Liceo Scientifico Statale Giordano Bruno Venezia-Mestre Physics
course Where is the heat? Teacher Francesco Minosso
Slide 2
Heat and electric charge The electrical work L done by a charge
q flowing through a heater across a voltage V during the time t is:
and that it is related to the heat Q produced by the heater by the
formula
Slide 3
Where is the heat? heat work Is the relationship between heat
and work general? To find the answer, you will measure the heat
produced in an electric motor working in different ways.
Slide 4
This project work is developed in three steps: a) heat produced
in an electric motor in neutral b) heat produced in an electric
motor when it lifts an object c) heat produced in an electric motor
when it lifts and drops an object heat Before starting, you will
need to measure the heat capacity capacity of the electric motor.
Where is the heat?
Slide 5
1 electric motor in an insulating box 1 chronometer 1
thermometer: min -20C max +50C T = 0.25C Equipment and
Materials
Slide 6
1 ammeter, max 10 A 1 voltmeter, max 20 V 4 batteries with four
cells, connected in parallel some wires Equipment and
Materials
Slide 7
Motor kit on a wooden support: 1 pulley connected to the motor
by a belt drive 1 peg 1 jerrycan partially filled by water
Equipment and Materials
Slide 8
Heat capacity of the electric motor When the motor is connected
to the batteries, the electric charge flows through the winding. If
you stop the axis with a peg, the motor behaves like a heater made
of copper wires. The only effect will be the heat production whose
amount equals the electrical work, if you express both the heat
produced and the work done by means of the same units.
Slide 9
Heat capacity of the electric motor By measuring the
temperature increase of the motor due to the electrical work, heat
capacity you can calculate its heat capacity.
Slide 10
Heat capacity of the electric motor Discuss your opinions a few
minutes with your mates to get the heat capacity of the about a
procedure to get the heat capacity of the electric motor and write
on your lab notebook: a sketch of the circuit the connections of
the multimeters needed to measure the electrical work a list of
materials and instruments a data collecting procedure tables and
formulas useful to calculate the heat capacity of the electric
motor
Slide 11
Heat capacity of the electric motor Write on the blackboard
some higlights about the previous discussion
Slide 12
Heat capacity of the electric motor Here is the sketch of the
lab apparatus the ammeter is connected in series the voltmeter is
connected in parallel the batteries are connected in parallel in
order to have a stable current
Slide 13
Heat capacity of the electric motor Here is a sketch to connect
the batteries and the formulas to solve the problem
Slide 14
Heat capacity of the electric motor This is a table you can use
to collect data
Slide 15
Heat capacity of the electric motor Look at the video where the
measurements to determine the heat capacity of an electric motor
are carried out. Collect the data on your lab notebook and carry
out the calculations to find the heat capacity of the motor. Start
the video Where is the heat? part one.
Slide 16
This is a table with the collected data Heat capacity of the
electric motor
Slide 17
Start calculating the average current and voltage Then
calculate the motor heat capacity Heat capacity of the electric
motor Remember to write the calculations and the considerations in
your notebook.
Slide 18
Where is the heat? Remember you are studying whether the
relation L = Q (where L is the elctrical work and Q the heat
produced) is valid for the motor working in three different
ways.
Slide 19
The motor in neutral Now the first step of your project work:
connect a pulley to the motor by a belt drive. How much work is
done in order to make the pulley rotate? How much heat is produced
while the pulley rotates? Which way can you measure heat and
work?
Slide 20
The motor in neutral Discuss your opinions with your mates
about which way you can do to verify the L = Q equation on your lab
notebook: and write on your lab notebook: the connections of the
multimeters needed to measure the electrical work a list of
materials, instruments and a sketch of the circuit a data
collecting and managing procedure how to measure the heat produced
and the work what are you expecting about L = Q equation?
Slide 21
Write on the blackboard some higlights about the previous
discussion The motor in neutral
Slide 22
Look at the video where the measurements to compare the
electrical work and the heat produced are carried out when the
motor works in neutral. Collect the data on your lab notebook and
carry out the calculations to find electrical work and heat. Start
the video Where is the heat? part two.
Slide 23
The motor in neutral This is a table with the collected data
The current and voltage data are negative because we inverted, by
mistake, the connections between the batteries and the motor. You
can use the absolute values for calulations.
