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# G485 OCR A2 Physics Long Explanations watch

1. This thread is for people to revise the long explanation questions in G485 paper for OCR A-level Physics. See next post for medical physics and modelling the universe.

Summary Notes of G485

Electric and Magnetic Fields
Mass Spectrometer

How Transformer's Work
how transformers work:

1. input ac current creates a changing magnetic flux
2. primary and secondary coils are magnetically linked, hence an e.m.f. is generated in secondary coil
3. low resistance wires allows a current to flow in secondary coil

Vp/Np = Vs/Ns

VpIp = VsIs (if transformer is 100% efficient.)

to reduce energy loss in transformers:
1. lamination minimises eddy currents
2. soft iron core lowers magnetic hysteresis
3. low resistance (thick) wires reduce heat loss

Capacitors
Use of Capacitors

Computer power backup:
Charge when laptop is on, and when power is cut off, it discharges slowly through a large resistor providing power to prevent data loss.

Camera:
When a pictures taken, Capacitor discharges extremely fast to provide a high current, through a tube filled with Xenon gas which is ionised proving a bright flash.

Nuclear fusion: Z machine in New Mexico has a very large number of high-voltage capacitors that are charged slowly and discharged in a few billionths of a second, generating around 300 terawatts of power.

Nuclear Physics
Rutherford's Alpha Particle Scattering

1. Alpha particles are fired at a thin gold target. A zinc sulfide screen that produces a flash of light when an alpha particle hits it is used.
2. Most of the time, there was no deflection - mostly empty space in atom
3. Some alpha particles were deflected by more than 90 degrees (i.e. backwards)
4. To get a large enough repulsion on a positive charge the alpha particle needs to get close to the nucleus.
5. The only way for the alpha particle to get this close is for a very small positive nucleus to exist; hence nuclear model.

Use of Radioactive isotopes - Carbon Dating

1. When Alive, plants absorb CO2 during photosynthesis.
2. Small proportion of the absorbed C is radioactive C-14 with a half life of 5570.
3. Once dead, no CO2 is absorbed, so ration between C-12:C-14 is decreased.

4. Ration is determined, and life of relic is determined by equation.

Use of Radioactive isotopes - Smoke Alarm

1. At the top of a smoke detector is an ionisation chamber, with americium-241, an alpha emitter with a half-life of 432 years.
2. This alpha source causes ionisation of the air particles and so an ionisation current runs through the chamber.
3. When smoke particles enter the chamber they become charged by the ions present, reducing the ionisation current.
4. Electronic circuitry detects the reduction in current and sounds the alarm.

Induced Nuclear Fission

1. thermal (slow-moving) neutron is absorbed by uranium nucleus
2. distortion in massive nucleus is such that strong force no-longer counter's electrostatic repulsion
3. nucleus splits into two daughter nuclei and one or more neutrons
4. Neutrons can collide & fission with further uranium nuclei
4. There is a decrease in mass, by E=mc^2, this mass is converted into heat energy (steam turbine => electricity)

Nuclear Reactor Layout
1. fuel rod contains the uranium nuclei / fissile material
2. control rod - boron to absorb neutrons, changing rate of reaction
3. cooling gas - H2O or CO2 => kettle way of generating electricity
4. moderator - carbon / D2O, slow down fast-moving neutrons to thermal neutrons
5. Slow moving thermal neutrons have a greater chance of causing fission, sustaining a chain reaction
Attached Images

2. Medical Physics
Production of X-rays

1. Electrons are accelerated through a high voltage
2. High speed electrons hit metal
3. Kinetic energy of electrons is transferred into x-ray photons

Absorption of X-rays

1. Photoelectric Effect:
Incoming photon strikes & absorbed by metal plate; an electron is ejected.

2. Compton Scattering:
Incoming photon collides with an electron, the electron is ejected and the photon is scattered / has lower energy (hence frequency decreased).

3. Pair production:
Incoming high energy photon (disappears and) produces electron-positron
pair

Image Enhancement of X-rays

1. Consume Barium Meal
2. increases attenuation coefficient of softer tissue that absorbs it
3. allowing imaging of tissue like intestines

Or

1. Phosphor screen is place behind the patient.
2. When it's struck by an X-ray photon, it is absorbed, and releases many visible photons.
3. Photons enter a photo-multiplier tube, exponentially increasing the number of photons untill an impulse is improved.
4. Relayed to computer where it produces a 3D image.

5. X-ray intensity/exposure is reduced as the detectors more sensitive to visible photons.

Medical Tracers

1. Radioactive isotope is attached to carrier molecule which targets organ
2. Tracer is injected into the body
3. Tracer is absorbed by organ
4. Gamma Camera is used to detect radiation from the body

Gamma Camera

1. lead collimator tubes absorb all photons except those travelling along the axis of tubes
2. sodium iodide crystal as scintillator absorbs a ɣ photon and emits thousands of visible light photons
3. photomultiplier tubes detect visible light, produces an electrical signal for computer
4. image quality improved by narrower lead tubes or longer scanning time

CAT Scan

1. X-ray beam rotates around the patient
2. A thin fan-shaped beam is used
3. Images of 'slices' through the patient (in one plane) is produced with computer software
4. The X-ray tube and detectors are moved along the patient for the next slice.
5. 3D image produced by computer.

