Category Archives: Introductory physics

Introductory physics: the concept of field

In a first course in physics, it is usually in electrostatics that one first encounters the concept of a field.

Everybody knows that like charges repel, while unlike charges attract. The quantitative version of this rule is Colomb’s law, which is the observation that the force between two electric charges A and B is given by F  =  k.q(A).q(B)/r2 where q represents electric charge and r is the separation of the charges (k is is a constant determined by the medium in which the charges are situated). Note that if the charges are like, the force comes out positive, so a repulsive force is positive in sign, which is what you might expect since work must be done to bring the charges together. On the other hand, if the charges are unlike, the force comes out negative (which also makes sense as the charges want to be together anyway).

However, the concept of force isn’t all that useful if one wants to know the effect of a given electric charge (A) on the world. It is clear from Coulomb’s law above that the force experienced by any charge B due to A will also depend on the magnitude of B i.e. each charge you bring up to A will experience a different force! Physicists get around this problem by defining the field due to charge A as the force a test charge brought up to A will experience divided by the magnitude of that test charge. Hence, while  every charge brought up to A will experience a different force, they will all experience the same field (F  =  k.q(A)/r2) – clever huh?

We can even draw pictures of the field due to A, simply by drawing lines representing the direction in which an electric charge will move. Unfortunately, the convention is that we draw the direction a positive charge will move (unfortunate because we now know that it is the negatively charged electron that’s doing the moving: the convention is historical)

Electric field between two unlike charges (red is +ve)

The concept of a field is not limited to electricity; it is used throughout physics. For example, you and I experience slightly different forces due to the earth’s gravity. This is because the gravitational force depends on the product of both the earth’s mass M and personal mass m (F = GMm/r2 where G is a constant). However we all experience exactly the same field: dividing by personal mass, the gravitational field due to the earth is given by GM/r2.


The earth’s gravitational field is directed towards its centre

Note

You may have noticed that the equation for gravitational force above looks very like that for the electric force, with charge replaced by mass (both forces decrease with the square of increasing distance). However, gravity is a much, much weaker force; the gravitation constant G is orders and orders of magnitude smaller than the electric constant k and you only notice a body’s gravitational field if it has the mass of a planet!

That said, it is now believed that there is a deep connection between electricity and gravity; indeed, particle physicists and cosmologists believe that all four of the fundamental forces of nature (gravity, electromagnetism and two nuclear forces) originally formed one superforce, which gradually split off into four separate forces as the uiverse expanded and cooled. We already have strong theoretical and experimental verification that two of the fundamental forces originally comprised one force, and it is one of the great ambitions of theoretical physics to describe all four forces in a single mathematical framework (unified field theory). Within that program, one of the great puzzles is why the gravitational force is so much weaker than all the others.

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Introductory physics: those ol’ gas laws

A nice way to finish our section on heat and temperature is to look at the so-called gas laws. The Ideal Gas Law is a famous equation that relates the pressure P, volume V and temperature T of a given gas by the very neat expression

PV/T = nR

where n is the number of moles of a gas and R is a constant known as the Rydberg constant.

For a physicist, the details of the right hand side of the equation is of little interest. What is important is that it is constant i.e. the product [pressure x volume divided by temperature] of a given gas remains fixed. Hence if any one (or all) of these three variables is changed, the others must adjust such that the total product remains the same.

The ideal gas law embodies three separate gas laws that were discovered by experiment many years ago. For example, you can see from the equation that if the temperature of a gas is held constant (isothermal process), the product of PV must remain constant. Hence the volume of a gas decreases with increasing pressure if the gas is held at a steady temperature – a law known as Boyle’s Law after the Irish scientist who first discovered it in the 17th century.

Boyle’s Law – a favourite 1st year experiment

On the other hand, you can see from the equation that if the pressure of a gas is held constant (isobaric process) the quotient V/T must remain constant. Hence the  volume of a gas must increase linearly with increasing temperature if the pressure is held fixed – a law known as Charle’s law after the English scientist who first observed it  (also known as thermal expansion).

Charle’s Law: volume increases with temp at constant pressure

Finally, if you increase the temperature of a gas while keeping the volume fixed the quotient P/T must remain constant. Here the pressure of a gas must increase linearly with increasing temperature, a process known as an isovoluic process. This process is the most dangerous one of the three, as pressure can build up unobserved and cause a nasty explosion.

Press increases with temp at constant volume: this can be used to estimate the temperature of Absolute Zero

Each of the three laws above were discovered empirically, many years ago.  Later, when the molecular structure of gases was understood,  the ideal gas law, embodying all three laws,  was derived from first principles from the kinetic theory of gases. This was a stunning achievment and marks one of the first unifications of the laws of physics.

Question

When a car is driven at speed, the risk of a tyre blowout is much larger than normal. Can you explain why in the context of the ideal gas law?

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Introductory physics: radiation

We saw in the last post that energy that can be transferred by conduction and convection, two very different molecular processes. But it is the third mechanism of heat transfer that is the most surprising.

