Experimental physics

Using a kettle to measure the latent heat of Ice

Introduction

In this experiment, you will be measuring the latent heat of fusion of water, Lw . You will do this by placing ice (alongside water) in your kettle and seeing how the time to reach boiling point changes.

This exercise is designed to give you an understanding of:

.     Calculating specific heats

.    How electrical power can be transformed into heat that changes the temperature of water

.     The importance of repeating a measurement

.     How to use the gradient of a straight-line graph to obtain an accurate value.

Theory

The temperature increase in a substance (as long as it does not undergo a change of state) is proportional to the heat supplied to it, given by Q  - mcΔT, where c is the specific heat capacity of the substance.

The heating power of a device like a kettle is the time rate that it adds heat to the substance being heated, that is, P = t/Q.

When melting Ice, the amount of heat that an ice-cube of mass m absorbs is Q  - mL

What you will need

Assemble this equipment before you start the exercise:

.    A measuring jug

.    An electric kettle

.    A stopwatch (you could use a smart phone for this)

.     Some amount of ice cubes

.    A scale for weighing mass

.    Optional: A thermometer. If you do not have one assume that the initial temperature of the water is 22.0°C

Maintaining the integrity of your work

Collaboration

You may either do this investigation alone or with up to one other person currently enrolled in the course (i.e. you may not work as a group of three or more).

  If you work with someone else, you need to acknowledge your collaborator (include their name and student number in the report and include them in your photo). In this case you must conduct the

analysis and answer the questions individually, even if your data is the same. If you have identical

answers, this will be considered plagiarism. So while it is OK to collect the data together, you need to answer the questions and plot the graphs etc. separately.

Photographic evidence

You must include two (2) photographs in your report:

.    A close-up image of your UNSW photo ID card with your name and photo clearly visible.

.    A photograph of yourself with your student card, the equipment, and a piece of paper clearly

showing the date and your name and zID. If you worked with a partner this photo must include both of you.

  You MUST provide these photos in your assessment. Assessments without the photos will not be marked and you will receive a zero.

<  Sample photo of equipment, faces of

people who completed the experiment, your student cards and paper with names, zIDs,   and date. You also need to include a close-up of your student card.

If the experiment asks you to include additional photos, such as to record and report observations, then you must include a label in the image showing your name and zID (just like in the photo above, except you do not need to be in it).

  If you do not include your name and zID in one of these photos, you may receive a zero for that part.

Risk Assessment

For this exercise, the risk assessment has been constructed for you. For future exercises you will need to construct one yourself.

An explanation for the risk ratings with controls is available on the Moodle page for this experiment.

By proceeding with the experiment, you are agreeing to follow these risk control methods and conduct the experiment safely. If you have any doubts about your ability to complete the experiment safely then you     should not proceed.

  In your report, you need to state that you have agreed to follow these risk controls. Without this statement your report will not be marked.

A note on this experiment

Procedure

This experiment has two stages. In Stage 1, you will be determining the power (P) of your kettle. In Stage 2, you will determine the latent heat of ice (L).

Stage 1

1.    Choose five volumes of water to boil (in litres).  Record your selected volumes in results table one. The volume of water in Litres is also the mass of the water.

2.    Determine the uncertainty in mass for your selected five masses.

3.    Fill your kettle with water to its minimum required volume and turn it on. Wait until the water has boiled and then pour it down the sink (later you will be asked to explain why this step is necessary).

4.    Use your thermometer (if you have one) to measure the temperature of the water coming out of the tap. If you do not have a thermometer you may assume this is 22.0o C.

5.    Fill your measuring jug to the first volume you have decided upon. Pour this water into the kettle.

6.    Turn on the power to the kettle and start your stopwatch. Record the amount of time it takes for the kettle to boil. (If your kettle automatically switches off when it reaches boiling point, this makes things much easier)

7.    Repeat steps 5 and 6 another 2 times so that you have three times recorded for your initial volume of water.

8.    Repeat steps 5-7 with the other volumes. Record all the results in your table.

Stage 2

1.   Fill your measuring jug to the same volume of water as your third (middle) volume of water in stage 1.

2.   Place the measuring jug and water on a scale, recording their mass.

3.   Place ice cubes in the measuring jug; the change in mass is the mass of the ice cubes, record this in results table 2

4.   Pour OUT some of the water from the measuring jug, until the mass of the water and ice is the SAME as the mass of the water in step 1.

