Having tried out all the voltage settings, some appeared too high, some were not accurate enough and some were better with low voltages. 20 was decided upon as it is the easiest unit to work with. 10 readings were taken at every half a volt, the lowest voltage being 0.50V and the highest 5.00V. We also decided to use water in the beaker rather than another substance because it isn't flammable and has a relatively high boiling point.
ASSURANCE OF PRECISION AND ACCURACY
To assure that the results are precise I must use an appropriate multimeter range and for accuracy repeat readings will be taken and the water temperature will be taken each time with a thermometer to ensure it remains the same.
SAFETY
As the experiment involves electricity and water there are a few safety precautions necessary. When not in use the power pack should be turned off to prevent the wire from overheating and voltage readings should not be too high.
EQUIPMENT LIST
● Multimeter x2
● Power pack
● 100ml beaker
● Coil of nichrome wire
● Thermometer
● 75ml water
PREDICTION
I predict that as the voltage increases, the power of the electrical heater will increase. This is displayed by the graph below.
Power
ANALYSIS
I have plotted my results from the experiment on a graph. The graph shows a positive correlation, but it is not a straight line as it would be if Ohms law applied.
Ohms law: I (current) is proportional to V (voltage).
The steeper the graph, the lower the resistance.
On this graph, the results show that as the voltage increases, the current does also but not at the same rate. This shows that the resistor does not remain constant.
A substance that gives a straight graph is called an ‘ohmic conductor’. All metals and copper will produce this result, if their temperature remains constant.
However in this experiment a filament lamp was used and a lamp is a ‘non-ohmic conductor.’ It does not obey Ohm’s law because its temperature changes. As more current flows, the metal filament gets hotter and so its resistance increases, eventually the wire will melt and no current will flow.
As the temperature of the filament increases, the protons move around more, making it harder for the electrons to flow through the wire. This is what causes resistance. The protons ‘resist’ the electrons.
Anything that supplies electricity also supplies energy, which is then transferred. For example when energy was passed around this circuit the electrical energy was transferred to heat energy in the lamp.
As proved earlier, the higher the voltage = the higher the current, which in turn produces more heat. A higher resistance means less current and therefore less heat is produced in the lamp. This explains why the line steepens, rather than staying constant. The resistance of the lamp increases.
EVALUATION
Having completed the experiment and drawn some scientific conclusions from it, I can now look back to see what could be done to improve the method or find an alternative method which could be used.
The results I gained fitted the prediction and gave some satisfactory graph lines which proved that as voltage increases, power increases. However the experiment I carried out could not have given me completely reliable results. The current readings were only measured to two decimal places and therefore they were not completely accurate, particularly the lower values. The current measured at 0.50V was only 0.08A and gave the lowest calculated value of resistance (6.25Ω).
One set of results showed a difference between the repeat readings of current. This occurred at the highest voltage and current and therefore the highest power, this also gave the highest resistance. This result suggests that using voltages greater than 5V may lead to a variation in the resistance of the wire as it heats up.
The difference between the highest and lowest resistance values was 0.6Ω which could have been due to temperature change or another affecting factor.
An alternative method for this experiment could be to use a datalogger in the place of a multimeter. In this experiment the equipment would be set up in the same way but the datalogger is attached to the computer, allowing the readings to be automatically plotted onto a graph of voltage against current. This method would be an improvement as it would be much quicker, therefore allowing more time for extra repeats to be taken and more averages calculated, making the overall results more accurate. The computer could also be used to plot other relevant graphs such as voltage against power.
