E-39-0 Electric Charges and Electric Fields ONLINE
6-21-2020 Adapted from manual from Dr. Kam Chu

Objective
To study the electric field and electric potential around different charges.

Equipment
PhET Simulation:

https://phet.colorado.edu/sims/html/charges-and-fields/latest/charges-and-fields_en.html

Theory
There is an electric field surrounding a charge, in which another charge would experience an electric force. The strength of the electric field at a distance from a point charge is given by:

(1)

Where is the Coulomb Constant, q is the charge, and r is the distance from the charge. The unit vector points away from a positive charge, and towards a negative charge.

The electric potential due to a point charge is given by the equation:

(2)

Where is the electric potential (in volts), and is a scalar quantity.

In this Lab, we will use a PhET simulation to study the electric field and electric potential surrounding single and multiple point charges.

Procedure
Play with the simulation (Charges and Fields) and get oriented with all the different options. This should help you understand the lab better. Note that you have positive and negative point charges, an electric field sensor (yellow circle), a tape measure and a voltmeter, that also makes the equipotential lines. For each case, take a screenshot and attach with your report. You may alsoturn on ‘gridlines’ if desired. Each small square of the grid is 10 cm wide and high.

Activity 1: Electric Field Lines and Equipotential Lines

1: Have one positive and one negative charge placed symmetrically in the field. Get the Electric field lines. Use the voltmeter to draw about ten equipotential lines (Figure 1 shows a related situation with a few equipotential lines)

2: Repeat with both charges being negative.

3: Repeat with both charges being positive.

4. Repeat with 4 positive charges (on top of each other, to create 4q) and one negative charge.

5. Parallel Plates: Put a large number of positive charges in a straight row (to look like a solid line). Make a negative line in the same way (parallel to the first). As an example, see figure 2. Get the electric field lines and Equipotential Lines between and surrounding the parallel plates.

6. Attach screenshot of the simulations in your report.

Figure-1

Figure-1: Parallel “plates”.

ACTIVITY 2
1) Turn on ‘gridlines’.

2) Select positive point charge of any magnitude (you do this by placing the point charges on top of each other). Place the charge at the intersection of two thick gridlines, somewhere in the left half of the screen.

3) Use the tape measure and Voltmeter to find the voltage at different locations along the horizontal line on which the charge is placed. Enter values in Table 1.

4) Plot a graph in Excel between the voltage (y-axis) and the distance (x-axis).

5) Use Excel to determine the value of the Coulomb Constant (see eqn. (2). Find the percent error between the calculated and accepted values.

6) Use the tape measure and the yellow Electric Field sensor to measure the electric field at different distances in the horizontal direction from the charge. Enter the data in Table 2.

7) Plot a graph in Excel between the Electric Field (on y-axis) and distance (on x-axis)

8) Use Excel to determine the value of the Coulomb Constant (see eqn. (1)). Find the percent error between the calculated and accepted values.

9) Attach the screenshots, graphs and calculations to your report.

DATA
Table 1

Charge = _________

1

2

3

4

5

6

7

distance

voltage

Value of k found from the graph: ___________

Percent error in k: ________________

Table 2

Charge = __________

1

2

3

4

5

6

7

distance

Electric Field

Value of k found from the graph: ___________

Percent error in k: