Chapter 21 Electric Field

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Chapter 21.1 The Electric Field

• Define an electric field• Solve problems relating to charge, electric

fields, and forces.• Diagram electric field• Energy and Electric potential• Storing charges: Capacitor

The Electric Field

Definition of the electric field:

Here, q0 is a “test charge” – it serves to allow the electric force to be measured, but is not large enough to create a significant force on any other charges.

The Electric Field

If we know the electric field, we can calculate the force on any charge:

The direction of the force depends on the sign of the charge :

for a positive charge – the same direction

for a negative - opposite

Example

• An electric field is measured using a positive test charge of 3.0 μC. It experiences a force of 0.24 N at an angle of 15 degree north of east. What are the magnitude and direction of the electric field strength at the location of the test charge?

The Electric Field

The electric field of a point charge points radially away from a positive charge and towards a negative one.

The Electric Field

Just as electric forces can be superposed, electric fields can as well.

Electric Field LinesElectric field line is the lines to represent the

actual field in the space around charge.

EF lines:

1. Start at positive charges or infinity

2. End at negative charges or infinity

3. Are more dense where the field is stronger.

4. Always perpendicular to the surface of a conductor.

5. EF is stronger where the surface is more sharply curved.

Electric Field Lines

The charge on the right is twice the magnitude of the charge on the left (and opposite in sign), so there are twice as many field lines, and they point towards the charge rather than away from it.

Electric Field LinesCombinations of charges. Note that, while the lines are less dense where the field is weaker, the field is not necessarily zero where there are no lines. In fact, there is only one point within the figures below where the field is zero – can you find it?

Electric Field Lines

Shielding and Charge by Induction

When electric charges are at rest, the electric field within a conductor is zero.

Chapter 21.2

Electric Potential and Electric Potential Energy

•Electric Potential

•Electric Potential Difference

• Electric Potential Energy

• Equipotential Surfaces and the Electric Field

Electric potential energy and Electric potential

• EPE is the negative work to move an electric charge to an electric field.

• In electricity, it is more convenient to use the EPE per unit charge, just called Electric potential or voltage.

r

qk

q

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)(: VoltV

C

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Electric Potential Energy and the Electric Potential

The electric field is related to how fast the potential is changing:

The Electric Potential of Point Charges

The difference in potential energy (EPD) between points A and B is the ratio of the work needed to move a charge to the strength of that charge

q

WUUU qonBA

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The Electric Potential of Point Charges

Therefore, the electric potential of a point charge is:

Equipotential Surfaces and the Electric Field

Equipotential surfaces are those on which the electric potential is constant.

The electric field is perpendicular to the equipotential surfaces.

Equipotential Surfaces and the Electric Field

For two point charges:

• P 571 # 16-25 even

Capacitors and Dielectrics

A capacitor is two conducting plates separated by a finite distance:

The capacitance relates the charge to the potential difference:

Capacitors and Dielectrics

Capacitors and Dielectrics

A simple capacitor is the parallel-plate capacitor. It consists of two plates of area A separated by a distance d.

The general properties of a parallel-plate capacitor –

•

• d

C

AC

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Capacitors and Dielectrics

A dielectric is an insulator; when placed between the plates of a capacitor it gives a lower potential difference with the same charge, due to the polarization of the material. This increases the capacitance.

Capacitors and Dielectrics

The dielectric constant is a property of the material; here are some examples:

• P 578, # 30-34

• Do prelab Page 580- Testable question- Hypothesis- Data table

Summary

• A spherical charge distribution behaves from the outside as though the total charge were at its center

• Electric field is the force per unit charge; for a point charge:

• Electric fields created by several charges add as vectors

Summary

• Electric field lines help visualize the electric field

• Field lines point in the direction of the field; start on + charges or infinity; end on – charges or infinity; are denser where E is greater

• Parallel-plate capacitor: two oppositely charged, conducting parallel plates

• Excess charge on a conductor is on the surface

• Electric field within a conductor is zero (if charges are static)