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# Physics

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on 12 April 2015

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#### Transcript of Physics

I V
Current, Voltage and Resistance
Current
Current is defined as the rate of flow of charge.
In an electrical circuit, the charge is carried through the wires by electrons.
Q=It I=Q/t
Charge, Coulombs (C)
One coulomb is defined as the amount of charge that passes in 1 seconds when the current is 1 ampere.
An ammeter always needs to be attached in series.
Potential Difference
Potential difference, or voltage, between two points is defined as the work done in moving a unit charge between the points.
V=W/Q
Work done, Joules (J)
The potential difference across a component is 1 volt when you convert 1 joule of energy moving 1 coulomb of charge through the component.
1V = 1 J/C
A voltmeter always needs to be attached in parallel.
Resistance
How much current you get for a particular potential difference depends on the resistance of the component.
R=V/I
Ohmic conductors (e.g. metals) obey Ohm's law.
Provided that the temperature is constant, the current through an ohmic conductor is directly proportional to the potential difference across it.
Ohm's Law
I-V Graph for an Ohmic Conductor:
Doubling the potential difference doubles the current.
Resistance is constant as the gradient is always a fixed value (gradient = 1/R).
Factors such as light level or temperature will affect resistance, so Ohm's law is only true for ohmic conductors at constant temperature.
Resistivity
What does Resistance depend on?
Length
(L) - Resistance is proportional to the length of the wire. It is more difficult for current to flow through a longer wire.
Area
(A) - Its is easier for electrons to pass along a wider wire.
Resistivity
( ) - A measure of how much a particular material resists current flow. It depends on environmental factors (e.g. temperature and light intensity) as well as the structure of the material. Resistivity is a property of the material.
What is Resistivity?
2
The resistivity of a material is the resistance of a 1m length with 1m cross-sectional area.
Measured in ohm-metres.
=RA
L
The lower the resistivity of a material, the better it is at conducting electricity, e.g. copper, at 25 C, has a resistivity of 1.72 x 10 ohm-metres.
-8
o
Resistance is a property of an object, and depends on the material and dimensions. Resistivity is a property of a material.
m
m
2
Superconductors
All materials have some resistivity, including good conductors (e.g. silver and copper). That resistance means that they heat up when electricity flows through them - some of the electrical energy is wasted as heat.
The resistivity of many materials, such as metals, can be lowered by cooling them down. For some materials, below a '
transition temperature
', the resistivity is completely gone and they become a superconductor. Without any resistance, no energy is wasted as none of the electrical energy is turned into heat.
Most conductors, such as metals, have transition temperatures below 10 kelvin (-263 degrees Celsius). It is expensive and difficult to cool materials to this temperature. Solid-state physicists are trying to develop room-temperature superconductor. They have so far managed to get some metal oxides to superconduct at 140K (-133 degrees Celsius), which is much easier to cool to.
Uses of Superconductors
Power Cables
- Can transmit electricity without any loss of power.
Strong Electromagnets
- They have lots of applications, such as in medicine and Maglev trains.
Electronic Circuits
- Can work really fast as there is no resistance to slow the current down.
Alternating Current
Oscilloscopes
An alternating current or voltage changes with time.
An oscilloscope is similar to a voltmeter.
The trace seen is produced by an electron beam moving across a screen.
The time base controls how fast the beam is moved across the screen, and can be set using a dial on the oscilloscope.
The vertical height of the trace at any point shows the input voltage at that point.
The grid on the screen can be set to how many volts per division you want the y-axis scale to represent, by using the Y-input control dial.
Some oscilloscopes have the height of each square on the grid as 1cm - the scale can be set in terms of V/cm.
Y-gain = 2V per Division
Time Base = 1 ms per Division
The width of each square represents 1ms, and the height 2V.
Analysing Oscilloscopes
Time Period/T
Peak Voltage/V
0
Peak-to-Peak Voltage
Frequency = 1
Time Period
f = 1/T
Describing an Alternating Current
An a.c. supply with a peak voltage of 2V will be below 2V most of the time.
Therefore, it will have a lower power output than a 2V d.c. supply.
To compare them, you need to average the a.c. voltage.
For the the power of an a.c. supply, Power = V x I .
rms
rms
It is usually the r.m.s. voltage that is stated on a power supply. The 230V stated for the UK mains electricity is the r.m.s. value.
Quarks and Antiquarks
Quarks
Up (u) and down (d) quarks make up protons and neutrons.
A strange quark (s) will give a particle the property of strangeness.
Name Symbol Charge Baryon No. Strangeness
Up
Down
Strange
u
d
s
+2/3
-1/3
-1/3
+1/3
+1/3
+1/3
0
0
-1
The properties of a particle depend on the properties of the quarks that it consists of.
Evidence for the existence of quarks came from hitting protons with high-energy electrons. The way that the electrons scattered demonstrated that there were three concentrations of charge (quarks) inside the proton.
Antiquarks
Name Symbol Charge Baryon No. Strangeness
Anti-Up
Anti-Down
Anti-Strange
u
d
s
-2/3
+1/3
+1/3
-1/3
-1/3
-1/3
0
0
+1
Quark Composition of Baryons
Baryons are made up of 3 quarks. Antibaryons are made up of 3 antiquarks.
The charge and baryon number of baryon is the total charge and baryon number of its quarks.
Proton Neutron
The total charge of a proton is 2/3 + 2/3 - 1/3 = +1.
The total baryon number of a proton is 1/3 + 1/3 + 1/3 = +1.
The total charge of a neutron is 2/3 - 1/3 - 1/3 = 0.
The total baryon number of a neutron is 1/3 + 1/3 + 1/3 = +1.
Quark Composition of Mesons
Mesons are made up of one quark and one antiquark.
Example Question:
The positive pion has a charge of +1, a strangeness of 0, and a baryon number of 0. What is its quark composition?
The only way to get a charge of +1 and strangeness of 0 with two quarks is up and antidown.
ss
S = +1

