ELECTRICAL ENERGY

Part I: Discovering Ohm's Law

"Electricity runs through the video, and I watch it from this hole I call home..."

John Mellencamp

Electrical energy is energy associated with the separation of charge, that is, a type of potential energy that results from the attraction or repulsion experienced by opposite or like charges, respectively. The quantity that reflects the magnitude of electrical energy is called potential, symbolized E, and is measured in units of volts, symbolized V (a volt is defined as 1 joule / coulomb). When separated charges are allowed to attract or repel one another, the result is the movement of charge; movement of charged entities is generally referred to as electricity. The rate at which charge is moving is called electrical current, symolized I, a quantity measured in units of amperes or amps, symolized A (an amp is defined as 1 coulomb / second). Charged entities usually encounter opposition to their movement (akin to frictional forces opposing movement of massive objects); the degree of this opposition depends on the medium through which the entities are moving and is called the medium's resistance, symbolized R. This quantity is measured in units of ohms, usually symbolized with an uppercase omega (an ohm is defined as 1 volt / amp)

Analogy is often made between these electrical quantities and the more familiar ones of water pressure and flow, where water pressure / potential and water flow / electrical current are analogous pairs. Imagine the relation between water pressure (how far open you turn the spigot handle) and water flow (how fast the water streams from the end of a garden hose attached to the spigot); the greater the water pressure, the greater the water flow. Likewise, for a given medium (a metal wire, for example), the greater the applied potential, the greater the electrical current. The mathematical relationship describing this phenomenon is known as Ohm's Law:

E = I x R

As with water flow, current flow may be harnessed to do work (cook food, dry laundry, etc.); electric companies make their money by charging the public to route this current flow through their homes and, as with the water company, they charge according to how much of their product you use. The rate at which any form of energy is used to do work is a quantity known as power, symbolized P, and measured in units of watts, symbolized W (a watt is defined as 1 joule / second). The relation between power and these other electrical quantities is

P = I x V

In this lab project, the relationships between power, potential, current and resistance are examined through measurements made using incandescent light bulbs.

Apparatus and Reagents

AC ammeter (250 mA) and voltmeter (1.5/15/150 V)
variable AC voltage supply
various wattage light bulbs and sockets
connecting wires and clips

Procedure

1. Prepare a sketch of the experimental apparatus assembled by the instructor in the prelab session, making sure that the sketch clearly shows how the various components are connected.

2. Assemble a similar apparatus in the laboratory, and have the instructor inspect it prior to plugging in the power supply.

3. For each wattage light bulb available:

a. record the relevant electrical information printed on the bulb top; and

b. make measurements of the voltages which result from applied currents of 50, 100, 150, 200 and 250 mA (adjusted via the power supply dial). Be sure to use the appropriate voltmeter range (1.5/15/150 V) for each bulb so the voltages may be most precisely measured.

current (A)	40 W voltage (V)	60 W voltage (V)	100 W voltage (V)	150 W voltage (V)
0.050		.	.	.
0.100	.	.	.	.
0.150	.	.	.	.
0.200	.	.	.	.
0.250	.	.	.	.

Data Treatment

Prepare plots of voltage (y-axis) versus current (x-axis) for each of the light bulbs examined (put all these plots on the same set of axes). What is the nature of the relation between voltage and current? What is the physical significance of these plots' slopes? How are these slopes related to the bulbs' power ratings (see part 3.a. above)?

ELECTRICAL ENERGY

Part II: Solar Cells

A photovoltaic cell (or "solar" cell) is a device constructed from semiconducting materials which serves to convert electromagnetic energy ("light") to electrical energy. These devices are used widely in society, for example, as power supplies for emergency telephones, calculators, etc. In this project, inexpensive solar cells will be used to examine this energy conversion phenomenon.

Apparatus and Reagents

AC (150 V) and DC (1.5 V) voltmeters
variable AC voltage supply
150 W light bulb and socket
photovoltaic cells

Procedure

Prepare a sketch of the experimental apparatus assembled by the instructor in the prelab session, making sure that the sketch clearly shows how the various components are connected.
Assemble a similar apparatus in the laboratory, and have the instructor inspect it prior to plugging in the power supply.
Measure the voltage produced by the photovoltaic cell as the voltage applied to the light source is increased, recording your measurement results in the table below. NOTE: Make these measurements as quickly as possible in order to minimize excessive heating of the cell by the light bulb.
Repeat step three for two photovoltaic cells connected "in series".

Bulb Voltage (V)	Single Cell Voltage (V)	Double Cell Voltage (V)
10	.	.
20	.	.
30	.	.
40	.	.
50	.	.
60	.	.
70	.	.
80	.	.
90	.	.
100	.	.

Data Treatment

Prepare plots of cell voltage (y-axis) versus bulb voltage (x-axis) for both the single- and double-cell configurations (both plots on the same set of axes). Describe the similarities and differences of the observed plots.