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INTRODUCTION

    In this lab you will measure the electric field around two oppositely charged electrodes (an electric dipole) at enough points to map the field.

BACKGROUND

    The power supply pushes electric charge onto one electrode and pulls it from the other, making one electrode positively charged relative to the other. There is an electric field caused by these charges, which pulls on the electrons in the (conducting) carbon paper, causing them to move. The force on the electrons is:

    The electrons in the carbon paper drift slowly in a direction opposite to the field (why?). The power supply remains connected to replenish the (–) charge as it seeps away from the negative electrode.

    An electron traveling through a displacement from one point in the field to another in the field will experience a change in potential energy equal to

    We can measure the potential energy per electron by measuring the voltage between two points separated by a distance d. The voltage between two points is defined as

so we can find between any 2 points as

or

, with direction determined from θ.

    We can use this relation to map the electric field around a dipole.

EXPERIMENT

1. We will use a multimeter throughout the semester to measure voltage, current, and resistance in circuits and their components. Familiarize yourself with the operation of the multimeter by measuring the voltage of i) a battery, ii) the DC power panel, and iii) the AC power outlet. Be sure that the meter is properly set to read DC or AC voltage; if you're not sure which you should use, ask for assistance.

2. Your circuit is wired in the following way:

   

   Make sure you can identify the various components of the circuit. The probes are 2 pencil points permanently mounted a distance d apart. Use the probe with a red "dot" on it as your reference point, and measure all voltages with respect to that probe, as shown in the figures below. Be sure that your hand does not rest on the conducting paper while measuring the voltage!

3. Check that the electrode nuts on the board are finger tight.

4. Draw a full-scale copy of the electrodes and carbon paper on centimeter graph paper, to use for recording your results. Do NOT write on the carbon paper.

5. Your instructor will show you how to use the voltmeter. Turn it on and move the probes around, getting a sense of how the potential varies at different positions in the field. What happens when you reverse the probe points?

6. Measure the voltage difference for at least 10 different positions in the field, by determining E_x and E_y as shown below, and calculating magnitude and direction of :

Measuring Ex	Measuring Ey

(Note: Since the electric field is proportional to the voltage, you can simply plot your voltage measurements!). Create a data table using the headers below, calculate θ, and then transfer your calculated to the graph paper copy of the board. Be sure to indicate the scale you are using (a good scale is 1 volt = 2 cm).

Coordinates:		Ex (volts)	Ey (volts)	E (volts)	θ	Escale (cm)
X	Y

7. Check your calculations for 2 or 3 positions by holding the grounded side of the probe at a constant position and rotating the + side until you get the maximum voltage reading. This will give you the direction and magnitude of at that position.

• Your conclusion should briefly summarize what you did, and what the results were. Compare your picture to figure E2.4 of Moore.

Return to Physics 152

©	St. Lawrence University	Department of Physics

Revised: 25 Jun 2003

Canton, NY 13617