Download Topic 2 X-rays and ECGs

Unit P3: Applications of physics
Topic 2 X-rays and ECGs
Student Notes
Unit P3: Applications of physics
Topic 2
X-rays and ECGs
We Are Learning To
2.2 Explain the key features of passing a current through an evacuated tube,
including:
a thermionic emission of electrons from a heated filament
b potential difference between the cathode (filament) and the anode (metal
target)
c why the vacuum is necessary
d possible production of X-rays by collision with a metal target
Note: only underlined parts are covered in this lesson
Experiments by J. J.
Thomson in 1897 led to
the discovery of the
electron, and the award of
the 1906 Nobel prize. He
detected electrons as they
landed on a fluorescent
screen.
Filament bulb
When an electric current heats up a metal filament – electrons are ‘boiled
off’.
We call this thermionic emission. The atom is so hot it changes to an
ion and emits the electron.
Although this occurs in a light-bulb filament, you are not aware of the
electrons being emitted. Unlike the light from the filament, the electrons
cannot pass through the glass bulb.
How the electron gun part of the cathode ray tube works
1. Negative electrons are boiled off (thermionic emission) at the filament (cathode)
2. An accelerating voltage of 4000 V between the cathode and anode accelerates
the negatively charged electrons towards the positive anode to high speeds.
3. Some of the electrons pass through a small hole in the anode and come out as an
electron beam on the other side.
Note: The apparatus must be in a vacuum, otherwise air molecules would stop the
electrons by getting in their way.
(Negative) filament cathode
(Positive) accelerating anode
A 6V supply
to the filament
Accelerating Voltage
4000V
Deflecting a beam of electrons
Parallel charged metal plates create an electric field and deflect the electron beam.
The path of the electron beam can be changed by:
1. Increasing the accelerating voltage - the electrons will move faster and will not be
deflected by the plates as much
2. Increasing the voltage between the deflection plates – this will deflect the electron beam
more
3. Changing the charge on the plates - if the top deflection plate is negative and the bottom
deflection plate is positive the beam will be deflected downwards
(Negative)
filament cathode
Charged
deflection plates
(Positive)
accelerating anode
+
A 6V supply
to the filament
Accelerating Voltage
4000V
Unit P3: Applications of physics
Topic 2
X-rays and ECGs
We Are Learning To
2.1 Relate the ionisation by X-rays to their frequency and energy qualitatively (E
= hf is not required)
2.2 Explain the key features of passing a current through an evacuated tube,
including:
a thermionic emission of electrons from a heated filament
b potential difference between the cathode (filament) and the anode (metal
target)
c why the vacuum is necessary
d possible production of X-rays by collision with a metal target
Note: only underlined parts are covered in this lesson
In 1895, a German physicist,
Wilhelm Röntgen discovered X-rays
The Nobel Prize in Physics 1901
"in recognition of the extraordinary
services he has rendered by the discovery
of the remarkable rays subsequently
named after him"
Cathode Ray Tube (X-Rays)
X-Rays, such as those produced by this machine, are produced using
an electron gun.
Tungsten
target
Lead
casing
Cathode
metal target
cathode
Anode
X-rays
X-Rays are produced by firing high energy electron beams at a
tungsten anode. Higher energy X-rays can be produced by
increasing the accelerating voltage.
X-rays
Increasing frequency
The higher the accelerating anode voltage the
greater the frequency of the X-rays that are produced
Higher frequency = higher energy = more ionising =
more danger!
Unit P3: Applications of physics
Topic 2
X-rays and ECGs
We Are Learning To
2.3 Explain why a beam of charged particles is equivalent to an
electric current
2.4 Use the equation:
current (ampere, A) = number of particles per second (1/
second, 1/s) x charge on each particle (coulomb, C)
I=Nxq
2.5 Use the equation:
kinetic energy (joule, J) = charge on the electron (coulomb, C)
x accelerating potential difference (volt, V)
KE = ½ mv2 = e x V
Think of a moving electron in a CRT.
e-
We know that kinetic energy is given by:
KE = ½ mv2
Where has it got this energy from?