In radiation, the transfer of energy is not a molecular process at all. Instead, the energy is carried as an electromagnetic wave, that is a wave consisting of oscillating electric and magnetic fields. The fields are self-perpetuating (and mutually perpendicular) because the changing electric field induces a magnetic field and the changing magnetic field induces an electric field.

The discovery of electromagnetic radiation emerged late in the 19th century. From Maxwell’s theory of electromagnetism, it was realised that light itself consists of an electromagnetic wave: however, it took Einstein to realise in 1905 that electromagnetic waves travel from the sun to earth through a vacuum i.e. do not need a medium in which to travel (unlike conduction or convection).

The rate of radiation from the sun (or any body hotter than its surroundings) is proportional to the fourth power of its temperature i.e. is extremely sensitive to temperature. Radiation also depends on a property of the body known as emissivity. Emissivity is a measure of how well a material emits radiation and is determined by atomic processes within the body. For this reason, a good emitter is also a good absorber, if it is placed in an environment where it is cooler than its surroundings (a perfect absorber is called a blackbody, because it will absorb all light incident on it). The opposite of a good absorber or emitter is a reflector, an object which can neither absorb nor emit heat. Polished metals and bright materials tend to be goodish reflectors: for this reason white clothes are worn when playing cricket and tennis in hot countries (they reflect both heat and light, keeping the player cool and easy to see).

A hot body does not radiate energy at a particular frequency, but at all frequencies – from waves of high energy and frequency (gamma rays) to low-energy ones (radiowaves). The low energy waves have low frequencies but long wavelengths since the wavelength of a wave is inversely proportional to its frequency. The full range of frequency (or wavelength) of radiation is called the electromagnetic spectrum. One of the great unifying moments in physics occured when it was realised that radiowaves, microwaves, infra-red heat, visible light, ultra-violet light, X-rays and gamma rays are all versions of the same thing – they are simply electromagnetic waves of different frequencies (and wavelengths).

Even a blackbody body dies not radiate equally at all frequencies. The distribution of radiation vs frequency (i.e. the spectrum of radiation) depends on the temperature. A body at extremely high temperatures will radiate predominantly at high frequencies, while a body at very low temperatures will radiate predominantly at much lower frequencies. Below is a picture of the emisson spectrum of a blackbody, measured at several different temperatures.

This spectrum is of great interest in fundamental physics, because it turns out that it cannot be predicted using the laws of classical physics. In the early years of the 20th century, Planck and Einstein showed that the blackbody spectrum could only be explained if it was assumed that light behaves as a stream of discrete particles in some circumstances. This duality i.e. light behaving as wave in some circumstances and as a stream of particles in others forms the basis of the famous quantum theory (and was later found to be true of matter as well as of radiation i.e. the tiniest ‘particles’ of matter such as electrons can exhibit wave behaviour!)

Question

In cosmology, the cosmic background radiation is a faint background radiation that permeates the entire universe.  It is radiation that is almost as old as the universe itself, dating back to the time after the Big Bang when the universe had expanded and cooled just enough for the first atoms to form, allowing radiation to travel freely (up to this point in time radiation was scattered by the different particles) . Do you think the cosmic background radiation will be hot or cold? At what frequency do you think it is observed? What kind of spectrum might be expected?

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Introductory physics: heat transfer

One the great surprises about heat energy is that the transfer of heat can occur by any or all of three very different mechanisms.

In conduction, heat transfer occurs by a process of molecular collision. If you heat up one end of a bar of iron, the energy is transferred from the hot end to the cold by atoms or molecules bumping into one another i.e. while there is a net drift of energy from hot to cold, the molecules do not change their respective positions. This is the primary method of heat transfer in solids and it works best of all in metals (because loosely bound electrons play a role).  It is also an efficient method of conduction in liquids, but occurs hardly at all in gases. In gases, a low density of atoms or molecules inhibits conduction very effectively – hence air is an excellent insulator.

This fact is used to good effect in double glazing; a layer of air between two panes of glass allows one to have good light and views in a house without too much heat loss. Similarily, modern mountaineers keep warm by wearing many thin layers as the air trapped between each set of layers acts as an effective insulator.

Conduction in a solid: the molecules do not change position much

Heat transfer can also occur by the process of convection. In this case, heat energy is transferred by a movement of molecules. The classic example is hot air rising: on a hot day, air close to ground absorbs heat from the earth’s  surface, expands, and rises because it has become less dense than other air. Cooler and denser air then rushes down from above to fill its place, only to be heated in turn and a cycle is set up. As you might expect, this an important method of heat transfer in gases, and convection currents are responsible for everything from sea breezes at shore to major wind patterns around the globe.

