Spring 2005 Name____________________

Physical
Science Experiment

Electric
Power

All questions on this first
page must be completed before coming to lab or you will lose points on your lab
score!

Use your text and this lab to
find out what each of the following represents.

1. P

2. I

3. R

4. t

5. H

6.

7.

8.

9. What is the unit of measurement for current? ___________________ What is its symbol? ______

10. What is the unit of measurement for power? ___________________ What is its symbol? ______

11, What is the unit of measurement for resistance? _________________ What is its symbol? ______

12. What is the unit of measurement for voltage? ___________________ What is its symbol? ______

13. In equation (4) of your lab, what does each
of the following symbols stand for?

(a) m_{c}

(b) s_{w}

_{ }

(c) s_{c}

(d) m_{w}

(e) T_{i}

(f) T_{f}

What special caution are you
asked to observe in part 4 of this experiment?

**Objectives:**

To learn how to set up
electric circuits from diagrams.

To show how electric energy
is converted to heat energy.

To learn how to use the
equations: (**Note that these equation numbers are used in the experiment.)**

**(1)
****P = I V**

**(2)
****P = I ^{2}
R**

**(3)
****V = I R**

** Theory: **In the
experiment on heats of fusion and evaporation we learned how to calculate the
amount of energy necessary to raise the temperature of water in a
calorimeter. We will compare this energy
with the electrical energy put into the system by the electric heater.

Power is defined as P = work / time, to the amount of
work (energy) put into the system is given by H (heat energy) = power x time.
(H = P x t) As the power is obtained
from equations (1) and (2), we can find the total electrical energy put into
the system by multiplying this by the time.

The heat energy necessary to raise the temperature of a
can full of water from an initial temperature of T_{i} to a final
temperature of T_{f} is given by:

(4)
**H = (m _{w}s_{w}
+ m_{c}s_{c})(T_{f} – T_{i}) **where m

is the heater is the ammeter is the voltmeter connection points

After completing the first three
parts of the next section you are to try to set up the apparatus as shown on
the above figure. You may ask for help
when you believe you have the proper
connections. DO NOT PLUG IN THE POWER
SUPPLY UNTILL CHECKED BY THE INSTRUCTOR.

PRECEDURE:

Be
sure that the correct units are given for all the measurements you make. Failure to do this will cost you points.

1. Record the specific heat of your inner cup. If nothing is stamped on it check with the
instructor, it is probably aluminum (0.22 cal/(g Cº) S_{c} = ____________________

2.
Record the weight of the empty and dry inner cup. M_{c} = ______________________

- Go to the front of the room and use the cold
faucet (closest to the blackboard) to fill the cup ¾ full and record the
weight of the can plus water = _____________________

Now
subtract the weight of the empty can (item 2) to obtain the weight of the water

m_{w} =
_________________________

4. Go back to page 2 and look at the
diagram. Make the connections as
shown. Call the instructor to verify and
to obtain a stopwatch. UNDER NO CERCUMSTANCES ARE YOU TO
PLUG IN THE POWER SUPPLY UNTIL TOLD TO DO SO BY THE INSTRUCTOR.

- Record the temperature of the water in the
can. T
_{i}= ___________________

6. Your instructor will turn
on the apparatus and tell you when to start the stopwatch. DO NOT TURN ON THE APPARATUS YOURSELF. You may turn it off when it reaches 30ºC,
be sure you record the time at which the current is turned off after stirring
the can.

7. Record the current and the voltage from the
meters. Have the instructor verify your
readings.

I = _______________ V = _______________

8. From equation 3 on the
previous page calculate the resistance of your heater.

R =

9. Use equation (1) to
calculate the power of your heater.

P =

10. Now calculate the power using equation (2). If this is not quite close to the result you obtained in section 9, you have made a mistake. When you finish have the instructor verify your results.

P =

11. When the temperature of the water reaches
30ºC, turn off the current and pull the plug) from the socket (and record the
exact time t = ________________________.
Stir the water and then record the final temperature T_{f} =
_________________

12.
To obtain the
heat energy that entered the system, multiply the time (in seconds) by the
power of your heater. Give the units of
your answer.

H =

Change these to calories by
using the fact that there are 4.2 joules/calorie

Heat energy in calories =
_______________________________________

13.
Use equation (4)
to calculate the amount of heat energy needed to raise the temperature of the
water and can. First write the formula,
then the equation with your data, then show your answer.

H=

14. We will now calculate the
percent difference between the amount of heat supplied by the electricity and
that the water received. Be sure to use
the units of calories for both heats!

Percent difference = __(heat put in by electricity) – (heat
received by the water)__ =__ Ans(12) – Ans (13)__

heat put in by
electricity Ans(12)

Percent
difference =

15. Reread the paragraph about power on page 2
and calculate the number of joules used by a 100-watt light bulb in an hour.