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 = I² 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_ws_w + m_cs_c)(T_f – T_i) where m_w is the mass of water, s_w is the specific heat of water (one calorie/gram), m_c is the mass of the can and s_c the specific heat of the metal from which the can is made.

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 = ______________________

3. 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.

5. 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.