# Physics P3 - Edexcel

GCSE Extension Science Edexcel - Physics P3

From the CGP revision guide and school notes.

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• Created by: Mia
• Created on: 22-05-12 20:32

## Kinetic Theory and Gas Temperature

Gases are randomly moving particles.

They collide of eachother and the walls of container.

Absolute Zero = -273C = 0 Kelvin.

Reduce Temp = Reduce Kinetic energy as particles are given less energy and therefore move around less.

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## Kinetic Theory and Gas Pressure

Colliding gas particles create pressure - collide with eachother and container more, exerting a force = pressure.

The higher the temp -> the more energy given to the particles -> the more collisions -> the higher the pressure.

(Pressure x Volume) / Temp = Constant

(P1V1) / T1 = (P2V2) / T2

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## Particles in Atoms

Alpha: Helium atom. Slow + Heavy. Very Ionising. Stopped by paper or a few cm of air.

Beta: An electron. Light + Fast. Ionising. Stopped by thin metal (aluminium).

Gamma: Electromagnetic Radiation. No Mass + Very Fast. Weakly Ionising. Only stopped by thin lead or very thick concrete.

Positron: A positively charged electron.

Neutron Radiation: More powerful than a, B or y, but not ionising. Absorbing a neutron makes the nucleus radioactive. Neutrons are difficult to detect because they're neutral so leave no ionising radiation behind.

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## Particles in Atoms

Beta- Decay - too many neutrons. Emission of an electron from the nucleus. One neutron is changed into a proton. Proton number increases by 1 + the nucleon number stays the same.

Beta+ Decay - too few neutrons. Emission of a positron from the nucleus. One proton gets changed into a neutron. Proton number decreases by 1 + the nucleon number stays the same.

Alpha Decay - nucleus is too heavy (82+ Protons). Proton number decreases by 2 + the nucleon number decreases by 4.

Gamma Radiation - nucleus has too much energy. After a or B decay the necleus has excess energy so releases a gamma ray.

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## Particles

Fundamental Particles (can't divide them into smaller particles) = electrons and positrons.

Protons and neutrons are made up of 3 Quarks each.

• Up quark. Charge = 2/3
• Down quark. Charge = -1/3

Proton = (2 x Up) + Down. Charge = +1.

Neutron = Up + (2 x Down). Charge = 0.

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## Electron Beams

Electron gun uses thermionic emission.

1) The heater heats the ctahose, giving more energy to electrons, making them boil off = Thermionic Emission.

2) Electrons accelerate as they're pulled towards the anode. Andode has a gap to channel electrons into a beam.

3) Electric field between each of the plates deflects the electron beam.

4) Phosphorescent screen glows when the electrons hit it.

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## Maths

Kinetic Energy(J) = Charge of electron(Coulombs, C) x Accelerating Voltage (V)

KE = e x V

Current = (number of electrons x electron charge) / Time

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## Electron Beams

Deflected by an electric field.

Electron guns are used in:

• T.V.s - electrons hit the phosphorescent chemicals on the screen and light up.
• Oscillosscope - device for displaying voltage and frequency of an electrical signal.
• Making X-Rays - have a positively charged target (anode), which once the electron beam hits, some of the kinetic energy turns into X-Rays.

Can create new particles and find out about the world if done on a huge scale.

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## Total Internal Reflection

Refraction is caused by waves changing speed as they have entered a different medium with a change in density.

Critical angle.

• If less than, most of the light passes out.
• If equal to, along the edge of the medium with some total internal reflection.
• If greater than, no light comes out it is all internally reflected.

Optical Fibres

Light waves are reflected off the sides of the inner core of glass or plastic. It is reflected all away along the tube until it emerges at the other end.

Must be narrow and not bent too sharply - to keep angles above the critical angle.

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## Medical Uses of light

Endoscopes use bundles of optical fibres, one to carry light inside the body, the other to carry it out so it can be seen on a screen.

Used in keyhole surgery, so less chance of infection.

Pulse Oximetres use light to check the oxygen % in the blood.

