1 / 25

# Chapter 21 - PowerPoint PPT Presentation

Electric Fields. Chapter 21. Electric Fields. A charge creates an electric field around it in all directions A second charge placed in that field will interact. Compare this to the gravitational field. Electric Fields -- Continued.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about ' Chapter 21' - nolen

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

### Chapter 21

• A charge creates an electric field around it in all directions

• A second charge placed in that field will interact.

• Compare this to the gravitational field.

• A satellite interacts with the gravitational field without touching the planet.

• Likewise, an electron interacts with the positive field created by the nucleus of an atom.

Let’s say you want to measure the electric field at different points around a charge.

• Always use a positive test charge, q.

• Draw vector lines showing size and direction of force. These are called “force lines”

Field Lines different points around a charge.

• Once the force lines are connected, they become “field lines” and they go to infinity.

• They always indicate the force on a positive test charge (so they begin at a positive charge and end at a negative charge).

Field Lines -- Continued different points around a charge.

• The spacing between the field lines shows the strength of the field

• The magnitude of the electric field is a vector measuring the force per unit charge.

Van de different points around a charge.Graaff Generator

• + charge builds up on comb at bottom.

• Charges move up insulated belt to top.

• Charges spread around sphere at top (why?).

Energy & Electric Potential different points around a charge.

• Remember gravitational potential energy from Newtonian mechanics?

• The larger the distance of something from Earth the more potential energy it has.

• If there were no gravity, there would be no GPE (the object would not “want” to fall to earth)

BB different points around a charge.

Bowling Ball

• Bowling ball has more GPE because…

• Could say that each BB-sized portion of the bowling ball has the same potential as the BB

• So PE/mass = “potential”. We don’t use this in Newtonian mechanics but we do when talking about electricity.

h

Now consider an electric field… different points around a charge.

B

A

• It would take energy to move a + charge from A to B.

• Amount of energy required to move charge is proportional to

Battery

Electric Potential Difference different points around a charge.

• Remember that work done on an object = F●d

• In the electric world…need to know the “electric potential difference” (∆V) which is the work required to move a charge from one point to another divided by the magnitude of the charge.

Units:

Electric Potential Difference -- continued different points around a charge.

• Imagine you have a negative charge with a positive test charge nearby

Electric Potential Difference -- continued different points around a charge.

• Which direction does the positive test charge want to move?

• If you moved the test charge away from the negative charge would you be doing work on it?

Voltage vs. Volts different points around a charge.

• The change in electric potential (electric potential difference) is called the “voltage”

• We can only measure the difference between electric potential (thus the ∆ in ∆V).

• The units for voltage are volts

Imagine two oppositely charged plates and the electric field they would create

• Except at the ends, the field would be the same between the plates.

• Move a test charge a distance, d, against the field direction.

• W=F●d

• Remember that they would create∆V = W/q = Fd/q = (F/q)d

• F/q is electric field strength so…

The potential difference in a uniform field is field strength ●distance.

Millikan’s Oil Drop Experiment they would create

• Fine, charged oil

particles were

sprayed between

two charged plates.

• Electric field was

adjusted so the particles were suspended between the plates

Millikan -- continued they would create

• Millikan used E=∆V/d to determine the electric field strength.

Millikan’s Results they would create

• Millikan

• Millikan is credited with finding

Electric Fields Near Conductors they would create

• Remember that electrons are free to move around on a conductor.

• Electrons will move around a conductor until they are as far apart as possible.

• If a closed, metal conductor (i.e. car) becomes charged, the charges are all concentrated on the outside of the conductor and the electric field inside the conductor is zero.

Charges on Irregular Surfaces they would create

• On an irregular surface, electrons will be closer together on the “pointy parts” so they will become more concentrated.

• If that part gets close to another object, a discharge may occur (and a spark created).

Capacitors they would create

• Provide a way to store electrical energy.

• Imagine taking two conducting plates and charging them oppositely (pushing electrons from one to the other).

Insulator