Radioactive radiation is dangerous because it is ionising.
An unstable nucleus (one with too many protons or neutrons) can emit alpha, beta, or gamma radiation. They do this in order to achieve stability.
Alpha
Beta
Gamma
Description
2 neutrons, 2 protons (helium nuclei – not atom)
High energy electron
High enery electromagnetic radiation
Electric Charge
+2
-1
0
Relative Atomic Mass
4
0
Penetration Power
Stopped by paper/ few cm of air
Stopped by a few mm of aluminium
Reduced by several cm of lead or several metres of concrete.
Ionisation Effect
Strongly ionising (has a lot of power due to its large mass)
Weakly ionising
Very weakly ionising
Effects of magnetic/ electric field
Weakly affected (due to relatively large mass)
Strongly deflected
No deflection (no charge)
Effect on nucleus when this decay occurs
Alpha decay: 2 neutrons and 2 protons emitted, so atomic mass decreases by 4, atomic number decreases by 2, and energy is released
Beta decay: a high energy electron is emitted, so the atomic mass stays the same, and the atomic number decreases by one
Gamma emission: high energy electromagnetic radiation is emitted, so the nucleus changes into a more stable shape. Both atomic mass and atomic number stays the same.
Source Just so you can visualise how the numbers will change as each of the decays occur
ATOM: made of a nucleus of protons and neutrons, with electrons orbiting.
ISOTOPE: an atom with the same number of protons and electrons and a different number of neutrons (same charge, different mass).
ION: an atom with the same number of protons and neutrons, but a different number of electrons (resulting a positive or negative charge).
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BACKGROUND RADIATION
This is a constant, low-level of radiation around us at all times.
Sources include:
CMBR (from space)
Elements such as radon gas
Rocks
Food
Nuclear tests eg nuclear bombs
Nuclear medicine
SAFETY
Radioactive sources can be dangerous because they are ionising – they can damage the body’s cells, which can lead to genetic mutations, causing diseases such as cancer.
Definition: a change in the wavelength and frequency of a wave caused by relative movement of the wave source and the observer.
This occurs in all waves → sound, electromagnetic, etc.
When the source of a wave is moving relative to the obeserver it means that the waves produced are compressed, as shown in the diagram, if the source is moving towards you.
This means that the wavelength has decreased.
As the speed of the wave remains the same, the frequency has to increase.
We know this by looking at the equation for wave speed:
v = f x λ
Remember, this only occurs if the source is moving at a different speed to the observer. If they are moving at the same speed, the effect doesn’t occur.
So, because the frequency appears to have increased, the sound is higher pitched.
If the source was moving away, the wavelength would be longer, so frequency would be smaller, meaning the sound would be lower pitched.
This effect is exhibited best when a siren drives past you; the pitch appears to change depending on how far away from you the siren is, and whether is is coming towards you or moving away.
Red Shift Definition: The increase in wavelength and so decrease in frequency of electromagnetic radiation from distant, receding galaxies due to the Doppler effect and the expansion of the Universe.
So, essentially, this is the Doppler effect, but with light or other electromagnetic radiation. In the same way sound appears more high pitched when the source moves towards us, light appears more “blue” when it is moving towards us, and more “red” when it is moving away.
Red Shift as Evidence of the Big Bang
The big bang theory states that a huge explosion occured about 13.7 billion years ago that released so much energy that energy became matter, and the universe was born. It also states that the universe continues to expand to this day.
We can use red shift as evidence for this. This is because when electromagnetic waves move away from us, they are red shifted as their wavelength appears longer. The light we see from stars across the universe is red shifted; this means they are moving away from us. Since they are moving away, it indicates that the universe is expanding, as if it was staying the same, the light would exhibit no red or blue shift.
For example, if 3 objects were floating in a balloon, and that balloon was inflated more, then the volume of the balloon would expand, and the objects would move further apart.
Also, the more red shifted a star is, the further away from us it is. So we can use the amount of red shift a star exhibits to find out how far away it is from us.
Insulator Definition: a thermal insulator acts as a barrier to the transfer of energy by heating.
Heating a house costs money. However, a lot of the energy we pay for is lost to the surroundings. So insulating a house can save money as less heat energy leaves the house.
Things we can do to improve how insulated our houses are include:
double- or triple-glazed windows → there is air (or sometimes argon) in between the sheets of glass; gasses are poor conductors, so less heat is lost.
cavities in walls → again, a layer of air is between the walls, preventing heat loss by conduction.
polystyrene foam insulation → polystyrene is an excellent insulator.
reflective surface behind radiators → reflects heat back into house and away from walls.
draught excluders → stop cold air getting in and hot air getting out.
U-VALUES
This shows how good a material or building component is as an insulator.
The lower the U-Value, the less energy the material transfers, and so the better an insulator it is. So, when insulating homes, we look for materials with LOW U-VALUES.
(It’s unit is W/m²°C, but I don’t think you need to know that.)
PAYBACK TIME
The components we use to insulate our homes also cost money. To work out if a method of insulation is cost effective, we look at the payback time.
This is the length of time it takes to save the amount of money that the improvement cost.
E.g. if insulating a hot water tank cost £60 and it saves £15 a year, then the payback time is 60 divided by 15, which equals 4.
The shorter the payback time, the more cost effective the insulation method is.
Definition: a system that transmits a force from one place to another through a fluid.
Hydraulic systems are classed as force multipliers.
For this topic, it’s important to remember about pressure in liquids → they are incompressible.
An example of a hydraulic system is in car brakes (please spell it brakes, not breaks).
Hydraulic systems are useful because they essentially convert a small force on a small area to a large force on a large area, meaning people are able to exert a huge force on a massive object, such as breaks in a car.