Slide 24
The motor in neutral Notice that the batteries contain four
cells in this and next experiments to maintain the current constant
all the time. Notice that data are not collected at regular time
intervals for the structure of the clip. Now calculate the average
current and voltage
Slide 25
The motor in neutral Then calculate the electrical work and the
heat produced in the motor and compare them: did you expect these
results? Calculate the percentage difference between work and heat
and notice that it is about 3%. What can you say about the L = Q
equation? Remember to write the calculations and the remarks in
your notebook.
Slide 26
The motor lifts an object Now the second step of the project
work: making the motor lift an object. How much work is done in
order to lift the object? How much heat is produced while the
objetc is lifted? What do you think about the ratio between work
and heat? Is it greater or less than 1? Discuss your opinions with
your mates and write reasons in your lab notebook.
Slide 27
Write on the blackboard some higlights about the previous
discussion The motor lifts an object
Slide 28
Look at the video where the measurements to compare the
electrical work and the heat produced are carried out when the
motor lifts an object. Collect the data on your lab notebook and
carry out the calculations to find electrical work and heat. Start
the video Where is the heat? part three.
Slide 29
The motor lifts an object This is a table with the collected
data Notice that the data are not collected at regular time
intervals for the structure of the clip.
Slide 30
The motor lifts an object Now calculate the average current and
voltage Then the electrical work and the heat produced Remember to
write the calculations in your notebook.
Slide 31
The motor lifts an object Calculate the percentage difference
between work and heat and notice that it is more than 30%! Remember
to write your remarks in your notebook. 4) What can you say about
the L = Q equation? 1) Did you expect a result like this? 3) Can
you explain this large lack of heat? 2) Where is the lacking heat?
Discuss your opinions with your mates
Slide 32
Write on the blackboard some higlights about the previous
discussion The motor lifts an object
Slide 33
Now the third step of the project work: making the motor lift
and drop an object. How much work is done in order to lift the
object? How much heat is produced while the objetc is lifted? What
do you think about the ratio between work and heat? Is it greater
or less than 1? Discuss your opinions with your mates and write
reasons in your lab notebook. The motor lifts and drops an
object
Slide 34
Write on the blackboard some higlights about the previous
discussion The motor lifts and drops an object
Slide 35
Look at the video where the measurements to compare the
electrical work and the heat produced are carried out when the
motor lifts and drops an object. Collect the data on your lab
notebook and carry out the calculations to find electrical work and
heat. Start the video Where is the heat? part four.
Slide 36
The motor lifts and drops an object This is a table with the
collected data
Slide 37
The motor lifts and drops an object Now calculate the average
current and voltage Then the electrical work and the heat
produced
Slide 38
The motor lifts and drops an object Calculate the percentage
difference between work and heat and notice that it is about 1%. 4)
What can you say about the L = Q equation? And about the lacking
heat? 1) Did you expect a result like this? 3) What happens if the
motor drops the object to the floor again? 2) Is the lacking heat
of the past esperiment connected with the fact that the motor lifts
an object? Discuss your opinions with your mates and write them in
your lab notebook.
Slide 39
Write on the blackboard some higlights about the previous
discussion The motor lifts and drops an object
Slide 40
Conclusions You know that the heat produced in a heater depends
only on the electrical work done, and it is the same number if you
use the joules as unit. Then: 3) the lacking heat is found again if
the object drops slowly from the same height at which it was lifted
2) in a motor lifting an object the electrical work produces less
heat, therefore L > Q 1)in a motor working in neutral the heat
produced equals the electrical work done, therefore L = Q The
lacking heat is not missing because by lifting an object we create
a situation which allows us to recover it.
Slide 41
Conclusions A given amount of work can either lift a mass or
heat up an object, and we can trade one change for the other by
letting a falling mass to produce heat. We say that both changes in
energy are alike. Since lifting a mass and heating an object are
different processes, we give the two energies different names:
gravitational potential energy and thermal energy. Maybe you know
that gravitational potential energy depends on the mass lifted and
the height reached by the mass. If not, you can verify it in
another project work.
Slide 42
Conclusions In other words, we can say that a fraction of the
electrical work done to run the motor has moved to lift the
container while the other one has heated up the motor. Maybe you
know that the relationship electrical work entering the motor =
work done by the motor + heat produced in the motor is known as a
statement of the energy conservation principle. If not, you can
verify it in another project work.
