Modern physics Flashcards
1
Q
3 properties of an electron
A
extremely small mass
has a negative charge
located in orbit around the nucleus of an atom
2
Q
thermionic emission
A
the emission of electrons from the surface of a hot metal
3
Q
electrons were originally called
A
cathode rays
4
Q
what are cathode rays
A
electrons emitted from a cathode
5
Q
example of electrons emitted from a cathode
A
an old television
6
Q
who suggested the name electron
A
G J Stoney, irish hysicist
7
Q
who and when was the charge measured on the electron
A
milikan in 1909 with his oil drop experiment
8
Q
in a cathode ray tube what does the cathode do
A
electrons are emitted from the cathode by thermionic emission
9
Q
in a cathode ray tube what does the control grid do
A
by being kept more or less negative (o to -50V) you can control the amount of electrons passing through ie. brightness control
10
Q
in a cathode ray tube what does the focusing anode do
A
converges the electrons into a narrow beam so that a well defined image is seen on the screen
11
Q
in a cathode ray tube what does the accelerating anode do?
A
has a potential usually over 1000V. accelerates the electrons to a very high speed
12
Q
in a cathode ray tube what does the flourescent screen do?
A
when the very fast electrons hit the screen, their kinetic energy is converted to light
13
Q
what chemical is used on the screen
A
zinc sulfide
14
Q
Y-plates
A
a pair of charged horizontal metal plates
15
Q
what do the y-plates do?
A
the electric fields of these plates can deflect the electrons vertically
16
Q
X-plates
A
a pair of charged vertical metal plates
17
Q
what do the x-plates do?
A
the electric fields of these plates can deflect the electrons horizontally
18
Q
2 parts of the deflecting system
A
x plates and y plates
19
Q
what does the coating of graphite inside of the tube do?
A
conducts the electrons from the screen back to the accelerating anode. The passage of electrons between cathode and anode is maintained
20
Q
2 uses of a cathode ray tube
A
an old television
a cathode ray oscilloscope
21
Q
2 medical uses of a cathode ray tube
A
ECG
EEG
22
Q
what does ECG stand for?
A
electrocardiogram
23
Q
what does EEG stand for?
A
electroencephalogram
24
Q
what does an ECG do?
A
displays electrical signals in the heart
25
what does an EEG do?
displays small varying electrical signals in the brain
26
kinetic energy of an electron
1/2 m v squared
27
work done to accelerate an electron
q V
28
how do you find the speed of an electron accelerated
let the gain in kinetic energy equal to the work done to accelerate
29
how to find the velocity of an electron in a circular orbit in a magnetic field
let the force of magnetic field equal the centipetal force
30
force of magnetic field on electron
B q v
31
centripetal force
m v squared
-----------------
r
32
set up of demonstration to show photoelectric emission
high frequency light falling on a negatively charged plate on a gold leaf electroscope
33
what happens in the demonstration to show photoelectric emission
gold leaf electroscope deflection decreases
34
in the demonstration to show photoelectric emission, why does the deflection of the gold leaf electroscope decrease
electrons are being emitted from the negatively charged metal plate due to the incident light
35
***** definition of photoelectric emission *****
the emission of electrons from a metal surface using light of a suitable frequency
36
what do you use to change the frequency of the incident light
using special filters
37
photoelectric emission AKA
photoelectric effect
38
demonstration to show photoelectric emission. what do low frequencies result in?
no electrons, the deflection remains constat
39
below what number is there no loss in deflection
the threshold frequency
40
graph of number of electrons emitted and frequency of incident light
straight line graph started at the x-axis and threshold frequency
41
graph of number of emitted electrons and frequency of incident light, what's on x-axis
frequency of incident light
42
graph of number of emitted electrons and frequency of incident light, what's on y-axis
number of emitted electrons
43
what does the kinetic energy of emmitted elecgrons due to photoelectric emisiion depend on?