S = 0

S = -1
Wave-Particle Duality
Is light a Particle or a Wave?
Diffraction
A beam of light spreads out when it passes through a narrow gap, and it is able to interfere with other waves.
Waves explain diffraction and interference.
If light was acting as a particle, the particles in the beam would not get through the gap, or would pass straight through with the beam unchanged.
The Photoelectric Effect
The results of the photoelectric experiments can only be explained by light being a series of particle-like photons.
If a photon of light is a discrete bundle of energy, it is able to interact with an electron in a one-to-one way. All of the energy in the photon is given to one electron.
The photoelectric effect and diffraction show that light behaves as a particle and a wave, known as wave-particle duality.
Wave-Particle Duality Theory
De Broglie suggested that if 'wave-like' light showed particle properties (photons), then 'particles' like electrons should show wave-like properties.
The de Broglie wave of a particle can be interpreted as a 'probability wave'.
Later experiments have confirmed the wave nature of electrons and other particles.
Electron Diffraction
When accelerated electrons in a vacuum tube interact with the spaces in a graphite crystal, diffraction patters are observed.
When they pass through the spaces, they diffract like waves passing through a narrow slit and produce a pattern of rings.
Evidence that electrons have wave properties.
Wave theory says the spread of the lines in the diffraction pattern increases in the wavelength of the wave is greater.
Electron diffraction experiments show that a smaller accelerating voltage gives widely spread ring.
This fits in with the de Broglie equation - higher velocity mean shorter wavelength (i.e. the spread of the lines is smaller).
The wavelength for electrons accelerated in a vacuum tube is usually about the same size as electromagnetic waves in the x-ray part of the spectrum.
Diffraction only occurs if a particle interacts with an object of about the same size as its de Broglie wavelength.
Electron Microscope
A shorter wavelength results in less diffraction.
Diffraction blurs detail on an image, so you need to use a shorter wavelength to reduce blur.
Light blurs detail more than electron waves do, so an electron microscope can resolve finer detail than a light microscope.
Used to view things like a single strand of DNA.
Density
The density of a material is its mass per unit volume.
p = m

