Review and Clicker QuestionsStudy guide for final

M. GoldmanSpring, 2011

1

Friday, April 29, 2011

Would the following people please see me about clicker problems after class?

2

Tips for final

• Get a good night's sleep the night before and have a high-protein (egg) breakfast the morning of the exam.

• Work on the exam sheet and scrap paper first and transfer your answers to the bubble sheet at the end.

• Write down what is given, what needs to be found, and the starting equation or equations in terms of symbols instead of numbers. Use scrap paper if necessary.

• Before you hand in your exam, check over your answers to ALL questions, including those you anwered quickly to find careless errors.

3

Study guide for 10 final questions on material of Midterms 1-3

• Chapters 16 and 17o Coulomb's Law for electrostatic force between charges. Energy.o Electric field vectors and lines; electric potentialso Capacitors and energy storage

• Chapters 18 and 19o Currents, Ohm's Law, DC circuits, resistivity, resistors, powero AC circuits RMS current, voltage.

• Chapters 20 and 21o Magnetic fields – how they arise and how they exert forces.o Magnetic forces on currents and particleso EMF, Faraday's laws, transformers and transmission lines

4

5

a

b

c

d

|F| = k|q1q2|/d2

6

abcd

abcd

7

abcde

+ --

v or +v

Direction:

General magnetic force on a + or - charge, q, moving with velocity v

8

vF =qvv×

vB

v

B

sinBvqF

Magnitude:

F =−|q|v×B= |q|(−v)×B

F =+|q|v×B= |q|(v)×B

Force on test current, itest exerted by magnetic field, B created from unseen currents (cont'd)

• Direction of force, F, on current segment itest due to B is given by right hand rule #2

• Magnitude of force, F on current segment itest due to B is given by

9

|F| = |i| ·|L| ·|B|·sinθθ = magnitude of angle < 180° between vectors i and B

itest

•θ

L itest

B

F

10

abcd

Clicker Question

11

i

A long wire has a current moving as shown.What is the direction of the B-field created by the wire at a point just below the wire?

B=?A) Into the PageB) Out of the PageC) To the rightD) DownE) None of the Above

r

12

Magnetic fields add like vectors!Does this magnetic field exert a force on either wire?

B = μ0i2πR

i = 10

i = 20

Review of how magnetic fields are produced by currents

• A DC-current-carrying wire creates magnetic field lines and vectors at every point in space, even far away from the wire.

• Magnetic field lines around a current-carrying wire are an infinite number of closed loops around the wire

• Use right-hand-rule #1 to find the direction of magnetic field, B

• Magnitude of B a distance R away from a long current-carrying wire

• Magnitude B at center of circular current-carrying wire of radius, R

13

i

X

X

X

•

•

•

X

X

X

•

•

•

B = μ0i2πR

B= μ0i2R

(out of page)

14

A light bulb is attached to a 120 V AC wall socket with a transformer, as shown. If the number of turns in the primary (Np) is increased while keeping the number of secondary turns constant, the bulb will get…

a) brighterb) dimmerc) remain constant brightnessd) The answer depends on the number of turns in the secondary coile) The answer depends on the resistance of the light bulb p

s

p

s

N

N

V

V

The brightness of the bulb goes as Vs2/Rbulb.

The voltage of the primary is fixed at 120, so Vs goes down and so does the brightness

15

p

s

p

s

N

N

V

V

abcd

Study guide for 20-25 multiple choice questions on final covering material after MT #3

• General properties of waveso Waveforms, wavefronts, rays, wavelength, frequency, amplitude, energy,

intensity

• Electromagnetic (EM) waveso Predicted from Maxwell eqns (conceptual form). o Speed of light in empty space (= λ·f), EM waves = linked oscillating fields.o Ray, E and B are all mutually perpendicularo Energy and intensity related to electric field in waveo Different kinds of EM waveso White light has all wavelengths present

• Emission and absorption of EM waveso AC current in an antenna launches radio waves. Second antenna receives

radio waves (lights bulb)o Emitting and absorbing light from electrons in atoms (quantum picture) 16

Cont'd

• Rays and opticso Pinhole camera. One ray from each object point to each image pointo Index of refraction, n. Speed of light in glass and water. Dispersion

(different n for different frequencies)o Reflection law and refraction law. Marching soldier analogy. o Critical angle for total internal reflection.o Flat mirrors and ray tracing. Virtual images.o Thin convex lense and ray tracing. Lens eqn. Magnification eqn.

Real and virtual images, inverted and upright. o Multiple lenses and intermediate images.

17

Cont'd

• Scatteringo Blue sky and red sunset

• Polarizationo Unpolarized light. Electric field vector direction jumps around

perpendicular to ray.= components have phase jumps. o Electric field vectorstays in one perp. direction if EM wave is polarizedo Polaroid filters and long molecules. Multiple polaroids.o Transmitted light intensity after polaroid filters is proportional to E2

o Brewster's angle for suppression of one polarization component

• Interferenceo Phase difference between two waves of same λ at same place. Electric

fields add as vectors. Resultant field may be larger or smaller or zero. o Double slit formulas and reasoning behind them. Optical path difference.o Many "slits." Transmission and reflection gratings. White light. 18

The wavelength, λ, is 10 m. What is the speed of this wave?

Clicker question

1Time (sec)

A) 1 m/s B) just under 7 m/sC) 10 m/s D) 15 m/sE) None of the above/not enough info/not sure

speed = wavelength × frequencyfrequency = 1/(period)So speed = wavelength/period

Clicker Question

A

B

C

D

x

x

Wave 1

Wave 2

If these two waves were moving through water at the same time, what would the water look like?

Cont'd

• Diffractiono Interference around an obstacle (e.g., two edges of single slit)o Formula for first dark fringe = boundaries of bright central maximumo Diffraction of light exiting a pinhole or lens. o Two light sources resolved if bright central maxima in diffraction patterns of

each do NOT overlap

• Modern quantum mechanic (early models)o Line spectra of hot gases observed using diffraction gratingso Rutherford model of electron circling proton doesn't give line spectrao Bohr-Planck: bound electrons restricted to discrete energy levels labeled by an

integer.o Electron jumps between energy levels accompanied by emission or absorption of

photons. Energy conservation photons⇒o Photon energy = hf. Light energy density =hf ×(# of photons/vol)o Line spectra explained by Bohr-Planck model 21

Clicker Question

22

P

Rao

d4 2π

Pa

Rod2

2

Pa

Rod

P

Rao

dπ 2R

detector ad

Po

A point source of radiation emits power Po isotropically (in all

directions uniformly). A detector of area ad is located a

distance R away from the source. What is the power P received by the detector?

A:

B:

C:

D:

E: None of these

Answer: At a distance R from the source, the power Po is now

spread out uniformly over a sphere of radius R, area 4πR2. So at distance R, the intensity or brightness, which is power/area, is Po/4πR2. The power received by the detector is given by answer A.