Answer: the accelerating voltage (V)
Kinetic energy = charge of electron x accelerating potential difference
KE = e x V
KE = ½
2
mv
=exV
e-
v = velocity of the electron (metres per second, m/s)
m = mass of an electron (kilograms, kg)
e = charge on the electron (coulomb, C)
V = accelerating potential difference (volt, V)
KE = kinetic energy (joules, J)
The charge on an electron is 1.6 × 10-19 C
The mass of an electron is 9.1 × 10-31 kg
Question 1
If the accelerating voltage in an electron gun is 3 kV, what is the kinetic
energy of the electrons in the beam?
The charge on an electron is 1.6 × 10-19 C.
KE = eV
KE = 1.6x10-19 x 3000
KE = 4.8
x10-16
J
(3 marks)
Question 3
What is the velocity of an electron that has been accelerated by
the 2.5 kV anode voltage of a television picture tube?
The charge on an electron is 1.6 × 10-19 C
The mass of an electron is 9.1 × 10-31 kg
KE = eV
KE = 1.6x10-19 x 2500
KE = 4.0 x10-16 J
KE = ½ mv2
4.0 x10-16 = 0.5 x 9.1 x 10-31 x v2
4.0 x10-16 = 4.55 x 10-31 x v2
4.0 x 10-16 = v2
4.55 x 10-31
8.79 x 1014 = v2
V = 29649973 m/s (2.96 x 107 m/s)
e
e
e
e
e
e
e
e
e
e
What is current?
Current is the rate of flow of electric
charge around a circuit.
e
What is current?
Current is the rate of flow of electric charge
around a circuit.
Electrons can flow in
an electric circuit ……
….. and an electron
beam.
Current
=
number of particles
flowing per second
I = N
x
charge on
each particle
x q
I = current in amperes, (A)
N = number of particles flowing each second (1/s)
q = charge on each particle in coulombs (C)
Question 4
2.2 x 1014 electrons reach the anode per second. What is the
current?
The charge on an electron is 1.6 × 10-19 C
I=Nq
I = 2.2 x 1014 X 1.6 x 10-19
I = 0.0000352 A
Or
I = 3.52 x 10-5 A
Exam Style Question:
(a) The acceleration voltage between the cathode and anode is
8 000 V. Calculate the kinetic energy of one electron when it
has reached the anode. Charge on 1 electron e = 1.6 × 10–19C
KE = eV
KE = 1.6x10-19 x 8000
KE= 1.28 X 10-15 J
(b) 5.2 × 1016 electrons reach the anode per second. Calculate
the current. I = N q
I = 5.2 × 1016 x 1.6 x 10-19
I = 0.00832 A
I = 8.32 × 10–3 A
Unit P3: Applications of physics
Topic 2
X-rays and ECGs
We Are Learning To
2.6 Demonstrate an understanding of the inverse square law for
electromagnetic radiation
Intensity (brightness)
The intensity depends on:
• The distance from the source
• The medium the radiation is travelling through
the word ‘radiation’ is used to
describe any form of energy,
e.g. wave or particle
originating from a source
the intensity of radiation will
decrease with distance from a
source and according to the
nature of the medium through
which it is travelling
Intensity
The strength of a radiation is called its
intensity. This is the power of the radiation
per square metre and is measured in W/m2
Intensity (W/m2) = Power (W)
Area (m2)
Investigating how electromagnetic radiation
spreads out as you increase distance
1metre
Measure
area
Now move the projector to 2 m
away and measure the area!
Inverse Square Law
Intensity = 1
d2
Source
When electromagnetic radiation is
emitted by a source, it spreads out
and its strength (intensity)
decreases the further you are from
the source. When you double the
distance from the source, the
strength decreases to a quarter.
The strength decreases according
to the inverse square law.
1m
2m
3m
When the radiations used for medical purposes pass through the body,
they are not in a vacuum, they are in a medium. However, there is a
similar pattern of decreasing intensity. Each material in the body reduces
the intensity of the radiation by different amounts. Bone is dense and
reduces the intensity the most. Muscle is less dense so it does not
decrease the intensity as much.