Density, kg m .
-3
V
You can work out density in g cm if you are given the mass is g and volume in cm .
-3
3
The density of a material depends on what it is made of. It does not vary with size or shape.
The average density of an object determines whether it floats or sinks. A solid object will float on a fluid f it has a lower density than the fluid.
1 g cm = 1000 kg m .
-3
-3
Volume of a Sphere:
Constituents of the Atom
A = Mass Number
Z = Atomic Number
Isotopes:
Same number of protons (atomic number), different number of neutrons (atomic mass).
Scalars and Vectors
Scalar
A quantity with no direction.
Just an amount.
Vector
A quantity with magnitude (size) and direction.
Examples
Mass
Temperature
Time
Distance
Speed
Energy
Examples
Displacement
Force
Velocity
Acceleration
Momentum
Newton's Laws of Motion
Newton's 1st Law of Motion
"The velocity of an object will not change unless a resultant force acts on it."
A body will stay still or move in a straight line at a constant speed unless there is a resultant force acting on it.
If the forces are not balanced, the overall resultant force will make the body accelerate.
This could be a change in direction, speed, or both.
R
mg
Weight (mg) = Reaction (R)
Newton's 2nd Law of Motion
The acceleration of an object is proportional to the resultant force acting on it.
F = ma
N
kg
ms
-2
The more force you have acting on a certain mass, the more acceleration you get.
a F
For a given force, the more mass you have, the less acceleration you get.
a
1
__
m
Applies to objects with a constant mass.
Newton's 3rd Law of Motion
"If an object A exerts a force on object B, then object B exerts an equal but opposite force on object A."
The two forces actually represent the same interaction, just seen from two different perspectives.
If you push against a wall, the wall will push back against you, just as hard. When you stop pushing, the wall stops pushing.
This is the law that every action has an equal and opposite reaction. But this wrongly suggests that both of the forces are applied to the same object. And in that case, there would be a resultant force of zero and nothing would ever move anywhere.
Disease
Infectious Disease
Can be passed between individuals.
Caused by infection with pathogens or parasites.
A pathogen is an organism that can cause disease.
A parasite is an organism that lives on or in another organism (the host), causing damage to that organism. Some parasites cause disease, so they are also pathogens.
HIV (Human Immunodeficiency Virus) is the virus that causes AIDs.
Mycobacterium tuberculosis is the bacterium that causes TB.
Trichophyton rubrum is a fungus that causes athlete's foot.
Plasmodium species are single-celled parasites that cause malaria (they are also pathogens).
Tapeworms are parasitic worms that live in the digestive system of vertebrates (animals with a backbone).
Fleas are parasitic insects which live off the blood of mammals and birds.
Non-Infectious Disease
Caused by genetic defects, nutritional deficiencies, lifestyle and environmental factors (e.g. toxic chemicals).
Coronary heart disease (CHD) is a non-infectious disease of the heart.
Emphysema is a type of non-infectious lung disease.
Acute Diseases
Usually only cause a problem for a short period of time.
Symptoms usually appear rapidly, e.g. a cold.
Chronic Diseases
Much more persistent (you can have them your whole life).
The symptoms often appear very slowly but get progressively worse over time, e.g. diabetes and chronic bronchitis.
Malaria
A parasitic disease caused by
Plasmodium
(a genus of eukaryotic, single-celled parasites).
Transmission
Plasmodium parasites are transmitted by mosquitoes (insects that feed on the blood of animals, including humans).
It is the female Anopheles mosquitoes that carry the parasites.
The mosquitoes are the
vectors
- they do not cause the disease themselves, but they spread the infection by transferring the parasites from one host to another.
Mosquitoes transfer the Plasmodium parasites into an animal's blood when they feed on them.
Infection
Plasmodium infect the liver cells (hepatocytes) and red blood cells (erythrocytes), and disrupt the blood supply to vital organs.
Plasmodium have a complex life-cycle.
Treating Malaria
There are a number of medicines that can treat malaria, but many people still die from malaria.
Scientists are trying to find a way of preventing the disease. The best solution would be to make a vaccine against Plasmodium, but this is proving to be trick for several reasons.
A Plasmodium parasite spends most of its life cycle hidden inside human cells. It is only exposed in the bloodstream for a very short period of time, which does not give the immune system very long to recognise and destroy it.
There are four different species of Plasmodium that cause malaria. Because of mutation and variation, each species has different antigens; therefore, different species will require different vaccines.
The Plasmodium life-cycle involves more than one stage (a liver stage and a blood stage). Different stages have different antigens, so a vaccine that protects against one stage will not protect against another. The parasite is in each stage for a short time, so there is not much opportunity for the immune system to act on each one.
Protecting Against Malaria
Mosquito nets and insect repellent (stop people being bitten by the mosquitoes which carry the parasite).
Pesticides (kill the mosquitoes which carry the parasite).
Educate people about the signs and symptoms of malaria so it can be recognised and treated early, before the parasite can spread.
Preventing Food Spoilage
Salting
Freezing
Pickling in Vinegar
Heat Treatment
Food spoilage can be prevented by killing the microorganisms or depriving them of the conditions necessary for their growth (which will slow down or stop their growth).
Salt inhibits the growth of microorganisms by interfering with their ability to absorb water, which is needed to survive.
Water moves into the cells of microorganisms by osmosis. Salting lowers the water potential of the environment outside the microbial cells, causing the microorganisms to lose water.
Used to preserve some meats, and tinned foods are often preserved in brine (mixture of salt and water).
Adding sugar also inhibits the growth of microorganisms by interfering with their ability to absorb water by osmosis.
The high sugar content of fruit jams reduces the growth of microorganisms, giving the jam a long shelf life.
Freezers keep food below -18 C, which slows down enzyme controlled reactions taking place in the microorganisms.
o
Freezing also freezes the water in the food so that the microorganisms cannot use it.
Freezing can preserve foods for many months.
The low pH of vinegar denatures the enzymes in the microorganisms, preventing the enzymes from functioning properly and inhibiting the growth of the microorganisms.
Vinegar is used to pickle foods like onions.
Heat treatment involves heating food to a high enough temperature to denature the enzymes and kill any microorganisms present.
Pasteurisation is an example of heat treatment. It involves raising liquids, such as milk, to a high temperature.
This kills any microorganisms present by destroying or damaging their DNA.
This can extend shelf life considerably.
The Global Impact of Malaria, TB and HIV
Malaria, TB and HIV are most common in sub-Saharan African and other developing countries.
Drugs are not always available.
People are less likely to be diagnosed and treated.
Blood donations are not always screened for infectious diseases.
Surgical equipment is not always sterile.
Limited health eduction to inform people about how to avoid infectious disease.
Fewer people know about the transmission of HIV and that it can be prevented by using barrier contraceptives.
Limited equipment to reduce the spread of infections.
Fewer people have mosquito nets to reduce the chance of infection of malaria.
There are overcrowded conditions.
Increases the risk of TB infection by droplet transmission.
Social and economic development is slowed down by the prevalence of malaria, HIV and TB in developing countries, such as sub-Saharan Africa.
This is because these diseases increase death rate, reduce productivity (less people are able to work), and result in high healthcare costs.
The importance of studying the global distribution of these diseases.
The information can be used to find out where people are most at risk.
Any data collected can be used to predict where epidemics are most likely to occur.
It is important for research (e.g. into how it is spread).
It allows organisations to provide aid where it is needed the most.
In developing countries, the steps that are taken to prevent a disease have to be balanced with the cost. If a vaccine against malaria was developed tomorrow, it might not make much difference because it is likely to cost more than these developing countries can afford.
Infectious disease can easily spiral out of control. Increased prevalence of the disease means higher health care costs, which means that fewer people are treated. As a result, more people will be infected and the prevalence will increase further.
Work
Work is done whenever energy is transferred.
Activity:

