Electric Fields - For Wednesday (C) and Thursday (E)

The electric fields lab will be started in class. The problems that follow are for practice. Make sure that you understand the basics of charge, Coulomb's law, proton neutron and electron charge, etc.

Electric Fields

In this lab, you will investigate electric field lines. Recall the sign convention:

Field lines point away from positive charges

Field lines point toward negative charges

With this in mind, draw field diagrams for each of the scenarios described below, as well as some of your own:

• Single positive charge

• Single negative charge

• One negative and one positive charge (equal magnitude of charge)

• Two negative charges and/or two positive charges

• One negative and one positive charge (unequal charges) – try a couple different configurations, with at least one attempt having very different magnitude than the other

• Multiple charges in weird configurations – go for broke here, and make several drawings (at least five)

The applets below may prove useful – or at least cool to play around with. Also feel free to do a Google search for ‘E-field’ and ‘applet’. This should prove fruitful.

Also, don’t confuse E-field lines with Equipotential lines (lines of constant electric potential, or voltage). Some applets will display both if desired.

http://www.cco.caltech.edu/~phys1/java/phys1/EField/EField.html

http://falstad.com/vector2de/

http://falstad.com/vector3de/

http://www.cbu.edu/~jvarrian/applets/efield1/elefi_z.htm

Electrostatics practice Problems.

1. How many protons are required to make a charge of 4 C? How many electrons are required to make a charge of -2.5 C?

2. Explain why a charged balloon sticks to a wall. Also explain how the rotating meter stick experiment worked.

3. If a Van de Graaf generator has an excess charge of -1 μC (that’s -1x10-6 C), how many excess electrons would you say it has?

4. On a Van de Graaf generator, or any conductor for that matter, where does the excess charge reside? What is the charge inside a Van de Graaf generator, or any hollow conductor?

5. What is the unit of charge?

6. What is Coulomb’s law?

7. What does “inverse square law” mean?

8. What is the force between two charges (+0.025 C and – 0.05 C), if they are 0.001 m apart?

9. What exactly are electric fields (E-fields)? Draw E-fields for a positive charge, negative charge, and 2 positive (or negative) charges.

10. For the scenario above (8), draw an appropriate E-field diagram.

11. Two equal charges experience a 10 N repulsive force when they are 0.05 m apart. What is the value of each charge? What is the sign of each charge? Can you tell?