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**Electrostatics, Circuits, and Magnetism 4/29/2008**

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

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**Overview Electrostatics Electric Potential Dielectrics and Capacitance**

Electric Current DC Circuits Magnetism

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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

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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!

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**Electric field lines A visual representation of an electric field.**

More lines = stringer force Point away from positive, toward negative.

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**Electric Fields and conductors**

The electric field inside any conductor is zero The electric field is always perpendicular to the surface of a conductor

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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.

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**Electric Potential Energy**

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

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**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!

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**Potential due to a point charge**

Remember: potential is a scalar!

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**Capacitance A capacitor is a device that stores electric charge.**

The capacitance of an object is defined as: Capacitance is measured in farads.

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**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

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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.

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

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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.

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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.

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**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

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**Resistors in series Voltage and resistance are additive**

Current is constant everywhere in a series circuit

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**Resistors in parallel Current additive**

Voltage is constant everywhere in a series circuit More resistors = smaller equivalent resistance

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Complex Circuits

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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.

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Kirchhoff’s Rules

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**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!

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**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

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Right hand rule

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**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.

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**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.

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