To see how they do this, we look at the idea of pressure.
Here is a diagram of a hydraulic system:
Now, we know that pressure is equal to force divided by area (shown below).
And due to liquids being incompressible, the pressure is equal throughout the whole liquid – it remains constant. This is why it is an issue when break fluid leaks in cars, as the pressure is not transferred through the liquid, and so the force exerted on the break pads is not transferred to the whole vehicle.
So if the pressure exerted by the small force on the small area is equal to the pressure transmitted by the liquid to the large area, we can conclude:
The pressure is the same as that exerted by the small force on the small surface area. The surface area is bigger. This means that the force is bigger too, due to the equation for pressure rearranging like this:
I know this is difficult to get your heads round, but to summarise:
A small force is exerted on a small surface area → this results in a pressure → this pressure remains constant as it is transmitted through an incompressible liquid to a larger surface area → the pressure is being exerted on a surface area which is larger than the first one → due to the equation force = pressure x area, the force exerted on the second surface is larger than the force exerted on the first.
Since it’s quite complicated, here’s an example question:
A force of 30N is applied to piston A with a surface area of 0.2m² in a hydraulic system.
The surface area of piston B is 1.0m².
What is the force exerted by piston B?
So, the pressure exerted on piston A is force/area : 30N divided by 0.2m² is 150Pa.
To find the force exerted by piston B, we rearrange the equation to get pressure x area. So 150Pa x 1 m² = 150N.
Until recently, it was accepted that mass can only be positive… that is to say, there can be no negative values for mass.
As they are so fond of doing, scientists have recently shown that this particular element of physics is not true.
Negative mass is essentially mass which, when it is pushed, goes in the opposite direction, completely defying Newton’s Second Law. In a way, it seems logical that mass could be both positive and negative, just like things such as charge; yet until now, we have only ever considered it positive.
This new concept was proven by a physicist Michael Forbes. He created a superfluidout of rubidium atoms, whereby particles move as one, acting like waves, and at very slow speeds. This was done by lowering the temperature of the atoms to just above absolute zero (nearly -273 degrees Celsius): at this point a Bose-Einstein condensate is achieved. Lasers were used to trap these atoms and alter their spin, and some of those released exhibited signs of negative mass.
Although physicists are unsure of how this idea can be used in the future, such as explaining wormholes, it is thought they will prove useful in providing a better understanding of astrophysical phenomena such as black holes.
Definition: the force exerted divided by the area on which it is exerted → measured in Pascals (Pa).
So,
We know that force is measured in Newtons, and area is measured in metres squared. So pressure is measured in newtons per metre squared. This is a unit called Pascals.
We can use the equations to conclude that a wider area means there is less force exerted per metre squared → a wider area = less pressure. This provides an explanation as to why flat shoes don’t sink as easily in mud as highheels.
The atmospheric pressure on earth is 100 000 Pa.
Pressure in Liquids
We describe liquids as being virtually incompressible. This is because there are no gaps and no space between the liquid molecules.
The pressure exerted by the liquid (shown in red) is the same in all directions.
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[as year 11s I am sure you are familiar with the sensation of pressure. you’ll be fine]
The time period is directly proportional to the square root of the length of the string:
The frequency of the oscillations is equal to 1 divided by the time period:
T = time period → unit: seconds (s)
f = frequency → unit: Hertz (Hz)
L = length → unit: metres (m) examiners often trip students up by using cm or mm. Ensure you have converted all units before beginning your calculations.
We can conclude by looking at the equations that:
the longer the pendulum → the longer the time period → the smaller the frequency
The time period of a pendulum can be affected by:
mass of the bob
length of the string
amplitude
Random Errors
Definition: an unpredictable variation around the true value, causing each reading to be slightly different.
So, basically, mistakes. These are often human errors. One very common, and unavoidable, human error is the reaction time error.
As you know, it takes us a moment to react, for example, there will be some time between releasing the pendulum to swing and starting the stopwatch.
This is on average 0.2 seconds in humans.
To reduce the impact of this reaction time on results, record the time it takes for several oscillations to occur, such as 10 or 15, and then find the average. This will make reaction time almost negligible.
Mechanics has a reputation for being difficult and making students panic, but it doesn’t need to be. We’ll start with the basics and work our way into the tricky stuff.
Definition: The point at which an object’s mass may be thought to be concentrated.
It’s very easy to find in regular objects, as you can see above.
In irregularly shaped objects, it’s a little more complicated than that, and some examiners enjoy asking you the method for finding it. It’s called the plumb-line experiment.
Definition: the combination of atomic nuclei to form a larger nucleus and release energy.
How it works: Essentially, two light nuclei (nuclei with only a small number of protons and neutrons) such as hydrogen fuse together, producing a heavier element (nuclei with more protons and neutrons.
It is a process which releases energy.
This process is important → it powers every star in the universe, including our sun (please remember that the sun is a star, people seem to forget despite it being the thing that gives us life).
In stars, it is the gravitational force which starts fusion in a star → light elements are literally pulled together, squashing them so strongly that their nuclei fuse together. The gravity also keeps the reaction contained. Fusion is maintained by a combination of high temperature and high pressure.
The conditions for fusion to occur are extreme: millions of degrees celcius are required (about 100 million degrees celcius for hydrogen). We say it occurs in plasma conditions.
Fusion would be the perfect energy source, as the only waste products are heavier elements such as helium → very easy to dispose of. However, the conditions are very difficult to achieve on earth → it is not currently a viable option as an energy source. Scientists are working very hard to try and make it work on earth, but it isn’t yet. (So please don’t say in the exam that it is.)
Two isotopes of hydrogen are deuterium (one neutron in the nucleus) and tritium (two neutrons in the nucleus).
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