ap-physics-b-review-electromagnetism elecrostatics circuits maagnetism

1. AP Phys B
Test Review
Electrostatics, Circuits, and
Magnetism
4/29/2008

2. Overview
 Electrostatics
 Electric Potential
 Dielectrics and Capacitance
 Electric Current
 DC Circuits
 Magnetism

3. 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

4. Coulomb’s Law
 This law determines the force of attraction or
repulsion between 2 charged objects
• ε0 is a constant – permittivity of free space
• Positive force = repulsive, negative force = attractive
• Remember: force is a vector!
F
Q Q
r
q =
1
4 0
1 2
2π ε

5. Electric field lines
 A visual
representation of
an electric field.
• More lines =
stringer force
• Point away from
positive, toward
negative.

6. Electric Fields and conductors
 The electric field inside any conductor is
zero
 The electric field is always perpendicular
to the surface of a conductor

7. 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.
Φ =
Q e n c l o s e d
ε 0
Φ = E A c o s θ

8. Electric Potential Energy
 Electric Potential energy can be determined
using mechanics
 Electric potential is defined as the electric
potential energy per unit charge
∆ U q E d= −
V
U
q
W
q
= = − ∆ ∆U q V= −

9. 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!

10. Potential due to a point charge
V
Q
r
=
1
4 0π ε
• Remember: potential is a scalar!

11. Capacitance
 A capacitor is a device that stores electric
charge.
 The capacitance of an object is defined as:
 Capacitance is measured in farads.
C
Q
V
=

12. 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
C
A
d
=
ε 0

13. 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 Q V=
1
2
2

14. Electric Current
 Electric current is defined as the amount
of charge that flows past a given point in
a second

15. 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.
R
V
I
=

16. 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.
R
L
A
= ρ

17. 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 I r= −

18. Resistors in series
 Voltage and
resistance are
additive
 Current is constant
everywhere in a
series circuit
R Re q i
i
= ∑
V Vt o t a l i
i
= ∑
I I It o t a l = = =1 2 . . .

19. Resistors in parallel
 Current additive
 Voltage is constant
everywhere in a
series circuit
 More resistors =
smaller equivalent
resistance
1 1
R Re q ii
= ∑I It o t a l i
i
= ∑
V V Vt o t a l = = =1 2 . . .

20. Complex Circuits

21. 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.

22. Kirchhoff’s Rules

23. 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!

24. 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 q v B= s i n θ
F I L B= s i n θ

25. Right hand rule

26. Ampere’s Law
 A moving charge (current) creates a magnetic
field.
• For a long wire, ∆l = 2πr
• Two wires can attract or repel due to this effect.
• A solenoid is a long coil of wire.
B l Ii
i
e n c l o s e d∆∑ = µ 0

27. 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 N
t
= −
∆ Φ
∆