IB 5.2 Electric Field & Potential. Electric Field Just like mass create gravitational fields, charges create electric fields With gravity the field strength.
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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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