the frequency of the incident light (Hz)
44
to show the effect of intensity of incident light experiment. describe the set-up
a wire anode and cathode in a photocell (vacuum inside) connected to a milliammeter. bulb connect to battery and rheostat shining on anode
45
to show the effect of intensity of incident light experiment. the brighter the light
the more current recorded
46
to show the effect of intensity of incident light experiment. the number of electrons emitted is proportion to
the intensity of the incident light
47
rheostat
variable resistor
48
to show the effect of intensity of incident light experiment how can you vary the amount of light
the rheostst
49
what do you measure photocurrent with
an ammeter
50
to show the effect of intensity of incident light experiment. graph x-axis
photocurrent (mA)
51
to show the effect of intensity of incident light experiment. graph y-axis
intensity of incident light
52
to show the effect of intensity of incident light experiment describe the graph
straight line through the origin
53
what is photocurrent an indication of?
the number of emitted electrons
54
if you wanted to be very precise what could you use to measure the intensity of light
light meters
55
to show the effect of intensity of incident light experiment. the brighter the light
the more current recorded
56
to show the effect of intensity of incident light experiment. the number of electrons emitted is proportion to
the intensity of the incident light
57
rheostat
variable resistor
58
to show the effect of intensity of incident light experiment how can you vary the amount of light
the rheostst
59
what do you measure photocurrent with
an ammeter
60
to show the effect of intensity of incident light experiment. graph x-axis
photocurrent (mA)
61
to show the effect of intensity of incident light experiment. graph y-axis
intensity of incident light
62
to show the effect of intensity of incident light experiment describe the graph
straight line through the origin
63
what is photocurrent an indication of?
the number of emitted electrons
64
to show the effect of intensity of incident light experiment. describe the set-up
a wire anode and cathode in a photocell (vacuum inside) connected to a milliammeter. bulb connect to battery and rheostat shining on anode
65
to show the effect of intensity of incident light experiment. the brighter the light
the more current recorded
66
to show the effect of intensity of incident light experiment. the number of electrons emitted is proportion to
the intensity of the incident light
67
2 things that happens to the energy acquired by the electron
used to escape from the metal surface and the rest is used as kinetic energy
68
what does the number of electrons emitted depend on?
the number of photons ie. the intensity of the incident light
69
energy of incident photon =
work function + max. kinetic energy of emitted electron
70
to show the effect of intensity of incident light experiment. graph x-axis
photocurrent (mA)
71
to show the effect of intensity of incident light experiment. graph y-axis
intensity of incident light
72
to show the effect of intensity of incident light experiment describe the graph
straight line through the origin
73
what is photocurrent an indication of?
the number of emitted electrons
74
what did planck propose?
that electromagnetic radiation consisted of discrete quantities of energy called a quantum or a photon
75
what year did planck make his proposal
1902
76
what was the classical idea of electromagnetic radiation
that it was a continuous wave
77
energy of the quantum is proportional to
the frequency of the radiation
78
E =
hf
79
E
energy of the photon
80
photoelectric emission experiment: why does the leaf not collapse when zinc covered by a piece of glass
uv light cannot pass through ordinary glass and photoelectric emission does not occurs
81
f
frequency of the radiation
82
when a photon collides with an electron at or just within the surface of a metal
it may transfer its energy to the electron
83
3 applications of photoelectric effect
burglar alarms
smoke alarms
reading barcodes
84
what does it mean to be an all or none process
either the electron gets all the energy of the photon or none at all
85
energy of the photon is proportional to what
the frequency of radiation
86
2 things that happens to the energy acquired by the electron
used to escape from the metal surface and the
87
what did planck propose?