5.   Pour the ice and water mixture into the kettle and boil, recording how long it takes to boil. Record this time in results table 2. (This is the ICE Boiling Time)

6.   Repeat steps 1-5 with two different masses of ice (use the SAME initial mass of water).

7.   In the ‘ No Ice Boiling Time’ section of Results Table 2, copy the average time for the third (middle) volume of water in stage 1.

Analysis of Results

Stage 1

1.    For each of the masses calculate the average time taken for the kettle to boil. You may want to also calculate the uncertainty in the average time by using:

uncertainty in average time = 2/longest time − shortest time

The uncertainty in the volume (and thus the mass) can be calculated by halving the smallest increment on the side of your measuring jug:

uncertainty in mass of water = 2/smallest measuring jug increment

2.    Now it is time to plot a graph. Plot time on the Y-axis and mass on the X-axis.

You can download the Excel template ‘Linear plot with uncertainties.xls’ from Moodle. Enter your values and obtain the gradient of the graph. With the Excel template, the equation for the line of best fit is

written below the graph in the form:

Y = AX + B

where A is the gradient.

  Include a screenshot of this graph in your report.

3.    Use your gradient to calculate the POWER (P) of your kettle If you have plotted time on the Y-axis and volume on the X-axis, then the gradient is equal to

gradient = P/cwΔT

where ΔT is the change in the temperature (final temperature (100°C) minus initial temperature (what you measured or 22.0°C)), cw is the specific heat of water and cw  is the specific heat of water; cw   =      4184

  If you find it difficult to type your working into your document, then you can do your working with pen/pencil and paper and insert an image of it into your report.

4.    If you have considered the uncertainties, then calculate the uncertainty in your result. To do this, work out P using each of the two lines of worst fit (given in the ‘Linear plot with uncertainties.xls’ template).  The uncertainty is then:

uncertainty in P = 2/largest P − smallest P

Stage 2

5.    For each of the rows in Results Table 2, calculate the extra time required to boil the water when ice is

present; this is the Ice Boiling Time minus the No-Ice Boiling Time. Record this in Results Table 2.

6.    Use Q = P*t and the power of your kettle (worked out in the previous stage) to determine the extra energy required in Joules to boil the ice-water. Record this in Results Table 2.

7.    Determine the Latent heat of Ice for each of your three rows, using Q  = mL. Record this in Results Table 2.

8.    Use your three values for Lw  to determine a latent heat with an uncertainty.

Extra Questions

1.    Why is it not safe to turn on the kettle without the element in the kettle being fully submerged?

2.    Why would you use a measuring jug rather than the markings on the side of the kettle to measure the volume of water?

3.    Why do you boil your kettle and then empty it before you start to record results?

4.    How accurate is your measured value for Lw? Consider your percentage uncertainty and whether or not the accepted value fits inside your absolute uncertainty range.

5.    Why is your value for Lw  incorrect? (Note: there is a deliberate flaw in the method for stage 2! Look for this).

Conclusion

Write a suitable conclusion that outlines what you have found.

What to submit

  You should type up your report to submit it to Turnitin. The report needs to be in a single file. It is better to submit a .pdf rather than a .docx Word document as Word documents can display different on different computers. Your tutor will mark the report as it is displayed on their machine. The report should include:

.    A statement of the aim of the investigation

.    A statement saying that you will follow the risk controls recommended in the risk assessment

.    A photograph of yourself and partner (if you had one), your student card, a piece of paper with the date, name and student number and the equipment.

.    A close up photo of your student card

.    All the results under the results section of the investigation

.     Details of your analysis of the results (ie, show how you did the calculations)

.    A copy of the graph you have plotted (you may use a screen shot of the graph)

.    Answers to the questions

.    A conclusion stating what you have found out

You report should not include:

.      The procedure (as this has been given to you, only comment if you make a change to this procedure)