the electron

Question 1

dicovered electron in 1897

Answer 1

J.J. Thomson

Question 2

named electron

Answer 2

G.J. Stoney

Question 3

measured charge on electron (oil drop expt.)

Answer 3

Robert Millikan

Question 4

thermionic emission

Answer 4

release of electrons from the surface of a hot metal

Question 5

cathode ray tube

Answer 5

uses thermionic emission to create beam of electrons (know how to draw)
invented by Karl Ferdinand Braun

Question 6

CRT process

Answer 6

1. low voltage heats up filament which heats cathode (-) causing thermionic emission
2. high voltage placed across anode and cathode (anode voltage) causing released electrons to accelerate towards anode
3. end of tube coated in phosphors (fluorescent) so when electrons strike it light is produced

Question 7

cathode ray

Answer 7

streams of high speed electrons moving from cathode

Question 8

properties of cathode rays

Answer 8

1. travel in straight lines
2. cause certain materials to emit light when struck
3. have kinetic energy
4. can be deflected in magnetic/electric fields
5. can produce x-rays when they strike metal target

Question 9

conservation of energy in electric field

Answer 9

loss in potential = gain in kinetic

eV = 1/2mv^2 // qV = 1/2mv^2

q = e = charge
e = charge on electron V = voltage

–> W = qV

Question 10

electronvolt

Answer 10

amount of energy gained/lost by 1 electron when it moves through a p.d. of 1V

1eV = 1.6x10^-19J

Question 11

uses of CRT

Answer 11

1. tv/computer monitor
2. cathode ray oscilloscope (CRO) - displays elec. signal

Question 12

Electrocardiogram (ECG)

Answer 12

displays the hearts elec. signals

Question 13

electroencephalogram (EEG)

Answer 13

displays brains elec. signals

Question 14

photoelectric effect

Answer 14

emission of electrons from the surface of a metal by electromagnetic radiation of a suitable frequency

Question 15

Heinrich Hertz

Answer 15

1887 discovered elec. spark occurred more readily when U.V. light fell on one of the electrodes

Question 16

Hallwachs

Answer 16

1888 discovered if UV light fell on neg. zinc plate it lost its charge rapidly

Question 17

photocell

Answer 17

device that conducts when light of a suitable freq falls on the cathode
- current ∝ intensity

(be able to draw)

Question 18

photons

Answer 18

packets of electromagnetic energy

Question 19

energy of a photon

Answer 19

E = hf
h = planck’s constant
-energy depends on freq. of light
-no. of photons per sec ∝ intensity of light
-energy of 1 photon only passed to 1 electron

Question 20

threshold frequency

Answer 20

definite frequency below which no photoemission occurs no matter how intense the light is
e.g. threshold f for zinc is UV light

Question 21

work function ϕ

Answer 21

minimum energy needed to remove the loosest electron from the surface of that metal

Question 22

einstein’s photoelectric law

Answer 22

hf = ϕ + 1/2mv^2(max)

• energy of 1 photon passed onto 1 electron = min energy required to remove electron + kinetic energy gained when released
• max kinetic energy increases with freq. but not intensity

Question 23

uses of photoelectric cells

Answer 23

• burglar alarms
• automatic doors
• counting items on conveyor belt
• monitoring + controlling flame in central heating burner
• reproduce soundtrack in film projector
• scintillation counter

Question 24

x-rays

Answer 24

high freq. electromagnetic radiation. produced when high speed electrons in CRT strike a metal target with high melting point.
- penetrating power ∝ freq.
- freq. ∝ V across tube
Hard xrays - high penetrating power
soft xrays - low p.p.

Question 25

discovered x rays 1895

Answer 25

wilhelm rontgen

Question 26

x ray formation

Answer 26

1. thermionic emission occurs at cathode
2. electrons accelerated towards anode where they strike metal target
3. <1% of kinetic energy converted to x rays
4. rest of energy converted to heat so coolant used and target must have high melting point e.g. tungsten
   (know how to draw)

Question 27

x ray properties

Answer 27

1. electromagnetic radiation (wavelength = 10^-9 and 10^-15m)
2. ionise materials as they pass through
3. penetrate material
4. not deflected by elec. or mag. field
5. produce fluorescence in certain materials e.g. zinc sulphide
6. produce interference patterns + undergoes diffraction
7. causes photoemission

Question 28

uses of xrays

Answer 28

1. medicine: -xray photos for damaged bones + diseased tissue. eating barium sulphate –> can see stomach + intestines
   - destroy cancerous cells
2. industry: -detect cracks + flaws in metals, welds, castings
   - photo inside machines
   - determine thickness of things + fullness of packages

Question 29

ionise

Answer 29

cause loss/gain of 1 or more electrons from an atom/molecule