Labs from Chicago, Winter 1994 :
Electrical Power.

Dr. Rich Kron, Dr. Heidi Newberg, and Luisa Rebull
Labs written for the CARA Space Explorers, Winter 1994

This is meant to be handed out to the students.

I. Introduction

Energy: Energy is a word we use in our everyday lives. In science, the word has a similar, but more specific definition. LetŐs use an example to illustrate the different way that scientists use the word energy. In the winter, the temperature drops below a comfortable living temperature, so people want to heat their homes. So, they buy gas, oil, coal, or electricity from a utility company. Gas, oil, and coal can be burned to produce heat. Electricity will heat resistors, like the filaments in your toaster that get red-hot. In any case, the energy stored in gas, oil, coal, and electricity can be converted to heat in your home.

Now, consumers talk about using energy up - a concept that is foreign to a scientist. You see, from the consumerŐs point of view, he or she must keep buying more energy to keep the house warm. The electric or gas company sends bills in proportion to how much gas or electricity was used up. A scientist would agree that the gas or electrical energy was used up, but would point out that the energy has just been converted to another form. It is now heat energy. Since the house is not perfectly insulated, the heat escapes from the house and goes outside. The consumer no longer has the gas or electricity, no longer has a warm house, and cannot tell the difference in the temperature outside, since the outside is so big. So, the consumer says the energy has been "used up." The scientist says the energy is still there, it has just been converted to a form that is not useful to the consumer.

We have now discussed the energy that is stored in gas, coal and oil (this energy of chemical bonds), electrical energy, and heat. Other forms of energy are kinetic energy (energy of motion), mechanical energy (energy stored in springs), and gravitational potential energy. All forms of waves, (including sound waves and light waves) carry energy. There are other more esoteric forms of energy, but the important thing to know is that energy can be converted from one form to another, and that no energy is ever lost in the process.

Power: Power is the rate of energy conversion. For example, suppose you have a high-powered space heater. It does not contain any energy itself, but it has the capacity to convert electrical power into heat at a very fast rate. A lower powered space heater will heat the room too, but it will take much longer. The total energy bill might be the same for both heaters (this depends on the efficiency, not the power rating). But the high power heater will use electricity faster, and heat the room faster. Written as an equation,

power = energy / time,

or P=E/t.

II. The Experiment

You will be assigned two resistors (between one and ten ohms) and two voltages.

Fill the dewar up with 150 ml of water. The water should be at room temperature..

Hook the resistor up to the variable power supply, but don't turn it on yet..

Place the resistor and the thermometer in the water.

Data-taking for this lab takes about 20 minutes, so it is important to make your measurements accurately and consistantly the first time. What you will do is set the power supply to a pre- determined voltage, and make one measurement temperature measurement every minute for ten minutes. It might help to gently swirl the water in the dewar before each measurement so that the temperature is even throughout the water. Repeat this experiment for your other combination of voltage and resistance. To save time, make your graphs at the same time you are taking your data.

Voltage	Time (min)		Temperature

_________	1			___________

		2			___________

		3			___________

		4			___________

		5			___________

		6			___________

		7			___________

		8			___________

		9			___________

		10			___________


_________	1			___________

		2			___________

		3			___________

		4			___________

		5			___________

		6			___________

		7			___________

		8			___________

		9			___________

		10			___________
Now, make two graphs, one for each experiment. You should plot temperature on the y-axis (vertical) and time on the x-axis.

Draw a line which is the best fit to the data.

Measure the slope of the best fit line, and fill in the following chart:

Voltage		Resistance		Degrees/min

____________	____________	____________

____________	____________	____________
Label the graphs with the voltage, resistance, and degrees per minute measured. Then, tape your graphs to the blackboard so we can compare them with graphs done by others.

Important Disclaimers and Caveats


Go back to the Chicago Winter 1994 Electronics home page.