that electromagnetic radiation consisted of discrete quantities of energy called a quantum or a photon
88
what year did planck make his proposal
1902
89
what was the classical idea of electromagnetic radiation
that it was a continuous wave
90
energy of the quantum is proportional to
the frequency of the radiation
91
what can x-rays also have to help the cooling process
A liquid cooling system
92
E
energy of the photon
93
h
planck's constant
94
f
frequency of the radiation
95
when a photon collides with an electron at or just within the surface of a metal
it may transfer its energy to the electron
96
what type of process is the transfer of energy
an all or none process
97
what does it mean to be an all or none process
either the electron gets all the energy of the photon or none at all
98
energy of the photon is proportional to what
the frequency of radiation
99
2 things that happens to the energy acquired by the electron
used to escape from the metal surface and the rest is used as
100
why is it good that there isn't much gas inside the glass in the x-ray
electrons are less probable to hit a gas atom on the way
101
Φ
work function
102
threshold frequency is related to
work function (Φ)
103
threshold frequency equation
Φ = h fo
104
fo
threshold frequency
105
other way of writing hf, when equal to
| Φ + 1/2 m v squared
hc
------ = Φ + 1/2 m v squared
λ
106
unit of energy in atomic physics
the electron volt
107
1 ev =
1.6 x 10 -19 joules
108
can you use electron volts in calculations
no, must convert to joules
109
photoelectric emission experiment: why does the leaf collapse when it is a negatively charged piece of metal
the uv light causes photoelectric emissions, ie electrons leave the surface of the metal, therefore the negative charge reduces and it collapses
110
photoelectric emission experiment: why does the leaf not collapse when the zinc is illuminated with green light
the frequency of the green light is lower than that of uv radiation and it is below the threshold frequency of zinc and photoemission cannot occur
111
photoelectric emission experiment: why does the leaf not collapse when the electroscope is charged positively
electrons emitted will be attracted back to the zinc plate, and if they could escape it would become even more positive and it would diverge more
112
3 applications of photoelectric effect
burglar alarms
smoke alarms
reading barcodes
113
3 steps in how x-rays are made
electrons emitted from the cathode by thermionic effect
accelerated to very high speed by high voltage
they hit the tungsten target and electrons are emitted
114
what does a lead shield do when making x - rays
protects the user from the harmful effect of x - rays
115
atomic view; 3 steps to making an x - ray
- high speed electrons hit the tungsten anode and some are energetic enough to penetrate
- outer electro falls into a lower energy level due to missing electron
- excess energy emitted as a photon - an x - ray
116
99% of energy of the electrons
becomes heat energy when hits the tungsten
117
does tungsten melt
no, has a high melting point
118
what conducts the heat to the outside when making x - rays
the tungsten is set in copper which conducts the heat to the cooling fins outside
119
why does the heat escape off the fins into the air
the cooling fins have a large surface area
120
what can x-rays also have to help the cooling process
A liquid cooling system
121
who and when was the x-ray discovered
1895 by the German Physicist, W Rontgen
122
what does the intensity of an x-ray beam refer to?
the number of x-rays in the beam
123
what does the number of x-rays required depend on?
the surface area of what is to be examined
124
intensity depends on (3)
number of electrons crossing the tube
this depends on temperature of the heating coil
this depends on the current from the low voltage battery
125
what does the penetrating power of the x-rays refer to?
the ability of the x-rays to pass through different thicknesses of material or different densities of material
126
penetrating power of x-rays depends on what (2)
speed of electrons crossing the tube
| which depends on the value of high voltage
127
2 uses of x-rays
damaged tissue in lungs is more dense, absorbs more x-rays, and shows up as a dense could on x-ray
can locate flaws in metal eg.crack in the body of an airplane
128
what's special about the gas in the x-ray tube
very low pressure inside, very little gas inside
129
why is it good that there isn't much gas inside the glass in the x-ray
electrons are less probable to hit a gas atom on the way
130
what would gas molecule collisions do to the electrons
would slow them down
131
relationship between x-rays and photoelectric emission
it is said that one is the reverse of the other
132
what goes into making x-rays
electrons
133
what goes out of making x-rays
photons
134
when electrons hit the tungsten target when making x-rays
photons are emitted
135
photoelectric effect what goes in
photon
136
photoelectric effect what comes out
electrons
137
photoelectric effect, what happens when photons fall on a metal
electrons emitted
138
speed of a photon
c, speed of light
139
we assume that all energy
is given to the electron (even though it isn't)
140
speed of an x-ray
same as speed of light when it's in a vacuum
141
what does it mean that x-rays are regarded as ionising radiation
they ionise the molecules of a material that they pass through
142
are x-rays deflected in electric or magnetic fields
no
143
what does it mean that x-rays aren't deflected in electric or magnetic fields
they are neutral
144
how are x-ray photographs possible
they cause a chemical reaction with they film in a similar way that light does
145
1 danger of x-rays
excessive doses of x-rays could lead to certain forms of cancer
146
what is a photon
a discrete quantity of electromagnetic radiation
147
what is an x-ray
a photon of electromagnetic radiation of a very high frequency
148
who discovered radioactivity
Becquerel
149
when did Becquerel discover radioactivity?
