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

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.

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

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!

Capacitance A capacitor is a device that stores electric charge. The capacitance of an object is defined as: Capacitance is measured in farads.

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

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.

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.

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.

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

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

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

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

Right hand rule

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.

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.