Posted in Physics Revision

Pendulums

P3 3.6

Definition: a bob (a mass) swinging on a string.

For example, a playground swing, with the mass being the person on the seat.

How it works: The swing of a pendulum can be described by its:

  • amplitude → how far from the vertical (the dotted line on the diagram) the string moves.
  • time period → time taken for one complete swing.
  • frequency → number of complete swings per second.

When a pendulum is swinging we say it is oscillating.

Image result for swinging pendulum
Source

The time period is directly proportional to the square root of the length of the string:t vs length

The frequency of the oscillations is equal to 1 divided by the time period:

f = 1 over t

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.

Posted in Physics Revision

Centre of Mass

P 3 3.3

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.

Image result for centre of mass of regular shapes
Source

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.

Source

1) Pin a piece of string through a point at the edge of an irregular shape, hanging a pendulum at the other end.

2) Draw a line where the string hangs.

3) Repeat several times on different points.

4) Where the lines cross is the object’s centre of mass.

Posted in Physics Revision

Nuclear Fusion

P2 6.2

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).

A typical nuclear fusion process in any star is:

deuterium + tritium → fusion → helium + neutron

Image result for fusion reaction equation
Source

 

Posted in Physics Revision

Nuclear Fission

P2 6.2

I think from our last tutoring session we established this is an area in which a few of you need some extra help, so here it is.

Definition: a reaction in which a large, unstable atomic nucleus splits into smaller nuclei, releasing energy (kinetic energy and heat).

How it works: All atoms are made of protons, neutrons and electrons → the number of each of these is what determines the element an atom is (so, an atom with 6 protons, 6 neutrons and 6 electrons is Carbon).

When nuclear fission occurs, the nucleus of an atom is split → this releases a lot of energy, and is how energy is produced at a nuclear power station, most commonly with uranium-235.

This is caused by two to three neutrons being fired at the nucleus of uranium-235. The nucleus absorbs these neutrons and becomes unstable, so it splits.

As it splits, it releases a number of neutrons, which then are absorbed by another nucleus, which also splits and releases neutrons.

As each reaction causes another one, we call it a chain reaction. It is said to be self-sustaining if one neutron from a fission reaction continues to cause the fission of another nucleus, and so on.

An out-of-control chain reaction is an atomic bomb. That is why they are so completely detrimental (as seen in WW2 in Japan).

As the nucleus has split, it has formed two new elements: Barium and Kryptonium.

 

Image result for uranium 235 fission diagram
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Unfortunately, naturally-occurring uranium contains less than 1% uranium-235, the rest being mainly uranium-238 – this is an issue as Ur-238 is not a nuclear fuel as it doesn’t easily undergo nuclear fission. Engineers have to enrich the uranium to about 3-5% uranium-235.

Plutonium-239 can also be used as a fuel → formed in nuclear reactors, it doesn’t occur naturally, and could be a good way of getting rid of radioactive waste. However, it’s considered dangerous → it was used in the atomic bomb that killed 50 000 people in Nagasaki.

Nuclear Reactors

Image result for nuclear reactor labelled diagram
Source

We can harness the energy released by nuclear fission and use it in the same way we use energy from coal, oil and gas powerstations.

Remember, a fuel is something from which you get energy, not something you burn.

The fuel used here is either uranium-235 or plutonium-239. This is provided in rods.

The rods are surrounded by a moderator → this is either graphite or D2O (heavy water). The moderator slows down neutrons, and so slow down the speed of the chain reaction. When they slow, they are called thermal neutrons.

It is essential that the chain reaction is controlled → remember how dangerous an out-of-control nuclear fission reaction is.

The coolant removes heat from the reactor to be used to turn water into steam to turn turbines (just like a regular powerstation). The coolant could be water or carbon dioxide.

So the main components are the fuel, coolant, and moderator.

PROS AND CONS OF NUCLEAR POWER

PROS:

  • no greenhouse gasses → no environmental impact (which we all know is extremely important due to the conclusive and undebatable evidence of the very real impacts of global warming).
  • it has the highest energy density of any other fuel.
  • some of the waste product can be used in nuclear medicine (e.g. radiotherapy)

CONS:

  • it is difficult to dispose of the nuclear waste → we just bury it → can be dangerous as it is highly radioactive.
  • the powerstations are very expensive to build.
  • the building has to be decomissioned (taken down) after 35-40 years, which is another expensive process.
  • there is the danger of nuclear accidents (e.g. Chernobyl 1986).

 

 

Posted in Physics Revision

Unit P11: Energy; How It’s Transferred

P1 1.1, .3, .4

The three ways energy can be transferred are conduction, convection (both require particles) and infrared radiation (does not require particles).

Infrared Radiation

Definition: “Electromagnetic radiation that we can feel as heat. IR has a longer wavelength than visible light, but a shorter wavelength than microwaves.”

How it works: thermal energy is transferred from one place to another as an electromagnetic wave (more on that in P15). These are emitted and absorbed by everything, but some materials are better at absorbing or emitting than others. This radiation can occur in a vacuum, and so does not rely on particles to transfer heat energy.

Light-coloured shiny surfaces → good at reflecting radiation → poor at absorbing it.

Dark-coloured matte surfaces → poor at reflecting radiation → good at absorbing it.

Infrared can be used on thermal imaging cameras to detect people in the dark. It is how energy from the sun reaches earth → it is often used to heat water in less developed countries.

Conduction

Definition: “The transmission of temperature changes through a material (usually a solid) by transfer of vibration from particles to their neighbours”.

How it works: If a heat source is applied to one end of a material (e.g. a metal rod), those particles will gain kinetic energy → they will start to vibrate more vigorously, but remain in a fixed position. These then bump into their neighbouring particles, which gain kinetic energy, and so on, until the whole material is heated.

Conductor → material which is good at transferring energy by conduction. Metals are good conductors as they have free electrons which can move through the material and pass on the energy → they move faster when heated as they have more kinetic energy. The same principle applies with electricity.

Insulator → material which is poor at transferring energy by conduction → often used to reduce unwanted heat loss (e.g. in the walls of houses).

Convection

Definition: “Movement of particles in a fluid (a gas or a liquid) depending on their temperature. Hotter, less dense regions float, and cooler, denser regions sink.”

How it works: when a fluid is heated, the particles that form it gain kinetic energy. This means they move around more, and so move further apart from each other. This results in them taking up more space, so the fluid becomes less dense. It’s really important to remember that the fluid is becoming less dense, not the particles which make up the fluid. The less dense fluid is then able to rise up above the cooler fluid around it. The cooler fluid is denser than the hotter fluid, so sinks down below the hotter fluid. Then the hotter fluid cools and sinks down again → it’s a cycle.