The movement of the electrical charge is called the current. The current is the rate the charge flows around the circuit. The more ‘push’ emitted from an electrical source e.g. battery, the larger the current. The current (I) is worked out by finding the charge (coulomb-Q) and the time,(t-seconds) and dividing charge by time. A current flows in a metallic conductor by a battery or the mains forcing energy into a circuit creating a movement of electrons. The conventional current flows in the opposite direction to the –e.
Most metals outside shell of electrons have relatively weak bonds so their electrons can easier be broken and move from atom to atom meaning they conduct electricity well. The electrons that have the ability to move from their original atom are called conduction electrons and the more of them there are in a set volume the more conductive that metal is and the less resistance it has.
Ohms law states that current, resistance and voltage are all relative. The size of the current is dependant on the amount of push or voltage emitted by the battery and how good a conductor the material it is travelling through is. Some electrical components are designed to have a specific resistance; they are called resistors, some of them can even vary their resistance: variable resistors. If we decided to carry out further experiments when given a specific resistor and had a volt meter in the circuit we could work out the relationship between current and resistance by using this formulae and measuring voltage. with the final results of this experiment we could also find out the voltage with the current and resistance. We could then switch the multi meter to read volts and see how accurate our recording was.
Factors to control: To make our experiment a fair test it is necessary to control the other factors to keep the results accurate and there fore the conclusion based on the graph correct.
As mentioned above;
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When the temperature increases the resistance of the metal increases. This is due to the increase of energy transferred into the atoms of the +metal. As they become hotter they vibrate more energetically. The free electrons trying to move around the atoms find it more difficult and collide more often as the atoms move in their flow this obviously increases the resistance. In the experiment we must perform it at room temperature and preferably on the same day, as temperature will vary from day to day. It is also important to use a low voltage so the current is low and the energy will not heat up the wires as the experiment progresses. Using different pieces of wire in each single test would prevent this occurring. A thin piece of wire would heat up quickly because there is less room for the electron the pass through the wire and so the energy in them would transfer to the mass of the metal as they move around in a small space trying to carry on through the gaps.
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As the length of the wire is increased the resistance in the circuit will increase. The electrons will have to travel further and through more atoms than on their path back to the back to the power source, creating more collision- more resistance. If the length accidentally differs (through being careless with measuring) during the experiment it will be difficult to evaluate exactly how this affected the resistance.
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All different materials will have different atomic structures and there fore different resistances. Different metals there for will have a unique resistance so it is vital that only one metal is used through out the experiment. Even if two metals of same length and thickness are used they will have different resistances, as one will be a more efficient conductor-lower resistance. This is due to the fact that that type of metal atom will have more electrons in their outer shells so more free electrons can move at one time. But if the metal is denser having more atoms in the same space than the other, then there will be more collisions and friction-higher resistance. All materials have resistance, if there wasn’t any substance for the current to travel through it would be infinite.
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Other less relevant factors like humidity hardly effect the resistance at all and are extremely hard to control because you need it be able to put it in an environment where you can vary and measure the amount of water vapour in the air. So it is not necessary to control it as it will stay constant during the experiment any way.
Preliminary tests and results:
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With out performing any tests we realised constantan would be the most suitable metal to use out of copper and nicrome. We decided that we would need a large selection of gauges/ S.W.G’s to make a graph with enough data plotted to create a sufficient pattern to draw conclusions from. There was a selection of 7 values for constantan. Nicrome had a smaller selection. Copper seemed more unreliable from research that stated that it had little resistance so the results would be very low and more random and produce a small and uninteresting graph with a larger percentage error.
- In the real experiment we will use S.W.G’s; 22, 24, 28,30,32,34 and 36, as these are the diameters available. You will notice the interval of two gauges turns to four between 24 and 28, this should pause no problem as the graph will be to scale and so this gap will be compensated for. These values are also good to use as the will have fairly large resistances which will limit the percentage error making the results more accurate.
- I will try and take several tests for each value in the real experiment to have a good spread of results when making the average to be as reliable as possible and try and even out the anomalies if there are any.