Sea breeze close to shore on a hot day

Convection also occurs in liquids: indeed, convection currents are of great importance in the oceans of the world. For example, the seas around Ireland are warmer than might be expected for our latitude. This warming is a result of the famous North Atlantic Drift, a huge ocean current that is part of a giant conveyor belt that delivers heat from the seas off South America all the way up to the seas near Greenland. One of the concerns of global warming is that as the ice caps melt, this current may weaken or even shut down: in which case Ireland and Britain could become very cold indeed!

The North Atlanic Drift keeps Ireland’s climate mild

What is the third process of heat transfer? Well, that is a different story altogether…

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Introductory physics: the fourth state of matter

The relation between heat and temperature (last post) is not always straightforward: in some cases, large quantities of heat can be supplied to a substance without any observable change in its temperature at all!

When does this happen? It happens when matter changes state i.e. when a solid melts to liquid, or a liquid changes to gas. In fact, it takes a lot of energy to convert a solid to a liquid even if the solid is at the melting point, and it takes even more energy to convert a liquid to a gas even at the boiling point. Neither of these ‘phase changes’ can happen unless energy is continually supplied and there is no rise in temperature rise during the phase change; for this reason the energy consumed is known as latent heat (from the Latin for hidden). It’s interesting physics and quite fundamental at the same time; for example the liquid/gas phase change requires much more energy than the solid/liquid because the intramolecular forces must be completely broken down (as opposed to being weakened in the solid/liquid case). It’s also fascinating to observe that as a solid gradually melts into liquid, the resulting liquid stays stubbornly at the melting point temperature until all of the solid has undergone the change of state (ditto for gas).

Heating curve for ice: the temperature stops rising at 0 and 100 degrees Celcius during the phase changes

A good example of an application of the physics of phase change can be found in an electric kettle: a sensor detects when the top layer of water begins to bubble and quickly switches off the heating element – as opposed to boiling water in an open saucepan, a wasteful process where a lot of water is converted into useless steam, meanwhile consuming a large amount of energy.

Electric kettle : clever device

A change of state can also happen in the reverse direction: when a gas changes to liquid, or liquid to solid, energy is released. This process is exploited in the refridgerator, for example.

In a fridge, a gas-to-liquid phase change extracts heat, keeping the fridge cold inside, while a liquid-to-gas change outside the fridge dumps heat

The whole business of state change raises interesting questions about the nature of matter. Why do some substances exist as solids at room temperature and pressure, others as liquid or gas? To answer this requires a discussion of molecular bonding. Another common question is this: if supplying enough heat to a solid gives you a liquid, and eventually a gas, what happens if you keep supplying heat to the gas?

The answer is that if you supply enough energy, you eventually get another phase change as the atoms of the gas become ionized i.e. electrons are stripped off the atoms of the gas. In this case, the gas becomes a plasma, the fourth state of matter. Plasmas are plentiful in nature: a star exists as a plasma, as does lightning and even fire. Plasmas can also be produced in the lab under extreme conditions, for example by laser bombardment or by particle collisions in accelerators.

A supernova is not a liquid or a gas but a plasma

Update

Here is a super little YouTube video on plasmas by rock band They Might Be Giants, many thanks MattW


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Introductory physics: heat and temperature

The teaching semester began again at 9.15 this morning. First day back, I’m always struck by how much I enjoy being in the classroom. I think it’s because lecturing is basically a performance, with never a dull moment; anything can go wrong and usually does! I also quite like big classes, it makes for a good atmosphere…

Then there’s the content. This morning 1st Science got to meet Heat and Temperature for the first time. This is my favourite kind of topic – quite simple but of fundamental importance. ‘Heat is a form of energy’, we tell our students, and ‘temperature is a measure of heat’. Actually, the discovery that heat is simply a form of energy was an enormous advance in science, possibly the greatest breakthough of 19th century physics.

And what sort of energy is it? Well, kinetic energy arises due to the motion of molecules (vibration in solids). But there is also potential energy;  since atoms in solids have more-or-less fixed positions in the lattice they have must possess an associated potential energy (so do atoms in liquids for a slightly more subtle reason). So heat is basically a type of internal energy. Except that it’s not always internal; there is also the whole business of heat transfer, a phenomenon that can occur by any or all of three very different mechanisms!

Then there is temperature; a philosopher would have a field day explaining the difference between a quantity that simply is (energy), it’s manifestation (temperature) and human temperature scales. Indeed, the relation between heat and temperature was only quantified with the intoduction of concepts such as specific heat capacity and specific latent heat. Temperature clearly has a fundamental aspect too; for example, what do really mean by absolute zero (or zero Kelvin)? ‘Absolute zero is the temperature at which all molecular motion ceases’, students are told. But what does this mean? Why can’t we reproduce this temperature in the lab? Is -30 Kelvin ( or -303 degrees Celcius) really a nonsense? Fundamental stuff indeed and a nice start to the term…

Achieving the impossible in the lab

Update:

Oops! I thought the dial read zero on the LHS but it doesn’t of course. Also, I’m not sure why it reads degrees Kelvin, there is no such thing.

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