Electromagnetic waves carry energy

Size: px

Start display at page:

Download "Electromagnetic waves carry energy"

Shawn Robbins
5 years ago
Views:

1 Electromagnetic waves carry energy E max X #1 X #2 E 1max =E 2max >E 3max f 1 = f 3 < f 2 X #3 Which barrel will heat up the fastest? a. 2>1>3 b. 1>2>3 c. 1=2>3 d. 1=3>2 e. 2>1=3 Intensity = power/area α E 2 max Does not depend on frequency/color!

2 Electromagnetic waves carry energy E max =peak amplitude c E max E(x,t) = E max sin(ax-bt) X Light shines on black tank full of water. How much energy absorbed? Intensity = Power = energy/time α (E max ) 2 α (amplitude of wave) 2 area area Intensity only depends on the E-field amplitude but not on the color (or frequency) of the light!

3 Today: The Photo Electric Effect

4 The photoelectric effect (~1900) The photoelectric effect is a phenomenon in which electrons are emitted from matter as a consequence of absorbing energy from light. The effect was only observed with UV light, but not so with red or IR light. à Effect is frequency dependent! But Maxwell told us that the light intensity doesn t depend on frequency! (Intensity only depends on E 2 )

5 Experimental apparatus: PE effect Metal surface Glass cylinder Vacuum

6 Experimental apparatus: PE effect Metal surface Glass cylinder Vacuum Adjustable voltage Current meter

7 What happens? A B - 10 V 2 ohms Two metal plates in vacuum with a voltage between them. How much current is flowing through the resistor? A) 0 A B) 0.2 A C) 5 A D) 10 A E) infinite current

8 What happens? A B - 10 V 2 ohms Two metal plates in vacuum with a voltage between them. The potential difference between A and B is: A) 0 V B) 5 V C) 10 V D) infinite volts

9 Potential difference between A and B is 10 V Without light, no electrons can get across gap. But if we put an electron close to the surface of plate A it 'feels' the electric field between the two plates. The electron accelerates towards the positive plate (B) and picks up the energy = q(10v) = 1 electron charge x 10 V = 10 ev Uniform E-field between plates Constant force on electron à constant acceleration 0V A F=qE 0V E - 10V 10Volts Current = = A 10V B

10 A note about units of energy Joules are good for macroscopic energy conversions. But when talking about energy of single electrons, Joules are inconvenient (way too big!) Define new energy unit: The electron-volt (ev) 1eV = kinetic energy gained by an electron when accelerated through 1 volt of potential difference 1eV J 0V E F path 1V 1V

11 Example: Different d, same ΔU Case 1: 0V distance: d e - 1V Case 2: 0V 1V d/2 e - Electron in both cases initially at rest. What can you say about the final kinetic energies KE 1 and KE 2 (case 1 and 2, respectively) of the electrons just before they hit the right plate? A) KE 1 > 2 KE 2 B) KE 1 = 2 KE 2 C) 2 KE 1 = KE 2 D) 2 KE 1 < KE 2 E) Something else Electric Field: E = U / d Force on e: F = q E Kinetic energy: ΔKE = F d Case 1: ΔKE 1 = (e U/d) d = e U Case 2: ΔKE 2 = (e U/(d/2)) (d/2) = e U à ΔKE 1 = ΔKE 2

12 Electron Volt: A convenient unit for energy Example: KE init = 2 ev 0V KE x=0.5 =? 10 V What is the kinetic energy half way between the two plates? KE final =? 12 ev 0VElectric field: E = U / d U = 10 V E = ½ U / ½d d

13 How to put the e - close to plate A? Shine light on the plate!! Metal surface Glass cylinder A Electrons get pulled towards Vacuum plate B by the electric field F = E q e- B Adjustable voltage Current meter

14 Experimental apparatus: PE effect Play with color and intensity. Measure current I. (I ~ #e- / s ) photoelectric_en.jar Measure the current!

15 First we could argue that the light heats up the plate à electrons pop-out Measure the current!

16 Hot plate. A few electrons get enough energy to just barely splash out. Assume electrons can only move in the horizontal direction. What is the current vs. battery voltage? A Current 0 Voltage C Current 0 Voltage B Current 0 Voltage D Current 0 Voltage

17 What s happening here? Each electron that pops out is accelerated and hits the plate on the right side. Here, electrons are repelled by neg. electrode BUT: # of electrons = constant sec So current is constant! Current not I = V / R!! 0 Battery Voltage reverse V, no electrons flow. Vacuum tube diode. Works! - early electronic device.

18 Vacuum tube diode Current photoelectric_en.jar reverse V, no electrons flow. voltage

19 Swimming Pool Analogy If no water slops over side of pool, no flow. Little pump or big pump, still no water current. If electrons stuck inside metal plate, no current for little or big V. Pump (Battery)

20 Pump (Battery)? Pool party: put bunch of energy into water, splash some out, get flow through pump. Put energy into metal by heating it very hot, gives electrons energy, some splash out. Gives current. q

21 What do you think does actually happen? Optical power P - frequency f Voltage U Now: Take out a piece of paper and draw the following graphs with what you expect will happen.) Current I 1. Current vs. Voltage with the lamp on (fixed color, say UV light, and fixed intensity.) 2. Current vs. Frequency (color) at a fixed intensity and voltage (right plate is on positive potential) 3. Current vs. Intensity for fixed color (right plate is at fixed, positive voltage)

22 Let's do the experiment! Play with color and intensity. Measure current I. (I ~ #e- / s ) photoelectric_en.jar photoelectric online Measure the current!

Part 2: Quantum* Mechanics

Part 2: Quantum* Mechanics Courtesy of IBM *We say something is quantized if it can occur only in certain discrete amounts. Quantum Mechanics is the greatest intellectual accomplishment of human race.