To Investigate the Variables That Affect Resistance in an Electric Circuit

Coursework on Resistance

Aim

To investigate the variables that affect resistance in an electric circuit.

Investigation Research

Ohm, Georg Simon (1787-1854), German physicist, best known for his research on electrical currents. He was born in Erlangen and educated at the University of Erlangen. From 1833 to 1849 he was director of the polytechnic Institute of Nurnberg, and from 1852 until his death he was professor of experimental physics at the University of Munich. His formulation of the relationship between current, electromotive force, and resistance, known as Ohm's Law, is the basic law of current flow. The unit of electrical resistance was named the ohm in his honour. His theories stated that current flows in an electric circuit in accordance with several definite laws. The basic law of current flow is Ohm's law, named for its discoverer, the German physicist Georg Ohm. Ohm's law states that the amount of current flowing in a circuit made up of pure resistance's is directly proportional to the electromotive force impressed on the circuit and inversely proportional to the total resistance of the circuit. The rule is usually expressed by the formula I = E/R, where I is the current in amperes, E is the electromotive force in volts, and R is the resistance in ohms. Ohm's law applies to all electric circuits for both direct current (DC) and alternative current (AC), but additional principles must be invoked for the analysis of complex circuits and for AC circuits also involving inductance's and capacitance's.

Resistance, in electricity, property of any object or substance to resist or oppose the flow of an electrical current. The quantity of resistance in an electric circuit determines the amount of current flowing in the circuit for any given voltage applied to the circuit, according to Ohm's law. The unit of resistance is the ohm, the amount of resistance that limits the passage of current to one ampere when a voltage of one volt is applied to it. The standard abbreviation for electric resistance is R and the symbol for ohms in electric circuits is the Greek letter omega, U. For certain electrical calculations it is convenient to employ the reciprocal of resistance, 1/R, which is termed conductance, G. The unit of conductance is the mho, or ohm spelled backward, and the symbol is an inverted omega, ~~. The resistance of an object is also determined by the nature of the substance of which it is composed, known as the resistively, the dimensions of the object, and the temperature. Resistively is expressed in terms of the ohms resistance per cubic centimetre of the substance at 20'C (68' F).

Prediction

I predict that if the length increases then the resistance will also increase in proportional to the length. I think this because the longer the wire the more atoms and so the more likely the electrons are going to collide with the atoms. Furthermore, doubling the length of the wire will result in double the resistance. This is because by doubling the length of the wire one is also doubling the collisions that will occur, thus doubling the amount of energy lost in these collisions.

Apparatus

1.5m constantan wire

Crocodile clips

Ammeter (0-5A)

Voltmeter (0-10V)

Power Supply

Connecting leads

Metre rule

Method

1. 1.5 metre length of constantan wire is fixed to a metre rule.

2. The first crocodile clip is clipped to the wire at the 0cm position on the metre rule.

3. The second crocodile clip will be clipped onto the relevant position depending on the required length of wire. The readings will start with 1.5 metre decreasing by 0.1 metres each time, the final reading will be 0.1 metres

4. The power supply is turned on. The voltage and current are then read off the ammeter and voltmeter, then recorded.

5. The power supply is then turned off and ohms law is used to find the resistance (R=V/I). The second crocodile clip is then moved to the next position.

The above steps are completed for each length and then the entire investigation is repeated three times for accuracy.

Safety

Whilst doing the investigation, it is important to keep safety into consideration. Before using the power pack, the pointer should point at 0 volts. It is important to be careful while using the power supply. While handling live wires, it is essential to be careful. The voltage should be kept low because of the safety factor and the wires heating up. Lengths lower than 10cm will not be tried, which also helped to avoid overheating.

Fair Test

In my experiment I am only changing one factor which is the length of the wire. To make my experiment a fair test I will keep the following factors the same:

* I will keep the surrounding room temperature the same or the particles in the wire will move faster and this will therefore have an effect on the resistance.

* I will also keep the material of the wire the same as different materials have different conductivity. This factor will be kept the same by using the same wire all of the way through the experiment.

* The current that will pass through the wire is to be kept the same also. If this is changed the temperature of the wire might change in a way that is not constant making the results incorrect.

* The thickness of the wire. This will be kept the same by using the same wire throughout the whole investigation

Results

Experiment 1

Length (m) Current (I) Voltage (V) Resistance ( )

1.5 0.7 10 14.3

1.4 0.8 9.8 12.3

1.3 0.8 9.6 12

1.2 0.9 9.5 10.6

1.1 0.9 9.4 10.4

1 1 9.2 9.2