PhET Electric Field Lab

Directions - Answer the questions in bold with the letters - label them on your lab when you turn it in

Part 1 - Introduction - E field and Equipotential Lines.

1. Open the Charges and Fields PhET

2. Check "Electric Field" and "Grid" and drag a positive charge into the middle of the screen onto an intersection of major gridlines. The simulation now shows the direction and magnitude of the electric field on the screen. The arrows point in the direction, and the magnitude is the intensity or darkness of the arrow.

3. A. Which way does the electric field point near a positive charge?

4. Grab the "Equipotential" tool (blue, crosshairs, 0.0 V) , and move it around until you find a voltage close to 5.0 V. (5.031 V is just fine - don't go crazy trying to get it exactly 5.00) Then click the little pencil. This will find every point that is at 5.0V making a green line called an Equipotential Line. These are points that are all at the same potential. Do the same for 10 V, 15 V, 20 V, and 25 V. You will get a series of concentric circles.

5. B. What is happening to the spacing as the voltage increases? (This is because V is non linear, V = kq/r)

6. One way to visualize what you have here is to imagine that the screen is a sheet of rubber, and a positive charge pushes the sheet up, and a negative, down. The green Equipotential lines are lines that are the same "height" on this sheet of rubber. A bit like topo lines on a map. The lines get closer because the sheet of rubber is getting steeper because the slope of sheet is the electric field, and that is getting bigger and bigger as you approach the charge.

7. Notice that the Equipotential lines are perpendicular to the Electric field . Always. (The same is true for topographical lines - if you are on a trail that is traversing sloping ground without changing elevation, downhill is either to your right or left - think about it)

Part 2 - Like Charges

1. Click the yellow/orange button in the lower right to start over.

2. Place two negative charges 4 major gridlines apart and check "Electric Field" and "Grid"

3. C. Which way does the electric field point near a negative charge? Draw a sketch of the field lines you see.

4. Do the same with two positive charges:

5. D. Draw a sketch of the field lines you see. (check "Electric Field")

6. Use the Equipotential tool to make contour lines at 5V, 10 V, 15 V, 20 V, and 25 V. (Again - get as close as you quickly can - within 0.1 or 0.2 V is fine) (The 20 V and 25 V lines will make little islands around the positive charges.)

7. What you are looking at is two peaks with a saddle between them, and the voltage lines are like topo contour lines on a map.

Part 3 - An Electric Dipole

1. Click the yellow/orange button in the lower right to start over.

2. Check "Electric Field" and "Grid" and drag a positive charge onto a major intersection of gridlines, and a negative charge onto a major intersection 4 major gridlines to the side:

3. E. Draw a rough sketch of the field lines you see

4. Now use the equipotential tool and mark equipotential lines at approximately -15 V, -10 V, -5 V, 0 V, +5 V, +10 V, +15 V. Notice that the circles are now non-concentric. You can imagine as you look at the picture that the positive charge is sticking up, and the negative charge is pushing the sheet of rubber down. The equipotentials you marked are all the same "height" like topographical lines on a map.

5. F. What would be the shape of the Equipotential line for 0 V if you could have gotten it at exactly 0.000000V? (Chances are you didn't) What should the shape be?

Part 4 - Parallel Plates

1. Click the yellow/orange button in the lower right to start over.

2. Check "Electric Field" and "Grid" and build a couple parallel plates that are 4 major gridlines apart out of 14 + and 14 - charges:

3. Use the Equipotential tool to mark the voltages +60 V, +40 V, +20 V, 0 V, -20 V, -40 V, and -60 V. Again - just get close to those voltages - don't spend a lot of time trying to get them exact. You can find all these voltages between the plates, but they are more spaced out in other regions, so if you are a perfectionist, do the |60| |40| and |20| to the side of the plates (not between them) and find the 0 volt line above or below the plates where the voltage gradient is not so steep...

4. Parallel plates are used to create a uniform field. Notice that between our "plates" the equipotential lines are all parallel and more or less evenly spaced. This is what a uniform electric field looks like. Imagine that we had a sheet of rubber, and we took something like a book or a whiteboard, and pushed the sheet up where the positive charges are, and we pushed the sheet down along the line where the negatives are, there would be a region between the two that would be flat and tilted.

5. G. What is the relationship between the direction of the Electric Field and the direction of the the Equipotential lines? (What is the angle in general between the two? (It's easy to see between the plates, but true everywhere)

6. Notice that the electric field is not as uniform at the edges of the plates. The longer the plates are, and the closer they are, the more uniform the field you get.

That's it - Turn in your answers to questions A-G as a photo. (Or if you can do screen shots, a document)