Graph showing the results of
an experiment to measure
intensity at different distances
through a medium.
Unit P3: Applications of physics
Topic 2
X-rays and ECGs
We Are Learning To
2.7 Relate the absorption of X-rays to the thickness of the material
through which they are travelling, quantitatively
2.8 Describe how X-rays are used in CAT scans and fluoroscopes
2.9 Demonstrate an understanding of the comparison of the risks
and advantages of using X-rays for treatment and diagnosis
X-rays can be used to diagnose some
medical conditions.
X-ray photography
X-rays can penetrate the body. The photographic
film is turned from white to black by X-rays, so the
more X-rays that are absorbed by an area of the
body, the whiter that area will appear. This is why
bones appear white on X-ray photographs.
X-ray machine
patient
X-ray film
Fluoroscopic (fluoroscopy)
An X-ray procedure that produces immediate
images and motion on a screen. The images look
like those seen at airport baggage security
stations.
X-rays - airport security
Fluoroscope
France bans Britain's migrant X-ray scanner...
because it might breach health and safety (Jan 2008)
British border guards in Calais have been banned from using X-rays to search
for illegal immigrants in lorries - unless they ask for the stowaways' written
permission. French authorities have blocked the use of the scanners, claiming
they could breach European health and safety laws.
baby girl having a
fluoroscope scan.
Fluoroscopes are used to show a patient’s
organs working. They consist of an X-ray
source and an X-ray detector attached to a
digital video camera. The patient is placed
between the X-ray source and the detector.
Computed (Axial) Tomography
CAT / CT scans
CAT scans use the different absorption of Xrays by bones and tissue and images taken
from a number of different angles and positions
to build up a 3D picture of the inside of the body
of superb quality.
Diagnostic and preventive scans:
• Investigate cancerous tumours
• Watch the heart in motion
• Check for colon (bowel) cancer
• Study haemorrhaging, bone
trauma and lots more…
‘Slices’ through the body
• CT scanners create a 2D image that is like a
slice through the body, a bit like a slice of bread.
• Many 2D images can be used to reconstruct a
3D image.
BBC News, Sept 27 2010 A food production
company was ordered to pay nearly £17,000
after a man found a dead mouse in a loaf of
bread as he made sandwiches for his children.
34
What’s it like to have a CT scan?
• A CT scanner is an X-ray machine rotated at high
speed – so it’s pretty noisy… (see open CT scanner clip)
• … but non-invasive.
• It takes between a few minutes and an hour.
• Sometimes you’re given an injection of a chemical to
increase the contrast of the images.
• You need to lie still with your
arms above your head for the
best image of the chest or body.
CT Scan - What to Expect (UW Medicine) - YouTube.flv
Dangers of X-rays
This woman, like thousands of others, suffered from burns,
scarring, and cancer after undergoing X-Ray "beauty treatments".
Early X-Ray technicians fell victim
to the horrible side-effects of
radiation.
Today we are aware of
the dangers of X-rays.
X-rays are ionising
Early protective suits
made of heavy aprons
and metal helmets
Ionising radiation can damage DNA.
This could cause…
1. Cancer
2. Inflammation
3. Cell death
4. Damage to genes can lead to mutations in offspring
Workers who work with radiation every day e.g.
hospital staff/nuclear power plant workers need to:
(1) monitor the dose they are being exposed to staff wear special badges that monitor their
exposure.
(2) take precautions to reduce the dose they are
exposed to - leave the room or go behind a lead or
thick concrete shield whilst a patient is being given
radiation treatment
Is ionising radiation harmful?
Ionising radiation can damage DNA.
This could cause…
1. Cancer
2. Inflammation
3. Cell death
4. Damage to genes can lead to mutations in offspring
Cells are more sensitive to radiation during cell division than at
other times.
Cells which divide frequently (e.g. the gut walls) are more
sensitive than those which rarely divide (e.g. nervous tissue).
X-rays can be used to treat some
medical conditions.
Radiotherapy
 The tumour is exposed to X-rays radiation at
different angles.
