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Home , Light, Physics

1230: and

Chuck Rogers Duane F-631 [email protected] (303) 492-4476 http://www.colorado.edu/physics/phys1230

Lecture 2: Electric and magnetic fields and LIGHT

HW2 on CULearn: DUE ON THURSDAY, 5PM.

1 Today

• Electric fields • Magnetic fields • Light is an ‘electromagnetic ’

E

2 B up the iClickers

• Swap clicker code to DC

• Hold down On/Off Switch 4 seconds • Flashing light: hit D C

• Should see light and you’re ready to go.

3 Last : QUALITATIVE things about electric . A) There are two types of B) There is only one type of charge C) There are more than two types D) None of the above.

A) Like charges attract B) Like charges repel And opposites C) Opposite types repel attract D) Depends on the situation.

4 QUANTITATIVE statement:

Coulomb’s Law: Depends on the charges

Q qQ r Fk qQ r2

q Constant of . What is the force that q feels from Q?

5 A question:

You are given three numbers, a, b, and c. True or false: The combination of numbers ab b is always equal to: a c c

A) TRUE : Try some actual numbers, like a=2, b=3, c=4 B) FALSE and it.

6 You are given four numbers, a, b, c, and d. Which of these are valid ways to rewrite ab b a cd A) cd 1 B) b a cd

1 ab  Experiment can be valuable C)    d here too… c   D) All of the above

E) None of the above 7 QUANTITATIVE statement:

Coulomb’s Force Law: Depends on the charges

Q qQ r Fk qQ r2

distance q Constant of nature. What is the force that q feels from Q?

8 Useful to rewrite things: qQ Coulomb’s Force Law: Fk qQ r2 Which is another way to write this force law? qQ A) Fk qQ r2 qQ Fk B) qQ rr22

Q C) F q k qQ r2  D) None of these

E) All of these 9 The divides into two parts:

Coulomb’s Force Law: Q FqQ  q k 2 r

Property of the local THING, Properties of say YOU. the rest of the universe. We Say: Fq r  qE ELECTRIC due to the rest of the universe. 10 Suppose we have 3 charges:

The force felt by q … A) Can still be F=qE B) Cannot be F=qE Q1 r1 Q2 C) MAY be F=qE if the charges are in a line. r2 D) Something else q happens.

qQ1 qQ 2 Q 1 Q 2 FqQ  k2 k 2  q k 2  k 2  qE r1 r 2 r 1 r 2

11 Electric Fields

We Say that charges due to the rest of the feel a force: universe.

Fq r  qE

The charge value. Properties of the rest of the universe. 12 What do you already know about ? A) come with only a north pole B) Magnets come with only a south pole C) Magnets come with both a north and south pole. D) Depends upon the type of . GUESS WHAT: A) Like poles attract The universe B) Like poles repel produces C) Opposite poles repel Magnetic Fields D) Depends on the situation. too. 13 Two separate topics?

Electricity Magnetism

For many years, people thought they were essentially unrelated.

14 changing observations(1):

Oersted (1821) found that moving charges create magnets.

AND

Moving charges feel forces from magnets.

15 Electric and magnetic fields

Electromagnetic, or Lorentz, forces Forces on charges arise from properties and properties of the rest of the universe:

Properties of the particle

Fq  q E  v  B

Properties from the rest of the universe. World changing observations(2): Maxwell (1861) found that changing electric fields create magnetic fields.

AND

Changing magnetic fields create electric fields (Faraday).

Electric and magnetic fields can create each other, EVEN WITHOUT Charges!

17 Hero worship:

“From the long view of the history of mankind – seen from, say, ten thousand years from now – there can be little doubt that the significant of the 19th century be judged as Maxwell’s discovery of the laws of electrodynamics. The American Civil War will pale into provincial insignificance in comparison with this important scientific event of the same decade.” Richard P. Feynman,

Lectures on Physics, Vol. 2, 1-11 Light is an oscillating E and B-field • Oscillating ELECTRIC and Electromagnetic • Traveling to the right at (c) Snap shot of E-field in time: At t=0 A little later in time E c

X

Function of position (x) and time (t):

E(x,t) = Emaxsin(ax+bt-bt)) Properties of light  with

Electric fields exert forces on charges (e’s and p’s in )

F=qE Force = charge x electric field + X E F= qE

F=qE X_ E

Light is an oscillating E-field (and B-field). It interacts with matter by exerting forces on the charges – the and protons in . How do you generate light (electromagnetic radiation)? a. Stationary charges b. Charges moving at a constant velocity c. Accelerating charges d. b and c e. a, b, and c E Stationary charges  + constant E-field, no magnetic (B)-field

Charges moving at a constant velocity  Constant current through wire creates a B-field B But B-field is constant I

Accelerating charges  changing E-field and changing B-field (EM radiation  both E and B are oscillating) How do you generate light (electromagnetic radiation)? a. Stationary charges b. Charges moving at a constant velocity c. Accelerating charges d. b and c e. a, b, and c

Ans. is c. Accelerating charges create EM radiation. The of sun- very hot! + + Whole bunch of free electrons + whizzing around like crazy. Equal number of protons, but + heavier so moving slower, less EM generated.

Go to radiowave sim

NOTICE:

Some things are ‘self-luminous’, like the SUN. They burn and release light.

Some things are NOT self-luminous. Their charges are forced to wiggle by light from other things. The MOON

25 EM radiation often represented by a sinusoidal curve.

OR

What does the curve tell you? a.The spatial extent of the E-field. At the peaks and troughs the E-field is covering a larger extent in b.The E-field’s direction and strength along the center line of the curve c. The actual path of the light travels d.more than one of these e.none of these. Making of the Sine Wave

What does the curve tell you?

