Physics 222 Exam Review

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

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.

Useful tip: Storing variables in calculator

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

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

This is the definition of Pressure: Force/Area

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.

Questions?

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

Electric field of a point charge.

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

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

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.

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.

Questions?

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

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.

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.

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

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.

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.

Definition of capacitance C=Charge Q/ Voltage drop

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.

Adding capacitors in PARALLEL.

Adding capacitors in SERIES.

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.

The capacitance of a single spherical shell of radius R

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

Definition of current.

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

Resistivity changes as a function of temperature.

Resistance changes as a function of temperature

Adding resistances in series.

Adding resistances in parallel.

Practice Problems

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?

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?

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

Final Questions?

Thank you, and good luck!