PET Scan
PET - Positron Emission Tomography

1. A positron / beta-plus emitting source is used (injected into patient)
2. The positron annihilates with an electron inside the patient
3. This produces two gamma photons
4. The photons travels in opposite directions
5. The patient is surrounded by a ring of gamma detectors
6. The arrival times of the photons (delay time) indicates the location (of tissue)
7. A 3-D image is created by a computer connected to detectors.

MRI Scan

Piezoelectric Effect

ultrasound (>20kHz) is produced by the piezoelectric effect
1. alternating pd applied to piezoelectric crystal (quartz) causes it to oscillate.
2. alternating pd is at resonant frequency to maximise amplitude
3. epoxy resin backing prevents backwards propagation of ultrasound & rapidly damps oscillations after alternating pd stops

Z= ⍴c
(acoustic impedance = density of tissue x speed of ultrasound)

Ultrasound Scan

ultrasound scanning
1. transducer is used to send pulses of ultrasound into the patient.
2. pulses are reflected at tissue boundaries
3. intensity of the reflected pulse depends upon the acoustic impedance
4. delay time of the retuning pulse is used to determine the depth

Ir / I0 = (Z2 - Z1)2 / (Z2 + Z1)2

A-scans are voltage peaks, where reflected ultrasound creates a p.d. by the piezoelectric effect
B-scan is multiple A-scans for 2d image

Ultrasound Doppler

Ultrasound Doppler:

1 - A piezoelectric transducer is placed at an angle to the artery / arm which sends pulses of ultrasound.

2 - These ultrasound pulses are reflected by the moving blood cells.

3 - The frequency (or wavelength) of ultrasound is changed (from the doppler effect)

4 - The change in frequency is related to the speed of the blood, (calculated by change in f / f = 2vcos(theta) / c, where c is ultrasound speed, v is speed of blood)

Comparison of Medical Scans

Modelling the Universe
Formation of Stars

End of Life for Low Mass Stars

End of Life for High Mass Stars

Big Bang & Formation of Universe

Significance of 3K CMB

Significance of 3K CMB

1. The standard big bang cosmology suggests that the universe was created from a point which expanded.

2. Initially the universe was hot & infinitely dense.
3. Expansion of the universe has led to cooling
4. The temperature of the universe is now 2.7K
5. As electrons and protons combined in the early universe to form atoms (3x10^5 years), universe became transparent to light.
6. The wavelength of gamma radiation [from matter / anti-matter annihilation] from that time has since stretched to microwaves by the expansion.
7. Hence an isotropic microwave background radiation which corresponds to a temperature of 2.7K is present; This is the MBR.

1. If universe is infinite and static
2. Then every line of sight would end on a star, so sky is infinitely bright
3. But the sky is dark....
4. Hence at least one of our assumptions is wrong.

Future of Universe (Density)

if density < critical density (white): open universe [current research, see Nobel Prize 2011, suggests this]

if density = critical density (blue): flat universe [OCR specification says this is the "current" theory, lol]

if density > critical density (red): closed universe

Attached Images

3. Significance of 3K CMB

1. The standard big bang cosmology suggests that the universe was created from a point which expanded.

2. Initially the universe was hot & infinitely dense.
3. Expansion of the universe has led to cooling
4. The temperature of the universe is now 2.7K
5. As electrons and protons combined in the early universe to form atoms (3x10^5 years), universe became transparent to light.
6. The wavelength of gamma radiation [from matter / anti-matter annihilation] from that time has since stretched to microwaves by the expansion.
7. Hence an isotropic microwave background radiation which corresponds to a temperature of 2.7K is present; This is the MBR.
4. Ultrasound Doppler:

1 - A piezoelectric transducer is placed at an angle to the artery / arm which sends pulses of ultrasound.

2 - These ultrasound pulses are reflected by the moving blood cells.

3 - The frequency (or wavelength) of ultrasound is changed (from the doppler effect)

4 - The change in frequency is related to the speed of the blood, (calculated by change in f / f = 2vcos(theta) / c, where c is ultrasound speed, v is speed of blood)
5. (Original post by piguy)
Often need to explain the required components

ie: strong magnetic field required to cause precession
RF coil pulses RF matching Lamors frequency, causing resonance from excitation to decitation, which is detected by RF receiver coil -> Computer
Gradient coil used to change time of resonation.
6. (Original post by piguy)
This thread is for people to revise the long explanation questions in G485 paper for OCR A-level Physics.

