage will be needed to provide an extra push for the electricity.

·Resistance opposes the flow of an electric current around a circuit so that energy is required to push the charged particles around the circuit. The circuit itself can resist the flow of particles if the wires are either very thin or very long.

Prediction

When the length of wire increases so does the resistance. The rate the resistance of the wire increases will be directly proportional to the length. This is because resistance takes place when current carrying electrons collide with fixed particles. If the length of the wire doubles, the fixed particles will double, which will therefore double the resistance.

Electric current is the movement of electrons through a conductor. In this experiment a metal wire (Nichrome will be the conductor). So when resistance is high, conductivity is low. Metals such as Nichrome conduct electricity well because the atoms in them do not hold on to their electrons very well. Free electrons are created, which carry a negative charge, to jump along the lines of atoms in a wire, which are in a lattice structure. Resistance is when these electrons, which flow towards the positive, collide with other atoms, they transfer some of their kinetic energy. This transfer on collision is what causes resistance. So, if we double the length of a wire, the number of atoms in the wire doubles. This increases the number of collisions and energy transferred twice, so twice the amount of energy is required. This means the resistance is doubled. My graph should show that the length is directly proportional to the resistance.

Preliminary Work

I did not do a preliminary experiment because I wanted to find out more research from textbooks and I also wanted to understand the experiment more.

Safety

The experiment is not particularly dangerous but precautions must be taken when handling electricity

üThe current/voltage must be kept low to avoid burning the wire.

üCaution when touching the wire, as it may get hot

üCaution when the wire is connected, as it will get hot.

üCaution when cutting the wire.

üThe mains to the power supply must be switched off when installing or removing the wire from the circuit.

üNever handle electricity with wet hands.

Fair Test

The length of the wire is the only factor that will be changed in this experiment. This will insure that any change in the resistance is caused by the altering length of the wire, and not by another variable. The other factors that will remain constant are as follows

·Diameter- Increase in the diameter of the wire would theoretically effect resistance in the same way as an increase in the length of the wire and must therefore remain constant. The same wire will be used throughout the experiment to insure this.

·Type of wire- Some metals have less resistance than others. Using wires made from different metals would affect the resistance. The same wire will be used throughout the experiment to prevent this.

Accuracy

To make sure this experiment is accurate

·The length of the wire must be measured precisely. It will be measured with a ruler to the nearest millimetre.

·The wire must be completely straight during the experiment, to avoid short circuits. Bends and kinks in the wire could also affect the resistance.

· The voltage reading must be taken as soon as the circuit is connected. This will minimalise the current's effect on the temperature of the wire and thus the resistance.

Reliability

·Each reading will be taken twice. Then both readings will be averaged to give the voltage. If two readings for the same wire length are very disparate, further readings will be taken to insure an accurate result.

·To make the experiment reliable, all apparatus must be checked to see if it is functioning properly and is giving a true reading.

·We will use the same apparatus throughout the experiment to make sure that nothing changes.

·We will have the lengths all 5cm apart so that the results are not too similar and so it will be reliable.

Experiment

Apparatus

ØNichrome wire

ØRheostat

ØPower supply

ØSix connecting wires

ØTwo crocodile clips

ØVoltmeter

ØAmmeter

ØRuler

Method

1.Adjust rheostat until the ammeter reads A.

.Record voltage on voltmeter

.Repeat the experiment with the following lengths of wire, connected between the two crocodile clips

-0 cm

-5 cm

-40 cm

-45 cm

-50 cm

-55 cm

-60 cm

4.Use Ohm's Law (Resistance= Voltage/Current) to find the resistance of the wire.

Results

Length 1 = 0cm

Volts (V)I 1 (A)I (A)Average 1 (A)Resistance (Ω)

0.60.400.400.401.50

0.80.540.540.541.48

1.00.670.660.671.4

1.0.800.80.811.48

1.40.70.70.71.44

Length = 5cm

Volts (V)I 1 (A)I (A)Average 1 (A)Resistance (Ω)

0.60.40.40.41.76

0.80.480.480.481.67

1.00.600.600.601.67

1.0.70.70.71.67

1.40.860.840.851.65

Length = 40cm

Volts (V)I 1 (A)I (A)Average 1 (A)Resistance (Ω)

0.60.00.0.0.00

0.80.410.410.411.5

1.00.10.10.1.

1.0.60.60.61.4

1.40.70.70.71.

Length 4 = 45cm

Volts (V)I 1 (A)I (A)Average 1 (A)Resistance (Ω)

0.60.60.70.7.

0.80.60.60.6.

1.00.450.450.45.

1.0.550.540.55.18

1.40.60.660.60.650.60.66..1

Length 5 = 50cm

Volts (V)I 1 (A)I (A)Average 1 (A)Resistance (Ω)

0.60.0.0..61

0.80.0.0..50

1.00.400.400.40.50

1.0.500.40.50.40

1.40.580.570.58.41

Length 6= 55cm

Volts (V)I 1 (A)I (A)Average 1 (A)Resistance (Ω)

0.60.0.0..7

0.80.0.80.00.0.00..67.76

1.00.60.70.7.70

1.0.440.440.44.7

1.40.50.50.5.6

Length 7 = 60cm

Volts (V)I 1 (A)I (A)Average 1 (A)Resistance (Ω)

0.60.0.0..7

0.80.60.70.7.6

1.00.0.40..0

1.0.410.410.41.

1.40.480.480.48.

Key

Unhappy with readings, taken again and no change.

Unhappy with readings, taken again and corrected. Incorrect readings are marked in red underneath the correct one.

Evaluation

In our investigation a few results were incorrect so we had to redo them. On length (40cm) on 1 volt the resistance was very different to the others we took it again but there was no change. Length 4 (45cm) 1.4 volts, we took the reading again and there was an accurate change. Length 5 (50cm) 0.6 volts, we took another reading but there was no change. Length 6 (55cm) 0.8 volts, we took again and there was an accurate change. When plotting the graph one of the averages was not accurate (40cm) so we took the reading again and got a more accurate result. Our results were as we predicted (see graphs). We used Nichrome (8swg) wire for our experiment but we did have the option of choosing Constantan wire but it probably would not make a difference. We did have a few anomalous results, which we could not put on the graph because they did not go through the line of best fit. We chose the lengths so that they would all be 5cms apart because we wanted to make sure our results were not too similar. We could have done the experiment better if we checked to see if our ruler was exact, it probably was not completely accurate. We repeated the results three times and one more time if we had any anomalous results. We used an analogue ammeter because digital ammeters readings change sometimes but an analogue ammeter reading stays the same unless you change the voltage. We used the same equipment throughout the experiment to make sure it would be reliable. To make our experiment better we could have turned our power pack off between readings to make sure the wire did not get too hot, this could have changed are results slightly. Overall I am happy with our results.

Conclusion

My prediction was correct, the resistance was directly proportional to the length. This is because of the scientific idea, stated in the planning that if you double length, you double the number of atoms in it, so doubling the number of electron jumps, which causes resistance The results support my predictions well, the results turned out the way I had expected, they match the predicted line well. I had predicted a straight line through the origin, which means resistance, is directly proportional to length. This also means that if the length was trebled or quadrupled the resistance would treble or quadruple. This is emphasised because the line of best fit is a straight line, which means the resistance is proportional to the length. This proves the fact that the longer the wire is, the more collisions there are between atoms and electrons. So if the wire increases in length, so does the resistance. If the wire decreases in length, so does the resistance.

Research

I got some information from GCSE textbooks but for my main source of information I used the search engines Google and Ask.

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