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?

hw

due Monday (C) and Tuesday (E):

1. Write and solve two different Coulomb's law problems, solving for something different in each case.

2. Research the phenomenon of lightning, jotting down some brief notes about the topic.

3. Define voltage.

HW

This is due for C block next Thursday. For E block, next Friday (do the previous one for wednesday).

1. What is Coulomb's law? Give a description and a mathematical expression - don't forget to identify what the variables represent.

2. If the charge of one proton is 1.6 x 10^-19 C, how many protons are required to make a coulomb of charge?

3. What is the charge of a balanced hydrogen atom? Does this answer change for deuterium (an extra neutron in the nucleus)?

4. What is the charge of a helium atom that is missing an electron?

5. What is the force that exists between two 5 coulomb charges that are 0.05 m apart?

6. Coulomb's law is an "inverse square" law. What does that mean? What happens to the force between charged ifmthe distance between them is doubled? Tripled? Halved?

Electricity HW

Questions to answer in your notes. Apologies for not posting earlier.

1. What is charge?
2. What is the unit of charge?
3. What is the charge on an electron? How about a proton?
4. How do we generally charge something?
5. How do you suppose the rotating stick demo worked?
6. Look up Coulomb's law. Describe briefly.
7. Can protons, electrons or neutrons be broken into smaller particles? How so?

Test review

HW for Weds/Thurs

Create a study sheet for the test. Things you believe will be important to know for the test. Test days are:

Friday - C block
Monday - E block

Some questions to try:

1. A 10-cm focal length lens is used in an experiment. An object is placed 30-cm away from it. Find the following:

a. location of image
b. type (R/V) of image
c. magnification of image
d. up or down?
e. What type of lens is this?
f. How would this problem be different if the focal length of the lens were -10 cm?

2. (Review on Snell's Law)

A light (wavelength of 700 nm; 700 x 10^-9 m) hits a rectangular piece of plastic, striking it an angle of 40 degrees (with respect to a normal line). The angle of refraction inside is 17 degrees. Find the following:

a. index of refraction of this plastic
b. speed of light OUTSIDE block
c. frequency of light OUTSIDE BLOCK
d. speed of light OUTSIDE BLOCK
e. speed of light INSIDE BLOCK
f. frequency and wavelength INSIDE BLOCK
g. critical angle of this plastic
h. picture to represent this problem

3. Consider the recent lab. You have a diffraction grating, 500 slits/mm. A 650-nm laser is directed at it. Find the following:

a. d (in m), knowing that the grating has 500 slits/mm
b. angle of diffraction for n=1
c. How many images could be seen? (This is a tricky one.)

Thanks!

HW

Read about holography - define it and find out aout practical applications

Hw

As mentioned in class -

Interpret/research the diffraction equation.

n lambda = d sin(Q)