Upload
sebastian-dixon
View
231
Download
1
Tags:
Embed Size (px)
Citation preview
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
VU
q
W
q 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
VQ
r
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.
CQ
V
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
CA
d
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 QV1
22
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.
RV
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.
RL
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 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 1
R Req iiI Ito ta l i
i
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