1896
150
intensity of radiation
number of emissions per second
151
the intensity of radiation is proportional to
the mass of the radioactive sample
152
3 types of radiation
alpha
beta
gamma
153
emission of radiation
spontaneous
| cannot be controlled
154
definition of radioactivity
the spontaneous disintegration of the unstable nucleus of the unstable nucleus of an atom with he emission of a B or y radiation
155
alpha charge
positively charged
156
beta charge
negatively charged
157
gamma charge
neutral, no charge
158
alpha
a
159
beta
B
160
gamma
y
161
what does the radiation do to the substance it passes through
it ionises it
| splits it into positive and negative ions
162
are all molecules encountered ionised
no
| not all
163
what happens to the ray or particle after each ionisation
it loses me kinetic energy
164
how far does a good ioniser travel
not far
165
how far does a bad ioniser travel
ver far
166
how are penetration and ionisation related
penetration and ionisation are inversely proportional
167
a is a
particle
168
B is a
particle
169
y is a
ray
170
ionisation of a
very good ionising power
171
penetration of a
very little penetration
172
how do u stop an alpha particle
a thick sheet of paper
173
how far do alpha particles travel
only a few centimetres in air
174
ionisation of B
much less ionising than a particles
175
penetration of B particle
100 times more penetrating power than a particles
176
what can B particles pass through
up to 5mm thickness of aluminium
177
ionisation of y rays
very little ionisation
178
penetrating power of y rays
very penetrating
179
what does it take to stop y rays
several cm of lead
180
what do alpha particles consists of?
2 protons and 2 neutrons
181
what is the alpha particle like
the nucleus of a helium atom
182
what does a beta particle consist of?
an electron
183
how is a beta particle formed?
a fast moving electron that comes from the nucleus of an atom
a neutron decays into a proton and an electrons and the electron is the B-particle
184
symbol for a particle
4
He
2
185
charge on an alpha particle
++
186
symbol for a B particle
0
B
-1
187
what is a gamma ray
a photon of electromagnetic radiation from the nucleus of an atom
188
how do you calculate how many a and B particles
- take the smaller atomic mass away from the larger atomic mass and divide by 4
- that's how many alpha particles
- if it is lower than the atomic number still after taking away the 2 protons per a particle. that's how many B particles were emitted
189
how does the atomic number get bigger when you take away B particles
misusing a minus
190
how do you construct a radioactive decay series
you put a new element in after you take away each alpha or beta particle
191
how do you demonstrate ionisation
```
charge a gold leaf electroscope
bring a radioactive source near
it ionises the air above it
some ions attracted to cap
neutralised
less deflection
```
192
demonstration to show penetration
bring the radioactive source near the geiger muller tube, you will notice an increase in count rate
193
distance for a particle count to drop
10cm
194
what to put between a particles and g m t for count to drop
a piece of paper
195
what happens when you take the b radiation source away
no difference
196
what to put between B particles and g m t for count to drop and why
sheets of aluminium. B particles cannot penetrate aluminium
197
what to put between y rays and g m t for count to drop
a few cm thick lead
198
who first performed Rutherford's experiment?
Geiger and Mardsen
199
what is the set up of Rutherford's experiment?
a particles bombard a very thin piece of gold foil
200
most a particles in Rutherford's experiment?
go straight through gold foil
201
some a particles in Rutherford's experiment?
deflected through a small angle
202
very few a particles in Rutherford's experiment?
deflected through angles greater than 90º
203
where was Rutherford's experiment performed and why
in a vacuum because a particles have a very short range in air
204
what detected the alpha particles?
a fluorescent screen
205
how did the fluorescent screen detect the alpha particles
they cause scintillations
206
what are scintillations
small flashes of light
207
why would an a particle be deflected through a small angle
if it is repulsed by the positively charged nucleus
208
why would an a particle be deflected through an angle greater than 90º
if the a particle was hiding straight for the nucleus
209
why is there a very small chance of being deflected through an angle greater than 90º
the nucleus is very small, very small probability of going straight for it
210
3 conclusions of Rutherford's experiment
- the nucleus is very small
- electrons orbit the central nucleus
- atom is mostly made of empty space
211
4 features of the Bohr model of the atom
- electrons only move in allowed orbits
- when electrons move to a higher energy level, the atom is in an excited state
- when the electron returns to its original position it emits a photon
- energy emitted by an electron:
hf = E1-E2
212
what acts as the cathode in the geiger muller tube
a metal cylinder
213
what acts as the anode in a geiger muller tube
a thin wire along the centre
214
what does the voltage applied across the geiger muller tube depend on? 2
the density and pressure of the gas in the tube
215
what is usually the gas in the geiger muller tube
argon
216
what is in the geiger muller tube as well as argon?
