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# 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.

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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).

### 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.