Diagrams:
Diagram showing all other apparatus before the heater is lowered into the water.
Method:
- The circuit and apparatus will be set up as in the diagrams on previous page. (the ammeter will be on the 0-5 setting)
- The temperature of the water will be read from the thermometer ant eye level. (and then recorded)
- The voltmeter and power supply will be turned on and turned up to 10 volts (checking the voltage on the voltmeter) and the ammeter reading will be recorded
- The immersion heater will be lowered into the water until it is 1cm from the base of the beaker and the stop-clock will be started simultaneously.
- After 15seconds the water will be stirred with the glass rod.
- After 30 seconds the voltage will be checked, if it has varied slightly off the 10volts reading this will be adjusted back
- After 45seconds the water will be stirred again with the glass rod.
- After the minute, the temperature will be read from the thermometer at eye level. (and then recorded)
- The same procedure will be repeated for 9 more minutes (i.e. stirring at 15 and 45 seconds past each minute, readjusting the voltage if necessary at 30 seconds past each minute and then reading and recording the temperature on each minute
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After the temperature has been recorded on the 10th minute the power supply will be switched off and the apparatus left for about 5 minutes to cool down.
- The immersion heater will be disconnected from the circuit and this, the beaker, the glass rod and the thermometer will all be rinsed with cool water to ensure everything has cooled fully.
- The apparatus will be set up in the same way it was originally with a fresh 200ml of water in the beaker. The complete procedure with the temperature measuring, etc. beforehand and cooling the apparatus afterwards will be carried out for a second and third time.
Fair test:
There are many variables that I will keep constant so my investigation is fair. The factors that effect energy (which to start off here is electrical energy) are
- voltage
- current and
- time
This is because energy = power time
and power = voltage current
so energy = voltage current time
I have decided not to vary power but instead to vary time so to keep the tests fair I must keep power constant. I must do this because if I don’t then I wont know if the change in thermal energy is due to the varying of power or time. Therefore I wont be able to determine any reliable results.
To keep power constant I must keep both voltage and current constant because these affect each other. On the power supply that I am provided with, it is easy to change voltage to a certain number and then keep it constant so this is what I will do. Because of the fluctuation in voltage that I had in my pilot study I have planned to check the voltage every minute and readjust it if it has changed slightly so that it is constantly 10 volts. This will keep the current constant all the way through my investigation too.
Also, I will keep the circuit and the wires used in it the same because I want to keep resistance constant. Voltage = current resistance so if I change resistance then voltage will be affected which in turn will effect the energy. As I just explained, current and voltage must be kept constant in order to keep the only factor affecting energy as time, my controlled variable.
Another factor that will effect my investigation is volume of water. This is because if there are more water particles at any time it will take longer for the thermal energy to be passed to those particles so overall it will take longer to heat-up. I will therefore keep the volume of water constant at 200ml. I have chosen this volume because this beaker size is deep enough for the immersion heater to be mostly underwater and there is enough space to stir as well. However if I had a larger volume of water, the temperature change each minute would be quite low for the reason I described above.
I will be using the same beaker throughout the investigation too, and not a different sized one, because to keep my results fair I must keep surface area the same too. This is because, as particles gain more energy, the ones with the most energy are able to break free, i.e. evaporate. If for example there is a larger surface area at one point, then there is a larger surface area to volume ratio, i.e. a larger proportion of the particles are close to the surface and so evaporation occurs quicker. (Unfortunately I will not be able to prevent evaporation by using a lid, because there are too many objects that have to sit in the beaker and would get in the way)
Lastly, as well as keeping control of all the variables, to keep my results fair I will try to be very careful to press start on the stop clock at exactly the same time that I lower the heater into the water. This is because as soon as the heater is in the water it will pass thermal energy to the water particles.
Accuracy and Reliability:
To keep my results reliable I will be calculating the temperature rise after a range of 10 different times (1 to 10 minutes) and will be doing three repeats for each. Therefore I will be able to notice if there is any anomalous data. (I have chosen only to calculate the temperature rise for up to 10 minutes because I have a restriction on the amount of time I have to carry out the investigation. Because I have to set up the apparatus, wait for things to cool down, rinse some objects, take 3 repeats of everything and clear away at the end, and I don’t want to rush because that may lead to carelessness and inaccuracies, I feel it would not be possible for the time range to be any larger.)
The stirring of the water at 30-second intervals (every 15 and 45 seconds after each minute) will also increase reliability. In the water, when the particles have more thermal energy and therefore vibrate more, the particles expand away from each other, i.e. the water will be less dense so it rises. The cooler, less dense water will sink, replacing, the rising warm water, therefore causing convection currents.
The stirring will ensure that the warmer particles are mixed evenly throughout the water in the beaker so that the temperature readings will be more reliable. (I have realised that by stirring the water this may increase the kinetic energy of the particles however this is not likely to be a large enough increase to affect my results.)
I will also be keeping my results reliable by carefully controlling all the variables mentioned above.
In addition there are quite a few things I am planning to do to keep my results accurate. I will try to keep my investigation accurate by using a digital stop clock that is accurate to every 100th of a second instead of a conventional clock, which is only accurate to the nearest second. As I will be stirring the water, checking the voltage, or reading from the thermometer every 15 seconds, it is easy to see why a digital stop clock is a better choice.
As well as having a stop-clock accurate to every 100th of a second, I will have a voltmeter that is accurate to every 100th of a volt and this will make it easier to keep adjusting the voltage to exactly 10. If for example the voltmeter was only accurate to the nearest volt, the voltage could be 10.49 without me knowing, and this would increase by a substantial amount the thermal energy of the water particles.
Lastly, when reading the temperature, it will be at eye level to avoid parallax errors, as well as from a thermometer with degree C markings.
Pilot study:
The main alteration from my pilot study is the voltage. During my pilot study this was 7 volts yet I found that the temperature sometimes didn’t even increase by a whole degree each minute. Therefore I increased the voltage to 10. This made the temperature increase each minute easier to determine which is why I am using this voltage for my investigation.
When calculating the increase in power when I altered this voltage, I realised I had doubled the power from 17.5 watts to 35 watts, this doubling the amount of energy the particles had each minute. (Because power = current voltage and when voltage was 7V, current was 2.5A so power was 7 2.5 = 17.5Watts, yet when voltage was 10V, current was 3.5A, so power was 10 3.5 = 35Watts.) I therefore learnt that only an increase of 3 volts doubled the power so during my investigation I am planning to be very careful about keeping the voltage the same – even a slight difference could effect my results, making them unreliable.
From my pilot study I also learnt that the starting temperature of the water (at 0 minutes, before the heater is lowered into the water) wasn’t always the same. This is why I have decided to calculate the temperature rise from this starting temperature when doing my investigation, and I will plot my graph using this.
