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Physics 222 Exam Review

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Outline Overview/Mind-map What each equation does Practice Problems

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Sorry about the boring theme. I couldn’t find a suitable theme that I liked, that didn’t mess up my text. However, looking at green is said to increase creativity and stimulate brain function. May use this later.

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Useful tip: Storing variables in calculator

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Charge Electric Force Electric Field Potential Energy Electric Potential Divide by q Multiply by q Dipole Interacts with dipole Capacitors Resistors

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Overview Fluids Electric force -> Electric field Potential energy -> Electric potential E->V and V->E Capacitors and energy stored inside them Resistors

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This is the definition of Pressure: Force/Area

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This is the continuity equation for fluid flow. In English, it means that the amount of stuff going through a pipe is constant, so shrinking the pipe means that the water will go faster.

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Questions?

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Just like electric force, but without the test charge q 0. It’s still a vector.

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Electric field of a point charge.

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Electric field lines go from + to -. Also, line density indicates field strength

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Dipole moment, BY DEFINITION Notice: Dipole moment points from negative to positive….opposite of the direction E points

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Torque on a dipole by the external electric field – Note that E is not the E produced by the dipole – E is external Torque is maximum when dipole moment and E are perpendicular.

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Potential energy is minimum (also called stable equilibrium) when two things are true: – Dipole moment is parallel to E – Dipole moment points in the same direction as E. Potential energy is maximum (also called unstable equilibrium) when two things are true: – Dipole moment is parallel to E. – Dipole moment points in the opposite direction as E.

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Questions?

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Also known as Gauss’s Law Really there are two equations here…but they’re both equally valid…always.

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Steps in solving Gauss’s Law Problems – Draw a picture of the object. Pick a good Gaussian surface. – Write down the expression of Gauss’s Law that involves the dot product between E and A. (If E is perpendicular to A, the flux is 0 for that surface. Otherwise, use symmetry to get rid of the integral.) – Write down the expression of Gauss’s Law that involves the total charge q. – Set the two expressions equal to each other and eliminate variables.

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This one is especially important. This is the electric field anywhere away from a large sheet of charge. Notice that the electric field doesn’t depend on distance, and always points perpendicular to the surface.

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These relations let you go from either E to V or vice-versa. If you know one, you can calculate the other.

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This is the defining relation between potential energy (U) and electric potential (V). Note: since q 0 can be positive or negative, U and V do not necessarily have the same sign. One more time: Electric potential (V) is not the same thing as electric potential energy (U) But let’s rewrite it.

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Electric potential of a point charge q, a distance r away, assuming V=0 at infinity. Potential goes up as you get closer to the point charge.

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Definition of capacitance C=Charge Q/ Voltage drop

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Special case of capacitance when you’re looking at a parallel plate capacitor. Notice that the capacitance doesn’t depend on the charge on the plates.

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Adding capacitors in PARALLEL.

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Adding capacitors in SERIES.

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Special case of capacitance when you’re looking at a two concentric spherical conducting shells. The radius of the smaller shell is a, the radius of the larger shell is b.

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The capacitance of a single spherical shell of radius R

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Relates capacitance without a dielectric (C 0 ) to capacitance with a dielectric.

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Definition of current.

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Ohm’s Law: A relationship between voltage, current, and resistance. Pretty fundamental.

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Resistivity changes as a function of temperature.

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Resistance changes as a function of temperature

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Adding resistances in series.

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Adding resistances in parallel.

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Practice Problems

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In the figure to the right, there are two charges connected by a massless insulating rod…and remember to use VECTORS when appropriate… Draw the electric dipole. Torque caused by the electric field= Dipole moment= Potential energy as it is right now= Which way will the dipole begin to rotate? (Clockwise/Counter-clockwise) How much work is done in rotating the dipole from its current position to the stable equilibrium position? What does the work in question f?

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A block of MagicFoam (length 10 cm, width 10 cm, height 3 cm) sits on top of a calm body of water. MagiFoam density=0.5 g/cm 3. How much of the block is submerged? A 10 kg block floats in the water. What is the buoyant force on it? A house with a roof of area 5 m 2 has winds of 50 m/s above it. What is the force on the roof caused by the pressure difference?

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If current is going from your hand to your foot, which direction are the electrons going?

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Final Questions?

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Thank you, and good luck!

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