Review

Coulomb’s Law

Units of Charge 1 Coulomb (C)

Direction of the electric force is always along the line joining the two objects. Direction of the force depends on whether the charges have the same sign or opposite sign.

Coulomb’s Law in terms of e 0, permittivity of free space.

Conceptual Example Two positive point charges Q 1 =50 m C and Q 2 =1 m C, are separated by a distance l. Which is larger in magnitude, the force that exerts on Q 1 exerts on Q 2, or the force that Q 2 exerts on Q 1 ?

The Electric Field

The electric field E at a point in space is defined as an electric force F, acting on a positive test charge q divided by the magnitude of the test charge

Units of Electric Field 1 N/C

Field Lines

1.At every point of electric field line, electric field E is tangent to this line. No two field lines can cross! 2.The line must begin at positive charge and terminate on the negative one unless go to infinity. 3.The number of line per unit area is proportional to the magnitude of electric field.

Electric Fields and Conductors

Important: electric field inside of good conductor is zero!!!!

Important: any net charge on a good conductor distributes itself on the surface!!!!

Electric field is always perpendicular to the surface of conductor:

q Q P d d Is it possible to place a charge at point Q such that the electric field produced at point P by the two charges will be add to zero?

Electric Potential and Electric Energy

It’s useful to define the electric potential (or simply the potential) as a potential energy per unit charge:

Units of Electric Potential 1 V = 1 J/C

Important: this simple formula for E may be used only for uniform field!!!!

Equipotential Lines

The equipotential surface must be perpendicular to the surface.

Important: the surface of a good conductor is always an equipotential surface!!!!

Electric Potential Due to Point Charges

Work to Force Two Positive Charges Close Together What minimum work is required by an external force to bring a charge q=3.00 m C from a great distance away to a point 0.5 m from a charge Q=20 m C?

Capacitance

Q = CV The constant of proportionality, C, in the last relation is called the capacitance of the capacitor.

Important: the capacitance C is a constant for a given capacitor; it does not depend on charge Q or voltage V.

Capacitance of a Parallel-Plate Capacitor

Energy Stored in Capacitor

When such a circuit is formed, charge can flow through the wires of the circuit, from one terminal of the battery to the other. A flow of charge such as this is called as electric current.

Units of Current 1 Ampere (A) = 1 C /1 s

Important: Current is not a vector, it’s a scalar!!!!

Definition of Resistance I=V/R

Ohm’s Law The current through a metal conductor is proportional to the applied voltage.

Resistivity

Effect of Temperature

a – temperature coefficient of resistivity is positive for metals and negative for semiconductors.

Electric Power

Average Power

Root-Mean-Square Values (RMS)