Download Lab 4: Resistive Heating

eeName: _____________________________________ Lab Section: _________ Group Number:
__________
Lab Partners: _________________________________________________________ Grade: ____________
Physics 203
Lab 4
Resistive Heating
Pre-lab
In this experiment, you will be heating water by sending current through a resistor. Read through the
introduction to the lab, so that you understand the outline of the experiment and the background physics.
Suppose you begin with room temperature water, and heat it up in the calorimeter. The calorimeter is, of
course, not perfectly insulated, so some heat will always be lost to the surroundings. This is a systematic
source of error, because the measured temperature change of the water will always be low, never high,
and this would not average out in repeated trials. That is not a good thing.
1) What could you do experimentally to compensate for this exchange of heat with the surroundings
to eliminate this systematic source of error?
You will not be able to measure the mass of the water by itself – it will need to be in some container
when weighed.
2) What measurements and calculations will you make to determine the mass of water? Consider
whether you should weigh the water in the calorimeter cup or some other container – will it matter?
3) Will the error in the mass of the water be the same as if you could measure its mass directly?
Greater? Less? Explain, and show the relevant error equation.
Electrical energy will be converted to thermal energy over an extended period of time in this experiment.
4) Given a constant rate of energy conversion (power), P, continuing for a time, t, write down an
equation for the total amount of energy converted to heat.
5) In this experiment, you will be applying a power of approximately 10 W to approximately 50 g
of water. How much time would you expect it to take to heat the water by 1oC? 10oC?
6) How often do you think you should take temperature readings to get a good idea of the
temperature vs time curve?
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Name: _____________________________________ Lab Section: _________ Group Number: __________
Lab Partners: _________________________________________________________ Grade: ____________
Physics 203
Lab 4
Resistive Heating
Introduction
Energy exists in many different forms, and physical processes often involve a conversion of one form of
energy into another form. In this experiment electrical energy is converted into thermal energy energy,
and by measuring the heat absorbed in a calorimetry experiment, we can determine the electrical energy
that is given off by a resistor.
Suppose that the energy generated in this resistor (heating coil) is measured by submerging the coil in a
calorimeter filled with water. As a result of heating by the resistor, the temperature of the water will rise.
This temperature rise may be measured so the heat transfer may be computed. An appropriate setup for
this experiment is shown in the figure. The calorimeter is partially filled with cold water. The heating
coil submerged in the water is connected to the power supply as shown. The voltmeter and ammeter
measure the potential difference across the coil, V, and the current through the coil, I, respectively.
When the circuit is closed, electrical energy in the heating coil is transferred as heat to the water in the
calorimeter. The amount of heat absorbed is given by the equation
Q = ( M w + M e ) c T
(1)
where Mw is the mass of water, Me is the mass-water-equivalent of the rest of the system, c is the specific
heat of water, and T is the rise in temperature of the water. The mass-water-equivalent of the heating
coil, electrodes, and thermometer probe and other things in the calorimeter (Me) may be taken as 3 g.
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In this experiment, the water will be heated by current flowing through a resistor. The rate at which
electrical energy is converted to thermal energy by the resistor is given by
P = IV
(2)
Procedure
Important:
1. Do not turn on the current unless the heating coil is submerged in water. It may burn out otherwise.
2. Keep the calorimeter cover dry.
Record the room temperature. Fill the calorimeter with cold water so that it covers the coil. Connect the
coil into the circuit. After inserting the thermometer probe you will be ready to take temperature
measurements.
You should start recording the temperature before you switch on the power supply and continue
throughout the process and for some time after turning off the electrical power to the resistor. You
should choose a reasonable time interval for measurement. You should adjust the power supply so that
the current and voltage yield a power of about 8-12 watts. Record V , I, and T as a function of time. Be
sure to gently stir the water while it is being heated, so that the temperature readings will be
representative of the whole volume of water. When you switch off the power supply, continue taking
measurements for a period of time until you feel you have enough data to determine the total heat
absorbed.
1) Did the temperature of the water continue to rise for a while after the power was turned off? Why
might this be?
Plot a curve of temperature vs. time. On this graph indicate room temperature and the exact times at
which the power supply was turned on and off.
2) What is the purpose of plotting the heating curve in this experiment? Would it suffice to take only
two temperature readings to determine T?
Determine the change in temperature of the water, and the amount of heat absorbed by the water using
eqn (1).
Calculate the rate of electrical energy conversion (power) using equation (2). Find the total amount of
electrical energy that was converted to thermal energy during the time the current was flowing through
the resistor.
3) Does your calculated amount of electrical energy converted by the resistor agree with your
calculated amount of heat absorbed by the water to within your measurement error? Explain.
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Name: _____________________________________ Lab Section: _________ Group Number: __________
Lab Partners: _________________________________________________________ Grade: ____________
Physics 203
Lab 4
Resistive Heating
Post-lab
1) Use your voltage and current values to determine the resistance of the heating coil.
2) If you were to use the same voltage on the power supply, but a higher-resistance coil, would the water
heat up more quickly, less quickly, or the same? Explain.
3) Did your current and voltage values vary during the heating? By how much? Do you think this is a
significant source of error?
4) If your current and voltage readings were to vary so much that you could not take the power, P, as
approximately constant, how would you determine the total electrical energy converted to heat from the
voltage and current values?
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