- We decided to use a multi-meter in a passive circuit as it will be simple to set up and limit inaccuracy and resistance in the other components, as there are less joins and length of wires to create the circuit than with an active circuit. The multi meter will also work out the resistance in ohms automatically as it measures both voltage and current. By measuring volts and amps with volt and amp meters you would also have to do the sums as well; resistance=voltage/current.
- In the preliminary we will vary the length of the constantan by 60, 80 and 100cm to observe which will be the most suitable for our experiment or if it makes much difference. We will also do this with the smallest diameter and the largest diameter to see the range of results we are likely to record in our experiment. Perhaps with the thickness=36S.W.G and the shortest wire the results will be too small to be considered reliable as the percentage error increases and maybe of two tests on the same piece they will turn out very different and there fore inaccurate. Maybe with the thinnest and the longest wire the results will be so large it would be extremely hard to make a graph large enough to fit all the results on to a reasonable scale.
The preliminary results show that all of the lengths have suitable ranges of resistance for a graph but obviously 100 cm has the widest and there fore has the smallest percentage error, as a 0.1 amp difference would be less of a percentage error on 100cm than e.g. 60cm. Also none of the results seemed unreliable as the only variation was of 0.1 amps, this sought of marginal error will probably decrease with practise setting up the equipment etc. I would expect that perhaps the connections were slightly different as we disconnected the circuit between each trial to ensure it was working properly or, perhaps in between the first and second tests some of the wires heated up enough to slightly alter the resistance by increasing as the temperature rose. I will choose 100cm as my length, although slight harder to measure correctly as the wire has bends in, I can tape it straight to a metre ruler and use it in this manor which will be more accurate. I have also decided to do three tests in the real experiment just to ensure the differences in results with the same components are more accurate when made into an average.
I also realised that the extra resistance created in the circuit must be taken into account as each recording will actually be of the whole circuit not just the wire component.
Method:
Apparatus
- Reels of constantan more than 1 meter long of 22,24,28,30,32,34,36 S.W.G.
- 1 meter ruler
- Tape
- 1 red and 1 black plastic coated lead with plug heads
- 2 crocodile clips
- Multi-meter
Safety considerations.
Remember to wear protective goggles to ensure no bits of wire fling into eyes whilst cutting it etc.
Keep all equipment on the table and safe distance from the edge to stop you hurting others and yourself and not damage electrical equipment.
Make sure all the equipment is undamaged because if there was bear wire that wasn’t coated in plastic (which is an insulator) the current could flow through you if your resistance was less than that in the circuit.
Generally be careful when using electrical equipment to prevent getting an electrical shock.
Ensure that all surfaces and equipment are dry because water is an excellent conductor and an electrical charge will flow through it and possibly to you if you touch the water, there fore wet hands are prohibited.
- Collect first SWG of wire and all other equipment to make the experiment run methodically and smoothly
- Switch multi meter on and check it is working properly
- Unroll over a meter of Constantan and stick it length wise against the meter ruler with tape to straighten the bends. Cut tape with scissors so tape does not become wet and disturb conduction
- At the appropriate markings on the ruler (0cm and 100cm) clip the crocodile clips to the wire and secure connection by wrapping the wire around the two teeth prongs, plug the red and black wires in the ends of the crocodile clips to its maximum depth for proper connection.
- Make sure the multi meter is switched off before plugging the other ends of the black and red wires in the first and second holes (left to right) in the row of terminals so it measures in ohms.
- Turn the dial so it is facing 180 and turn it anti clockwise one notch so it has a reading in ohms. Record this
- Disconnect wires from the power source and do another test with same equipment. Repeat this if necessary bearing in mind if you do three results for one SWG to find an average you must do it with all the others to make it a fair test.
- Repeat this same method with all the other SWG’s. It is not necessary to cut the wire because the current will flow in the circuit and not travel down the wire as it takes the easiest route back to the power source the rest of the metal is attached to earth anyway.
- let all the equipment cool slightly before each separate test to ensure they all start at the same temperature(room=approx 20) so resistance is not increased between tests by a higher amount of thermal energy gathering in the wire(particularly thinner ones) because of previous resistance.
Table of results, with three tests and the average.