IB 5.2 Electric Field & Potential

Electric Field

• Just like mass create gravitational fields, charges create electric fields

• With gravity the field strength is measure as Newton per kilogram

• What do you think Electric fields are measured in?

Newton's per Coulomb

Measuring the Electric Field

• If the unit is Newton's per Coulomb, what is the equation?

E = F / qo

• Simple enough, right.• E = Electric Field• F = Force• qo = charge in the field

• Alternative Equation:

• It is the surrounding charges that creates an electric field at a given point in space.

• Look at Example 8

Summing electric Fields

2

2

r

Qk

q

rQqk

q

FE

Picturing the Electric Field

Click on the picture to open an applet

Electric Field Rules

• Fields start at positive and end at negative, or start and end at infinity.

• This is by convention. The field is said to predict the movement of a positive charge.

• The density of lines should represent the strength of the field.

• A positive charge will have a velocity tangent to a field line.

• Field lines do not actually exist since the are an infinite number of paths a test charge can take.

Parallel Plate Capacitors

• Used to store charge.• The most common type is a

parallel plate capacitor.• Charge is spread uniformly, not

really, throughout the plate• The electric field between two

parallel plates is 100 N C-1.• What acceleration would a 2.0

C charge with a mass of 10-3 kg experience if placed in the field?

Electric Potential

• Consider a positive charge Q and a test charge q.• Work must be done to move q closer to Q since like

charges repel.

• V = Voltage (Potential measured in Volts) q

WVVqWV

r

Qk

r

QqkWd

r

QqkFdW

r then d if ,2

Work and Route

• Regardless of the path taken to move the charge the work is always the same.

• Electric Potential Energy is U = qV• Potential Difference is U

Electric Fields Inside Conductors

• Excess electric charge moves to the surface of a conductor

• At equilibrium the electric field inside a conductor is zero– This comes from the fact that free

electrons will always be on the outside of the conductor.

• So the electric field lines don’t penetrate the conductor

• The electric field outside a conductor is perpendicular to the surface

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