Electric and Magnetic Fields
Mass Spectrometer
How Transformer's Work

Capacitors
Use of Capacitors

Nuclear Physics
Rutherford's Alpha Particle Scattering
Use of Radioactive isotopes - Carbon Dating
Use of Radioactive isotopes - Smoke Alarm
Induced Nuclear Fission
Nuclear Reactor Layout

Medical Physics
Production of X-rays
Absorption of X-rays
Image Enhancement of X-rays
Medical Tracers
Gamma Camera
CAT Scan
PET Scan
MRI Scan
Ultrasound Scan
Ultrasound Doppler
Comparison of Medical Scans

Modelling the Universe
Formation of Stars
Formation of Red Giant
End of Life for Stars
Big Bang & Formation of Universe
Significance of 3K CMB
Future of Universe (Density)
Use of Radioactive isotopes - Carbon Dating - When Alive, plants absorb CO2 during photosynthesis. Small proportion of the absorbed C is radioactive C-14 with a half life of 5570. Once dead, no CO2 is absorbed, so ration between C-12:C-14 is decreased.
Ration is determined, and life of relic is determined by equation.

How Transformer's Work - A changing magnetic flux in induced in the primary coil from an AC current, which is linked by the iron core to the primary coil which will have a different magnetic flux - producing an EMF (EMF = (delta)N(phi) / t

Use of Capacitors - CP power backup: charge when laptop is on, and when power is cut off, it discharges slowly through a large resistor providing power to prevent data loss.
Camera - When a pictures taken, Capacitor discharges extremely fast to provide a high current, through a tube filled with Xenon gas which is ionised proving a bright flash.

Image enhancement: Phosphor screen is place behind the patient. When it's struck by an X-ray photon, it is absorbed, and releases many visible photons. Photons enter a photo-multiplier tube, exponentially increasing the number of photons untill an impulse is improved. Relayed to computer where it produces a 3D image.
- X-ray intensity/exposure is reduced as the detectors more sensitive to visible photons.
8. how transformers work:

1. input ac current creates a changing magnetic flux
2. primary and secondary coils are magnetically linked, hence an e.m.f. is generated in secondary coil
3. low resistance wires allows a current to flow in secondary coil

Vp/Np = Vs/Ns

VpIp = VsIs (if transformer is 100% efficient.)

to reduce energy loss in transformers:
1. lamination minimises eddy currents
2. soft iron core lowers magnetic hysteresis
3. low resistance (thick) wires reduce heat loss
9. Brilliant
10. Nuclear Reactor Layout:

1. fuel rod
contains the uranium nuclei / fissile material

2. control rod - boron to absorb neutrons, changing rate of reaction

3. cooling gas
- H2O or CO2 => kettle way of generating electricity

4. moderator - carbon / D2O, slow down fast-moving neutrons to thermal neutrons
5. Slow moving thermal neutrons have a greater chance of causing fission, sustaining a chain reaction
11. ultrasound (>20kHz) is produced by the piezoelectric effect
1. alternating pd applied to piezoelectric crystal (quartz) causes it to oscillate.
2. alternating pd is at resonant frequency to maximise amplitude
3. epoxy resin backing prevents backwards propagation of ultrasound & rapidly damps oscillations after alternating pd stops

Z= ⍴c
(acoustic impedance = density of tissue x speed of ultrasound)

ultrasound scanning
1. transducer is used to send pulses of ultrasound into the patient.
2. pulses are reflected at tissue boundaries
3. intensity of the reflected pulse depends upon the acoustic impedance
4. delay time of the retuning pulse is used to determine the depth

Ir / I0 = (Z2 - Z1)2 / (Z2 + Z1)2

A-scans are voltage peaks, where reflected ultrasound creates a p.d. by the piezoelectric effect
B-scan is multiple A-scans for 2d image
12. (You don't need to quote exact times or temperatures [except for 2.7K & 13.7bn years], just the general order of things)

13. [Note for no. 1 that Lead tubes == Collimator]
Attached Images

14. Attached Images

15. PET - Positron Emission Tomography

1. A positron / beta-plus emitting source is used (injected into patient)
2. The positron annihilates with an electron inside the patient
3. This produces two gamma photons
4. The photons travels in opposite directions
5. The patient is surrounded by a ring of gamma detectors
6. The arrival times of the photons (delay time) indicates the location (of tissue)
7. A 3-D image is created by a computer connected to detectors.
16. (Original post by piguy)
Formation of star:

Different regions of density in space - Denser regions (filled with interstellar dust and gas) towards one point under gravitational force - they loose GPE, thus gain KE.
Due to this, pressure and friction accumulate as more and more mass in drawn to one point. Friction is proportional to Temp, thus temperature rises also.
When Temp & Pressure are significant enough, fusion occurs and H + H => He in the core - Producing a radiation Pa > G Force, thus the star expands.
As it expands, GPE incr, thus radiation pa decreases, and once radiation Pa = G Force, expansion stops - main sequence star is formed.
17. Piguy, I love you. Thank you so, so much for this.
18. (Original post by piguy)
(Original post by piguy)
x
Cheers for making all of that... Just a quick q, are the pdf notes basically everything that you have written in the thread posts?
19. And what do people think will most likely come up this Thursday?

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