AP Phys B Test ReviewElectrostatics, Circuits, and Magnetism 4/29/2008
Overview Electrostatics Electric
Potential Dielectrics and Capacitance Electric Current DC Circuits Magnetism
is carried by subatomic particles (protons, electrons)transfer
99% of all charged effects caused by electron Physical contactby Conduction by Induction
No physical contact
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
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
A capacitor is a device that stores electric charge. The capacitance of an object is defined as:
Q C = V
Capacitance is measured in farads.
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 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 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 =
V to ta l = V 1 = V 2 = . . .
1 = R eq Ri i
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.
magnet has two poles: north and
south Magnetic field & magnetic field lines: analogous to electric field
Direction: points north to south
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
A moving charge (current) creates a magnetic field.
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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