Electric fields
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Chapter 21 PowerPoint PPT Presentation


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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.

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Chapter 21

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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

  • 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

  • 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

  • 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 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

  • 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

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…

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

  • 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

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


Electric Potential Difference -- continued

  • 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

  • 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 ∆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

  • Fine, charged oil

    particles were

    sprayed between

    two charged plates.

  • Electric field was

    adjusted so the particles were suspended between the plates


Millikan -- continued

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


Millikan’s Results

  • Millikan

  • Millikan is credited with finding


Electric Fields Near Conductors

  • 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

  • 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).


What is the point (get it?) of a lightning rod?


Capacitors

  • Provide a way to store electrical energy.

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


  • Put an insulating layer in between to keep the electrons from jumping from one plate to the other.

  • If a wire is placed between the two of them, electrons will flow until the two plates are charged equally.

Insulator


  • Capacitance = net charge on one plate/potential difference between the plates.


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