Scientific Knowledge:
Resistance is the property of a substance that restricts the flow of electricity through it. The unit of resistance is the OHM.
Resistance is measured in Ohm’s. A resistor has a resistance of 1 Ohm (1 ) if a voltage of one volt across it will push a current of 1 ampere through it.
The more resistance a resistor has, the more volts are needed to push ach ampere through.
If the voltage across a resistor and the current through it both has been measured, the resistance(r Ohms) can be calculated using the equation:
R =V
I
If we look at Ohms law, discovered by German physicist “Georg Ohm” in 1827, it states:” The current flowing through a resistor is directly proportional to the voltage across it. That is, if the temperature remains constant (The resistance has an affect on the flow of electricity)
From this we get the equation:
Voltage = current x resistance.
V = I x R
Resistors are devices specially made to provide resistance; placed in a circuit they reduce the low of current. Resistors are often made from, wire wound coils or pieces of carbon.
Rheostats and petentiometers are variable resistors.
Rheostat is a resistor; it usually consists of a high resistance wire wound coil, with a sliding contact. It is sometimes used to vary electrical resistance with out interrupting the current, e.g. dimming the lights.
Pententiometer is an electrical resistor that can be divided so we can compare measure or control voltages.
Apparatus:
Wires
Power pack
Voltmeter
Ammeter
Piece of wire (acting as a resistor)
Variable resistor.
Diagram:
Method:
- Connect the red wire to the ammeter, and the to the power supply.
- Connect black wire to the power supply, and then to the variable resistor.
- Connect black wire to the ammeter and then to the resistor (The piece of wire)
- From the other side of the wire, connect to the voltmeter.
- Finally, record readings by turning on the power supply.
To keep the current exactly the same throughout the experiment, we altered the dial and the voltmeter. (I found this quite hard.)
When working with electricity, we must always remember certain rules.
Make sure your hands are ALWAYS dry. The electric circuit is know where near water Also always turn power pack off between recordings, as the apparatus might get too hot, causing us to burn if touched. Another safety precaution is to check the circuit works before turning it on.
To make this experiment a fair test we must ensure that:
- We use the same current through out the experiment.
- The power pack is turned off between readings. (So there are no false results)
- Always have a constant heat.
- Everything as to be exactly the same throughout the experiment. E.g. Same wires, same resistor, same ammeter. (The only thing that should be changed is the wire on the resistor.)
Results Table:
Results 1:
The set of results for “results 1” was more or less the same. (Take 1 or 2 either way) The results may be high due to the wire getting too hot.
Results 2:
Check results 2:
The readings are exactly the same, showing how accurate the results are.
See graph paper for graph.
Conclusion:
As I had predicted, the more we increased the wire the more the resistance increased, causing the current to decrease.
The reasons for my predictions being accurate are due to my researches on Ohms law, and how Ohms law has a big affect on the current flowing through the resistor. (The piece of wire)
The graph tends to go up quite steadily, although towards the end it jumps a bit. The reason for the ammeter reading to go up suddenly is due to the wire getting too hot. (So unfortunately the results aren’t fully accurate.) Although saying this, looking at the graph we can definitely see a set pattern occurring. That is, as the length gets shorter, the resistance gets less, causing the ammeter to record a higher current.
Evaluation:
In my opinion my results are fairly accurate, and I didn’t get any results that didn’t fit my set pattern. I found recording the results was very easy, and I had no difficulties. The only fault was the setting up of the experiment. I had connected everything right, but for some reason the ammeter was reading “minus”. We changed the power pack, the ammeter, and the voltmeter, but the ammeter was still reading “minus”. The only left to do was to check all the wires. We took each wire out one by one till eventually we found the loose connection.
The only thing I think id change about the experiment, was to maybe turn off the power pack a bit sooner, because as I said earlier, towards the end the results seemed to jump drastically.