-For Water Waves? -For Wave? -For E/M Waves? Made it to here, Lecture 2.

• Phet sim drew lots of questions • People wanted to know: How do you send (we talked about AM and FM) • Radiation in all directions was noticed. • Water and Sound waves drew lots of ‘Ahah’ type responses.

28 EM radiation often represented by a sinusoidal curve.

OR

What does the curve tell you? a.The spatial extent of the E-field. At the peaks and troughs the E-field is covering a larger extent in space b.The E-field’s direction and strength along the center line of the curve c. The actual path of the light travels d.more than one of these e.none of these. Our first simplification: Light Rays. 1. Concentrate on the directions of light travel. Straight lines. 2. Only the light that OR reaches the observer causes a response.

Light Rays! Light rays are invisible unless they enter directly into our or are scattered by , fog or some object into your eye! MANY reflected rays come from all parts of Alex, including Flashlight his nose - a diffuse object

Light Incident from a light bulb bulb

Bob sees Alex's nose because a reflected light ray enters Bob's eye! Rays bounce when they reflect off a or shiny surface

• This is called specular .

• How is it different from ? Mirror http://phet.colorado.edu/sims/string-wave/string-wave_en.html

Scientific and metric • Powers of 10 give a • don't use feet or miles to indicate shorthand notation for • They use very large numbers. – meters (m) • 103 = 1000 • 1 meter = 39.4 inches • 102 = 100 – kilometers (km) • 1 km = 1000 m = 0.625 mi • 10 1 = 10 – centimeters (cm) 0 • 10 = 1 • 1 cm = 10-2 m = 0.394 inches • Or very small numbers – millimeters (mm) • 1 mm = 10-3 m -1 • 10 = 0.1 – nanometers (nm) • 10-2 = 0.01 • 1 nm = 10-9 m • 10-3 = 0.001

Clicker question

• The of green • wavelength = 500 nm = light is around 500 nm. 5 x 102 x10-9 m = How many 5 x 10-7 m of green light fit into one • Hence 1 m = cm (0.4 inches, or a wavelength/(5 x 10-7) = fingertip)? 107/5 wavelengths = a) 20 thousand 2 million wavelengths b) 50 thousand • Since 1 cm is 1/100 of a c) Two million meter there are 2 x d) Two billion 106/100 = 20,000 e) 5 billion wavelengths in a cm

We see color when waves of different wavelengths enter enter our !

Light with wavelength of 650 nm appears when it enters a viewers eye

Light with wavelength of 520 nm appears green when it enters a viewers eye

Light with wavelength of 470 nm appears blue when it enters a viewers eye

The speed of light in empty space is the same for all wavelengths Clicker questions

• Which of the light waves has the longest wavelength? a) • Which of the light waves is brightest? • Which of the light waves has the highest speed in b) empty space? a) b) c) d) They all have the same speed c)

Clicker question

• What does Alex see when the wave at left with wavelength 650 nm goes by him? a) Red b) Blue c) Green d) e) Nothing What happens when two or more waves with different wavelengths reach your eye?

Light with both wavelengths 650 nm and 520 nm appears when it enters a viewers eye

Light with only wavelength 580 nm ALSO appears yellow when it enters a viewers eye (A DEEPER YELLOW THAN FOR THE CASE ABOVE) What is white light?

Light which is a mixture of 650, 520 and 470 nm wavelengths (and possibly more wavelengths) appears WHITE when it reaches your eye No single wavelength (mono- chromatic) wave appears white when it reaches your eye! A spreads out the over- lapping wavelengths in white light into different spatial locations where they can be seen as .

When does an electric field exert a force on a charge?

A. Always B. Sometimes (changing field) C.Sometimes (depends on the charge) D.Sometimes (depends on many things) E. Something else Chapter 1 Overview

• Light belongs to a family of • EM waves are created and waves called electromagnetic destroyed by emission and (EM) waves (Physics 2000) absorption by moving charge – Other waves: rope waves, – Classical picture (Phys 2000) water waves, sound waves, • wiggling electrons radiate etc. radio waves or waves • electrons in an atom are • Sometimes EM waves are resonant with emitted or called EM radiation absorbed light waves or X- rays – Radio waves – picture (Phys 2000) – Radar and similar waves • change of state of electrons • in atoms when bundles of • cell phone waves wave energy () are emitted or absorbed – or waves • Light sources – Ultra- (suntan) waves – – X-rays – Neon light – Gamma rays – Fluorescent light Waves

• Rope waves • Google search under – Created by oscillation of keyword "physics" my hand holding the rope – Water waves (circular – Finite speed of wave, but ) rope segments do NOT – Stadium waves move in direction of wave – Rope segments move up and down, not along wave – Note the change that occurs when I oscillate my hand faster • transmitter – 3 meters wavelength – (100 Mhz ) Rays (a single beam of light, for example)

• Single light ray • What about light coming – Ray from a laser acts like from everything in this a single light ray room? Two kinds of – Illustrate by laser light through fog objects: – Bounce off mirror – Self luminous objects – Bounce off white card () – Put through water – Objects which are not self- (bending) luminous are seen because • We only see light when of light reflected off them a ray enters into our • Turn out the light and we eye don't see anything in the – Laser light is visible from room side because it is • It's all reflected light with scattered into our eyes many rays coming from • Rays from a flashlight diffuse • Rays from a light bulb