a small amount of bromine
217
function of bromine in geiger muller tube
a quenching agent
218
how does the particle or ray enter the geiger muller tube
through a micra end window
219
what happens as the particle or ray enters the geiger muller tube
it ionises an argon gas molecule
220
what does the amount of molecules ionised depend on?
depends on the radiation
221
what happens to the negative ions in a geiger muller tube
they are rapidly accelerated towards the wire anode
222
what causes further ionisation in a geiger muller tube
high speed electrons
223
what is it called when high speed electrons causes further ionisation in a geiger muller tube 2
gas amplication or avalanche
224
in an avalanche, one electron can make how many electrons?
1 x 10 o the power of 8
225
when the very large number of electrons in the geiger muller tube, reach the anode what happens
they pass through the external circuit as a fairly sizeable current
226
what happens each time a current flows through a resistor
a potential drop occurs
227
what counts the potential drops in a geiger muller tube
a counter (scaler or ratemeter)
228
what can the counter in a geiger muller tube
can count the number of particles that enter the tube
229
what is there a risk of when positive ions hit the cathode of a geiger muller tube and why is that bad
risk of electrons being knocked off the cathode and would register as a count on the scaler
230
what does bromine do in a geiger muller tube
stops the electrons being emitted (quenching agent )
231
what does a scaler give
total number of particles that entered the tube
232
what does a ratemeter give
reading in counts per second
233
law of radioactive decay
the rate of decay of a radioactive isotope is proportional to the number of atoms of the isotope present
234
A is proportional to
N
235
A =
λN
236
N
number of atoms present in the sample
237
λ
the decay constant
238
unit of the decay constant
s to the minus one
239
half life
the time required for half the nuclei in a radioactive sample to decay
240
symbol of half life
T and a small half beside it
241
unit of half-life
second (s)
242
connection between decaying constant and the half life
1 ln2
T - = -----
2 λ
243
if they give half life in hours for a maths question?
get seconds by dividing by 60x60
244
if they want you to find amount left after a certain time but they give you the decaying constant?
find the half life first and then count out the hours
245
if they give the half life in years
divide by 365x25x60x60
246
how do you writ an equating to represent the decay of an isotope
original element -> B/a partices + new element
247
rate of decay unit
bequerel
| Bq
248
1 bequerel =
1 a/B
-------
1 second
249
nuclear fusion definition
the combining of two small nuclei to form a larger nucleus with a loss in mass and a release in energy
250
does nuclear fusion involve atoms and why
no because that would be chemical bonding. its nuclei
251
example of nuclear fusion
2 2
H + H --->
1 1
4
He + energy
2
252
2
H is also known as
1
deuterium
253
deuterium is an
isotope of hydrogen
254
2 problems with nuclear fusion
nuclei have positive charges and it is difficult to overcome the force of repulsion between them
controlling such high temperatures has not yet been achieved
255
whats the speed problem with nuclear fusion
the very high speeds would require very high temperature, as hot as the sun
256
what is a hydrogen bomb
a fusion reaction that has gone out of control
257
1 advantage of fusion
very little radioactive waste to dispose of
258
what is the required fuel for fusion and why is it good
deuterium, can be extracted from water, there would be no fuel shortage
259
E = (einstein)
mc squared
260
m
change in mass
261
c
speed of light
262
c =
3 x 10 to the power of eight m/s
263
definition of nuclear fission
a nucleus is bombarded with a neutron, it splits into two nuclei of approximately equal size, two or three neutrons are emitted and the resulting loss of mass is released as energy
264
2 main isotopes in natural uranium
u 238 and u 235
265
percentage of u 238 in natural uranium
99.3% of world supply
266
percentage of u 235 in natural uranium
0.7% of world supply
267
if you increase the percentage of u 235
you get enriched uranium
268
chain reaction
where one fission reaction causes another reaction which causes another reaction and so on
269
a fission reaction in uranium is likely to
be followed by another one
270
when do chain reactions not occur
when the hydrogen escapes through the surface of the sample
271
how do you achieve the escaping of electrons?