 This gives normal cells a low dose of radiation,
while the tumour receives a high dose.
 However, levels have to be carefully monitored
so that healthy cells are not damaged as well.
Rotating X-ray
source
tumour
In some patients radiation treatment may not be able to destroy the cancer.
Sometimes it is used only to reduce suffering (palliative care).
Unit P3: Applications of physics
Topic 2
X-rays and ECGs
We Are Learning To
2.10 Explain how action potentials can be measured with an
electrocardiogram (ECG) to monitor heart action
2.11 Relate the characteristic shape of a normal ECG to heart
action
2.12 Use the equation:
frequency (hertz, Hz) = 1/time period (second, s)
f = 1/T
2.13 Describe the use of a pacemaker to regulate the heart action
Right
Atrium
Left
Atrium
Right
Left
Ventricle Ventricle
An electrocardiogram (ECG) measures the
electrical activity of the heart.
An ECG trace can then be used to calculate a
patients heart rate and to check for abnormalities.
Action potential
When a heart beats, each muscle cell must contract at exactly the
right moment in order to push the blood out of the heart. An
action potential (electrical signal) is sent to each muscle cell to
tell it when to contract. This electrical signal starts in the SA node
(pacemaker).
The human body contains a high proportion of water with salt
dissolved in it, which means that it will conduct electricity. The
action potentials are conducted through the body to the skin,
where they can be detected and used to produce an
electrocardiogram (ECG).
Pacemakers
Some people’s hearts do not beat properly. The action
potentials do not spread across the heart properly. A
pacemaker can be attached to the heart. The
pacemaker detects the action potentials, amplifies them
and transmits them to other parts of the heart, so that
the chambers of the heart contract correctly.
1. The SA node (pacemaker)
produces an electrical stimulus
A
V
5. Cells in ventricle
relax.
A
V
2. The cells in the atria
contract.
3. The AV node delays
conducting the impulse from
the atria to the ventricles. This
delay allows the atria to fill
the ventricles with blood
before they contract.
4. Cells in the ventricles
contract.
QRS-wave is due to the
contraction of the ventricles
P-wave is due to the
contraction of the atria
T wave is due to the
relaxation of the ventricles
frequency (hertz, Hz) = 1/time period (second, s)
f=1
T
0.8
Frequency =
1
time period
Frequency = 1
0.8
1.25 bps
1.25 bps x 60
75
Unit P3: Applications of physics
Topic 2
X-rays and ECGs
We Are Learning To
2.14 Describe the principles and use of pulse oximetry
the word ‘radiation’ is used to
describe any form of energy,
e.g. wave or particle
originating from a source
the intensity of radiation will
decrease with distance from a
source and according to the
nature of the medium through
which it is travelling
Pulse Oximetry
Used to measure:
1. Heart rate
2. Oxygen concentration in blood
Non-intrusive/non-invasive technique
(no cutting/no surgery/no needles)
A fit, healthy person should have an oxygen saturation level
between 95% & 100%.
If the blood is 95% saturated. This is written 95 SPO2%.
Pulse Oximeters are routinely used on the wards and
in casualty department.
Hypoxia (medical) a shortage of oxygen in the body
Hypoxia (environmental) a condition
in high altitudes such as mountains
where the reduced partial pressure of
oxygen available leads to hypoxia
Photodetector
Oxygenated blood absorbs
more IR light than red light.
IR
LEDs
LEDs
L
Beam of red light
Beam of infrared li
Red
IR
Red
LEDs
Photodetector
Photodetector
Poorly oxygenated blood absorbs
more red light than IR light
Amount of red and IR detected by photodector
Oxygenated blood
Poorly oxygenated blood
Red
IR
Red
IR
Emission
Absorption
Transmission
How does a pulse oximeter calculate heart rate?
During a pulse the arteries stretch, their volume increasing in the moment
after the heart contracts and pushes out blood. As the pulse of blood
passes through the beam of light from the LEDs, the blood absorbs more
of the light and the photodetectors receive less light, so the reading
changes. Each change is detected as a pulse.