1. Electrostatics, Circuits, and Magnetism 4/29/2008
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
e0 is a constant – permittivity of free space
Positive force = repulsive, negative force = attractive
Remember: force is a vector!

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.

8. Electric Potential Energy
Electric Potential energy can be determined using mechanics
Electric potential is defined as the electric potential energy per unit charge

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
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.

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

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.

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.

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.

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

18. Resistors in series Voltage and resistance are additive
Current is constant everywhere in a series circuit

19. Resistors in parallel Current additive
Voltage is constant everywhere in a series circuit
More resistors = smaller equivalent resistance

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

25. Right hand rule

26. Ampere’s Law A moving charge (current) creates a magnetic field.
For a long wire, Dl = 2pr
Two wires can attract or repel due to this effect.
A solenoid is a long coil of wire.

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.