keeping the samle size below a certain size, the critical size
272
in a nuclear reactor
a chain reaction happens but the reaction is controlled
273
in a nuclear bomb
a chain reaction happens but it is purposely let go out of control
274
2 isotopes used in nuclear bombs
u235 or plutonium
275
in a nuclear reactor what are the fuel rods made of 2
natural uranium or enriched uranium
276
in a nuclear reactor what is the moderator usually made of
grahite or heavy water
277
heavy water
D2O
278
function of the moderator in a nuclear reactor
the neutrons are travelling too fast, but the moderator slows them down to a suitable speed for further fission
279
what are the control rods in a nuclear reactor made of 2
boron or cadmium
280
what can the control rods do
absorb the emitted neutrons
281
function of the control rods
prevent further fission and it getting out of control
282
how do you regulate the energy output in a nuclear reactor
by raising and lowering the control rods
283
what does the coolant do in a nuclear reactor
pumps carbon dioxide around the system that carries the heat to the heat exchange unit
284
where does the coolant in a nuclear reactor absorb the heat from?
very high kinetic energy creates temperature rises in a nuclear reactor
285
what happens in the heat exchange unit in a nuclear reactor
the heat carried by the CO2 gas is used to boil water
286
what generates the electricity in the heat exchange unit in a nuclear reactor
steam from boiling water is used to turn a turbine which creates electricity
287
what do conventional power stations do?
boil water to make steam using fossil fuels
288
what is the purpose of the shielding in a nuclear reactor
a thick concrete safety shield gives protection from neutrons and gamma rays to the outside world
289
impact on miners, mining for uranium
the workers are exposed to radiation from materials like radon which can cause lung cancer
290
what can an accident at a nuclear reactor do
release radioactive chemicals into the atmosphere
291
why do used nuclear rods have to be carefully stored
because they are radioactive
292
3 uses of radioisotopes
medical imaging
food irradiation
smoke detectors
293
explain how medical imaging is a use of radioisotopes
radioactive chemicals with very short half lives are inserted into the body to trace blood flow or to help obtain an image of a particular organ
294
explain how medical therapy is a use of radioisotopes
radiation from certain radioactive chemicals can be used to kill cancerous cells
295
explain how food irradiation is a use of radioisotopes
if food is placed in a sealed container and bombarded with radiation , any harmful bacteria is killed and it stays fresh for a very long time
296
explain how agriculture is a use of radioisotopes
the passage of chemicals through a plant can be traced by making the chemicals radioactive beforehand
297
explain how radiocarbon dating is a use of radioisotopes
by examining the amount of C-14 in old samples of dead matter, their age can be determined
298
explain how smoke detectors are a use of radioisotopes
they contain radioactive material that ionises the gap between the two electrodes, allowing current to flow. if smoke gets between it cuts it off and triggers the alarm
299
explain how industry has a use of radioisotopes
manufacturing of paper: thickness can be checked
| leaks in underground pipes can be detected
300
4 factors that the harmful effects of radiation of on the human body depend on
nature of the radiation
the part of the body being exposed
how active the radioactive material is
the dose that the body reiceves
301
a particle source outside the body
no damage, cannot penetrate skin
302
B particle on the body
most radiation absorbed by surface tissue, a few mm of aluminium could protect you
303
y rays on body
present greatest external radiation as they can penetrate deep into the body
304
3 most susceptible parts of the body to harm by radiation
reproductive organs, blood forming organs eg. liver, and eyes to a smaller extent
305
less vulnerable organs 4
hands, forearms, feet and ankles
306
cosmic radiation
from outerspace, bombards the earth and we are constantly exposed to it
307
what rock gives off radiation
granite has a minute traces of uranium which decays to radon gas
308
2 things you can do to prevent radiation if your house is built above granite
proper ventilation so that the gas can escape and good insulation to stop gas entering
309
how to measure background radiation
use a geiger muller counter and a ratemeter and take a count each minute for 20 minutes. it will vary a lot but get an average value
310
4 ways to protect yourself from radiation
correctly dispose of radioactive waste
wear protective clothing (gloves, glasses)
do not eat or drink near radioactive material
measure radiation levels near you to ensure your safety