Lifting up a box.
Pushing a chair across level floor.
Pushing two magnetic north poles together.
Stretching a spring.
Work Done Against:

Gravity.
Friction.

Magnetic Force.

Stiffness of Spring.
Final Energy Form:

GPE
Heat

Magnetic Energy

Elastic Potential Energy
Usually, you need a force to move something as you are having to overcome another force.
The thing being moved has kinetic energy while it is moving, which is transferred into other forms of energy when the movement stops.
Work means the amount of energy transferred from one form to another when a force causes a movement of some sort.
W = Fs
Work done in J
Force causing motion in N
Distance moved in m.
Work is the energy that has been changed from one form to another - it is not necessarily the total energy. For example, moving a book from a low shelf to a higher one will increase its GPE, but it had some GPE to begin with. In this case, the work done would be the increase in GPE, not the total GPE.
The force, F, will be a fixed value in any calculations (it is constant or averaged).
The equation assumes that the direction of the force is the same as the direction of movement.
One joule is the work done when a force of 1 newton moves and object through a distance of 1 metre.
Waves
A
progressive wave
carries energy from one place to another, without transferring any material.
A wave is caused by something making particles or fields (e.g. electric or magnetic fields) oscillate/vibrate at a source.
The oscillations pass through the medium as the wave travels.
A wave carries energy, and transfers it away from its source. This means that the source of the wave loses energy.
The Energy that Waves Carry
EM waves cause things to heat up.
X-rays and gamma rays knock electrons out of their orbits, causing ionisation.
Loud sounds make things vibrate.
Waves power can be used to generate electricity.
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