Placed on finger or ear lobe, then the amount of light detected at the other end gives and indication of how much of the heamoglobin contains oxygen or not (as it changes colour, therefore changing the absorbancy of light)

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## More Maths

When a force moves an object, energy is transferred and work is done, so:

Work done = energy transferred, so:

Work done = Force x Distance

Power is the rate of 'doing work', so:

Power = Work Done/Time Taken

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## Metabolic Rate

= rate at which your body uses energy (for exercise or just for body functions e.g. tissue repair)

Basal Metabolic Rate (BMR) is the rate you burn energy at rest. The minimum amount of energy to keep the body working properly/alive.

Factors that affect BMR include:

• Age - kids = higher as more energy to grow, reduces as you get older.
• Body fat % = lower body fat % the higher your BMR as muscle needs more energy than fat. Men generally have a lower % of body fat than women so a higher BMR.
• Body Surface Area - increases heat transfer. More body surface = more place from heat to be lost from so needs more energy to keep warm.
• Diet - sudden calorie reduction can reduce BMR as it needs less energy.
• External Temp - exposure to cold makes the body increase it's BMR to keep warm.
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## Electricity and the Body

Muscle cells can generate potential differences (a voltage) between the inside of the muscle cells and the outside. When the muscle cell is stimulated by a small electrical signal the potential difference increases - this increase is called action potential.

Electrocardiographs measure action potential of the heart.

When the heart beats, an action potential passes throught the atria, making them contract. A tiny bit later, another action potential passes through the ventricles making them contract too. Once passed the muscle relaxes.

These action potentials produce weak electrical signals on the skin which can be detected by electrodes and measured by an electrocrdiograph which produce an electrocardiogram (a graph).

Heart Rate:

Frequency (hertz) = 1 / time period (seconds)

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Radiation is energy emitted by a source, such as light.

The more intense the radiation the more energy it carries, the more energy gets transferred when it hits an object.

Radiation Intensity = Power / Area

Intensity of radiation depends on the distance from the source.

Intensity of radiation also depends on what medium it's passing through (absorbtion).

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## Nuclear Bombardment

Splitting of U-235 (used in atomic bombs):

• Fire a slow moving, low energy (if going to fast they bounce off) neutron at the nucleus to make it unstable. It creates U-236.
• U-236 splits into two smaller atoms and 2 or 3 fast moving neutrons.
• These fission products have a nucleus with a larger proportion of neutrons making them unstable and radioactive.

Proton Enrichment:

• A proton is absorbed by the nucleus (needs a lot of energy so done in a particle accelerator). This increases the proton number so you get a new element.
• Positron emitters are used in hospitals in PET scanning, although, they have a short half life so have to be made on site.
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## Momentum Conservation

Momentum is always conserved. The total momentum after = the total momentum before.

Momentum = Mass x Velocity

E.g. In Positron/Electron Annihilation.

• When positrons and electrons meet, they collide head on at the same speed with the same mass but opposite velocities so the momentum = zero.
• Momentum is always conserved so the gamma rays produced have a total momentum of zero - they have the same energy but exactly opposite velocities.
• Mass/energy is also conserved in the reaction. All the mass has been converted into energy (E=mc2)
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PET Scanning can detect damaged tissue in the heart by detecting areas of decreased blood flow, can detect active cancer tumours by showing metabolic activity in tissue, and can record blood flow and activity in the brain.

PET Scanning:

• Inject the patient with a substance used by the body (glucose) containing a positron emitting radioactive isotope with a short half life - a radiotracer.
• Positrons emitted by the the radioisotope collide with the electrons in the organs emitting gamma rays.
• Detectors around the body record the gamma rays and build up a map of radioactivity.
• The distribution of radioactivity matches up with metabolic activity.
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Should be limited because:

• Can destroy cells
• Can damage a cell so it cannot divide
• Can alter the genetic material of the cell causing mutations or it to grow and divide uncontrollably - cancer.

But sometimes it's the best choice because:

• Radiotherapy can be used to treat cancer (destroys cancer cells much more than normal cells)
• Can reduce the suffering of a patient who is close to death - Pallative Care.
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## Medical Research

Ethical Arguments

• Just because we can, doesn't mean we should.
• Nothing is morally or ethically right or wrong, black or white, so the best we can do is get a consensus from society.

New techniques have to be tested on people, ethical arguments:

• Could have harmful side effects, patients should be made aware of risks, but you cannot be certain of what they are.
• Places are limited.
• How long is the process before a successful trial treatment is available to everyone.

Environmental and economic arguments:

• Disposal of radiation, how? where?
• Companies unlikely to pay unless there's a profit.
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