Ohms Law
Ohms law clearly states that
Resistance = Voltage
Current
Which would mean as the current increases in a circuit so would the voltage because the resistance stays the same and for example if there is higher voltage if the current doesn’t increase then the calculation for resistance would be higher so the current has to increase at the same time. A graph comparing voltage against current should look something like this:
Ohms law is only an approximation and works well for many components. This however would only be for components such as wires and resistors. A component will only obey ohms law if it is kept at a constant temperature.
Components such as thermistors and light bulb with filament lamp. Below is a graph for 2 components that don follow ohms law.
Insulators
Is a material that doesn’t conduct electricity. Materials such as wood, plastic and sand are known as insulators in electric current. Insulation means putting something which doesn’t conduct electricity between a live wire and yourself. Any non-metal is a insulator except for graphite.
Conductors
Is a material that does conduct electricity. The electrons flow freely through it. Conductors are always metal.
Insulation and Conduction in Wire
Wires we use which conduct electricity are made of a high purity copper metal, which is an excellent conductor.
The insulator which covers the wires is a plastic called pvc, it is flexible enough to bend.
Warning
Insulation can become unsafe if it is damaged or wet
because impure water will conduct electricity.
Preliminary Experiment
I am doing this experiment to find out what temperature to start and stop measuring current at (and in real experiment voltage) , this is important because I don’t want to start measuring when there is no significant change between current and voltage when the temperature is changed. Also I am going to work out when to measure different readings for example when the temperature is increased by 5ํ°C I may want to take a reading of current and voltage if they are significantly different however if there is a huge gap or a very small one I may change the step I take.
Also doing preliminary work I can decide how I am going to take the readings or example I may take a reading every time the current changes by 0.2amps and then see what the temperature is at these points, or I may do as I have explained above and take a reading e.g every 5ํ°C depending on how the current changes
In this preliminary experiment I am not going to include the voltmeter because all I am trying to work out is start/stop temperatures and the step I am going to take so all I need to do is concentrate on these two factors and voltage will be added in real experiment.
(i) is a diagram of the circuit used and (ii) is a diagram of what the actual experiment will look like.
(i) (ii)
Preliminary Results
What I did…
I set up my experiment as previous page shows I measure the current each time the temperature increased by 5°C and started to measure at 5°C. I measured up to 100°C and then turned off the lab pack and the Bunsen burner and recorded results.
I have decided to start measuring results at 25°C because this is when the current was significantly changing. Also I have decided to measure the current and voltage each time the temperature increases by 5°C because the current changes significantly within a step of 5°C any more time could give varied results without an obvious pattern and smaller differences in temperature could give reading of current that are the same because there is not enough difference between temperature.
I decided to start measuring at 5°C because I wanted to find out when the current changed significantly and I may have missed it if I didn’t start from when it was first possible, and I stopped measuring the temperature at 100°C because this is the maximum temperature of water.
Through these results I have decided to make the step 5°C each time because there is a significant change in amps. I have also decided to start measuring the change of current when the temperature have reached around room temperature at 25°C because again this is where the current starts changing obviously.
Plan
(i) Circuit and (ii) Set–up of apparatus:
(i)
(ii)
List of Apparatus
Wire
Cell/Lab pack
Voltmeter
Thermistor
Bunsen burner
Stand
Heat Proof Mat
Griddle
Water
Thermometer
Ammeter
Paper Towels
Matches
Crocodile Clips
Method
- Set up apparatus as diagram shows with ammeter anywhere and voltmeter parallel to thermistor and with crocodile clips hooking the wires onto the wooden block suspending the thermistor into the water.
- Turn on the ammeter, voltmeter and lab pack.
- Place the Bunsen burner underneath the stand that the beaker is placed on, and then light it with a match to a blue flame.
- Change the initial voltage onto 12V because this is where the current is significant at 0.1amps
- Check the temperature just before you turn on the Bunsen burner and then keep checking the temperature every 30 seconds.
- Write down the first readings of voltage and current when the temperature is 25°C then every 5°C from this point onwards.
- When the temperature has reached 100°C turn of all apparatus including voltmeter, ammeter, lab pack and Bunsen Burner
- Remove hot beaker of water and refill will cold water and repeat method twice more.
What I will measure
I am going to control the temperature that the water is at, and then measure the current and voltage that are independent variables. I will measure these independent variables when my control variable rises 5°C from 25°C each time
These start, stop and step temperatures were all decided through preliminary work. I decided on these because it was the step temperature and start temperatures that were decided because of the amount of current that could be recorded because if the temperatures (start or step) were set lower, then as the temperature there would be such a low current to start with and such an insignificant change in current because of the low step then it would be impossible to record with the equipment that we had to use.
Repeat Measurement?
I am going to repeat my recordings at least 3 times so that I get results that are accurate in case anything I do is wrong for example missing a reading for a certain temperature, I could take my other reading and estimate the current or voltage for a temperature in that set of readings. Also if within a set of results there are anomalous results (for any reason) then the other results will make that more in the trend of the results and this will be shown in a graph of the final results when all the averages of all the final readings has been taken.
Fair Test?
What will I keep the same?
- Voltage from lab pack will always be as near to 12v as I can make it so the results will always be the same.
- I will use the same thermistor for all readings because the only thing I want to change is the temperature and I want to be able to say “the reason why the current and voltage changed is because the temperature changed and everything else was exactly the same”
- The same thermometer or at least the same type of thermometer I will use a liquid thermometer all the time because the readings I get could be different because of a thermometer being too or not accurate enough compared to other thermometers I used previously but in the same investigation.
- When I repeat results if I have time I want to wait until the thermistor cools down so this doesn’t affect the results because the heat of the thermistor could be different from that of the water and the results I get aren’t correct because the current and voltage results I am taking aren’t in relation to the temperature of the water because doing this experiment I am assuming he thermistor is the same temperature as the water.
Safety
- I have to make sure none of the wires are in the water or are on something hot in case they burn etc.
- When taking apparatus apart I need to make sure that I am careful I don’t burn myself
- That I wear goggles in case of any fires or water boiling and bubbling into eyes.
- That I don’t fill the water up too high on the beaker in case it splashes out and burns someone
Prediction
Scientific Theory
