ap-physics-b-review-electromagnetism
TRANSCRIPT
AP Phys BTest Review
Electrostatics, 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)• 99% of all charged effects caused by electron
transfer Charging by Conduction
• Physical contact Charging by Induction
• No physical contact
Coulomb’s Law This law determines the force of attraction or
repulsion between 2 charged objects
• 0 is a constant – permittivity of free space
• Positive force = repulsive, negative force = attractive• Remember: force is a vector!
FQ Q
rq 1
4 0
1 22
Electric field lines A 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
Gauss’ Law: The total electric flux passing through a closed surface is proportional to the charged enclosed in that surface.
Q enclosed
0
EA cos
Electric Potential Energy Electric Potential energy can be determined
using mechanics
Electric potential is defined as the electric potential energy per unit charge
U qEd
VUq
Wq
U q V
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
VQr
1
4 0
• Remember: potential is a scalar!
Capacitance A capacitor is a device that stores electric
charge. The capacitance of an object is defined as:
Capacitance is measured in farads.
CQV
Parallel plate capacitors and dielectrics 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
CAd
0
Capacitors and energy A charged capacitor stores an amount of
electric energy given by
• This energy can be thought of as stored in the electric field between the plates.
U QV12
2
Electric Current Electric current is defined as the amount
of charge that flows past a given point in a second
Ohm’s Law Ohm’s Law related the resistance of an
object to the decrease in electric potential across a point and the current flowing through that point.
RVI
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.
RLA
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 Req i
i
V Vto ta l ii
I I Ito ta l 1 2 . . .
Resistors in parallel Current additive Voltage is constant
everywhere in a series circuit
More resistors = smaller equivalent resistance
1 1R Req ii
I Ito ta l ii
V V Vto ta l 1 2 . . .
Complex Circuits
Kirchhoff’s 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.
Kirchhoff’s 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
The force in a current carrying wire immersed in a magnetic field
F qvB s in
F ILB s in
Right hand rule
Ampere’s Law A moving charge (current) creates a magnetic
field.
• For a long wire, l = 2r• Two wires can attract or repel due to this effect.• A solenoid is a long coil of wire.
B l Iii
enclosed 0
Faraday’s Law A changing magnetic field induced an emf.
• A current produced by an induced emf moves in a direction such that its magnetic field opposes the original change in flux (Lenz’s Law)
• A coil rotating in a magnetic field is a good example of this.
E Nt