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# AP Physics B Review - Electromagnetism

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AP Phys B Test ReviewElectrostatics, Circuits, and Magnetism 4/29/2008

Overview Electrostatics Electric

Potential Dielectrics and Capacitance Electric Current DC Circuits Magnetism

Electrostatics Charge

is carried by subatomic particles (protons, electrons)transfer

99% of all charged effects caused by electron Physical contactby Conduction by Induction

Charging Charging

No physical contact

Coulombs Law

This law determines the force of attraction or repulsion between 2 charged objects

1 Q 1Q 2 Fq = 4 0 r 2 0 is a constant permittivity of free space Positive force = repulsive, negative force = attractive Remember: force is a vector!

Electric field linesA

visual representation of an electric field.

More lines =

stringer force Point away from positive, toward negative.

Electric Fields and conductors The

electric field inside any conductor is

zero The electric field is always perpendicular to the surface of a conductor

Gauss Law

Electric Flux: The amount of an electric field passing through an area

= E A cos

Gauss Law: The total electric flux passing through a closed surface is proportional to the charged enclosed in that surface.

Q e n c lo s e d = 0

Electric Potential Energy

Electric Potential energy can be determined using mechanics

U = qEd

Electric potential is defined as the electric potential energy per unit charge

U W V = = q q

U = qV

Equipotential lines or surfaces An

equipotential surface is a surface over which all points have the same potential.

An equipotential surface must be

perpendicular to the electric field!

Potential due to a point charge Remember: potential is a scalar!

1 Q V = 4 0 r

Capacitance

A capacitor is a device that stores electric charge. The capacitance of an object is defined as:

Q C = V

Parallel plate capacitors and dielectrics

0 A C = d

For a parallel plate capacitor (two conducting plates with a vacuum between the plates)

Often, an insulator known as a dielectric is placed between the plates to enhance capacitance

Dielectric constant: measures the strength of the dielectric

Capacitors and energy

A charged capacitor stores an amount of electric energy given by

1 U = Q V 2 2

This energy can be thought of as stored in the electric field between the plates.

Electric Current Electric

current is defined as the amount of charge that flows past a given point in a second

Ohms Law

Ohms Law related the resistance of an object to the decrease in electric potential across a point and the current flowing through that point.

V R = I

Electric Resistance

Electric resistance is the innate ability of a material to inhibit the passage of electrons.

Measured in ohms. Given by the resistivity as well as the geometry of the object.

L R = A

Circuits emf and terminal voltage

A device that transforms one type of energy into electrical energy is a source of electromotive force

emf: the potential difference between the terminals of a battery when there is no current flowing to an external source. A battery has some internal resistance The real voltage of a battery is then

V = E Ir

Resistors in series

Voltage and resistance are additive Current is constant everywhere in a series circuitR eq =

V to ta l

Ri i = Vii

I to ta l = I 1 = I 2 = . . .

Resistors in parallel

Current additive Voltage is constant everywhere in a series circuit More resistors = smaller equivalent resistanceI to ta l =

i

Ii

V to ta l = V 1 = V 2 = . . .

1 = R eq Ri i

1

Complex Circuits

Kirchhoffs rules Junction

rule: At any junction point, the total current into the junction has to be equal to the total current out of the junction. Loop rule: The sum of changes in potential around and closed loop is zero.

Kirchhoffs Rules

Magnetism Every

magnet has two poles: north and

south Magnetic field & magnetic field lines: analogous to electric field

Direction: points north to south

Electric

current (moving charge) produces a magnetic field!

Force due to magnetic fields

The force on a charged particle moving through a magnetic field

F = q v B s in

The force in a current carrying wire immersed in a magnetic field

F = IL B s in

Right hand rule

Amperes Law

A moving charge (current) creates a magnetic field.

i

B li = 0 I e n c lo s e dFor a long wire, l = 2r Two wires can attract or repel due to this effect. A solenoid is a long coil of wire.

A changing magnetic field induced an emf.

E = N t

A current produced by an induced emf moves in a direction such that its magnetic field opposes the original change in flux (Lenzs Law) A coil rotating in a magnetic field is a good example of this.

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