Thermodynamics - Heat moves, get used to it!

THERMAL PHYSICS

THE FIRST LAW OF THERMODYNAMICS

Energy is Absolute

The First Law of Thermodynamics (Thermodynamics I) states that the total energy in the universe is a constant.  This means, what is there is there, it does not depend on how you look at it, or on a variation of another quantity.  Temperature is not absolute, it’s an average which depends on the amount of energy in a given mass of a given substance.    You probably have come across Thermodynamics I before but not called it this, it's often stated as energy cannot be created or destroyed just transferred from place to place or from form to form.  This is actually more useful to you in your exams.  It's an important law, the first conservation law that you will come across and the key to understand a lot of quite complicated situations.  It is the concept that you cannot get something for nothing, and without understanding it James Watt would never have invented the steam engine, and you wouldn't have electricity or a car....

Heat

Thermodynamics refers to heat moving, thermo meaning heat and dynamics meaning to move.  When we are applying the law solely to heat we state it in another way, in a closed system things tend towards thermal equilibrium.  This in fact defines heating, it says basically that if a bunch of objects are left alone, and start from different temperatures, they will eventually reach an equal temperature, it's an idea of balance, of heat spreading out, and it's a really useful one to remember for your exams, heat dissipates into the surroundings, heat spreads out and becomes too small to use.

Energy Transfers

In all transfers of energy some energy is wasted, and it always ends up in the form of heat dissipated to the surroundings.  We'll return to this idea later.  But for now, although it is a massive simplification in the complexity of the transfer of energy, it will suffice to give names to certain macroscopic manifestations of the influence of energy, be it stored, or in the act of being transferred.  These energy transfers are often represented by simple or more detailed flow diagrams, and later, quantitatively in Sankey diagrams.

Potential Energies

When energy is stored we call it potential energy, in other words, it has the potential to do work, to be transferred.  Chemical potential energy is energy stored when atoms are bonded together, it's basically the energy released or absorbed in chemical reactions.  Gravitational potential energy is energy given to an object when it is raised in a gravitational field, (in the same way you could have a magnetic potential energy or an electrical potential energy (static electricity), but let's come back to that at A Level,) indeed the saying "what goes up, must come down" is a consequence of the law of conservation of energy, of Thermodynamics I.  You can have elastic potential energy, which is the type of energy given to an object when it is stretched or squashed and has the ability to return to its original shape.  Lastly there is nuclear potential energy, and this is a proportionally vast amount of energy compared to the others, energy stored within the nucleus of atoms, which is released during radioactive decay or nuclear reactions.

Transferring Energy (Work)

When energy does a job for us... or for anything... when it is transferred from place to place or form to form we call that "work".  Although remember that work is not always useful, for instance work done against friction is usually a nuisance and inhibits the efficiency of most machines.  It causes energy to be wasted as Heat.  Heat is an important form of energy to understand, and that is why we have chosen to start this book with it, but on the microscopic level, or you might say the nanoscopic, level heat is actually just the random kinetic energy of the particles which make up a substance.  Kinetic energy is the term we usually give to the larger scale ordered movement of many particles, that is to say a whole ball worth of particles, and you should see kinetic energy as just that, movement energy.  Electrical energy, current electricity that is, is another example of particles moving, however this time the kinetic energy belongs to particles which carry charge.  Usually it is good enough to say electrical energy is electrons moving.  Sounds are when particles move, (again,) in a different ordered manner, this time as oscillations, as pressure waves, and this is why we can say that sound does not travel in a vacuum, it needs particles.  The last type of energy is probably the most mysterious, at the point in Physics where you are now, that is light, it too is a wave, but it is not carried in a medium made of particles, but upon the electromagnetic field which pervades the universe.

THE SECOND LAW OF THERMODYNAMICS

Heating and Cooling

Heat moves from hot to cold doesn't sound like much of a Law, but it is central tenet in Physics which has implications beyond the obvious.  Importantly for yourself, whenever you are asked about a heat transfer just always remember to work out in which direction heat is going to travel.  The number of children who have told me that a hot drinks cup should be matt black so it absorbs heat from the surroundings and stays warm for longer is huge, the hot drink is warmer than the surroundings so we need to limit the transfer from it to the room, rather than increase the flow in the other direction.  The Second Law of Thermodynamics, Thermodynamics II, also states that the rate of energy transfer is proportional to the energy difference, and that is the reason for these pretty natty heating and cooling curves.  But it most importantly instructs us that it there is no such thing as cold energy, that to cool something down we need to think of a way to take heat out, rather than put cold in, and that is the reason you have air conditioning in your car and a cold can of coke from the fridge.

Conduction

Conduction is heat transfer in solids.  Conduction is heat transfer by contact.  Conduction is heat transfer by collision between particles.  These statements are all true and hold at the macroscopic level, but it will be important for you to understand what is happening to the particles during conduction, by understanding this you will be able to make predictions about which material will conduct heat better or insulate heat better.  It will allow you to understand experimental evidence and it allows chemists to make materials particularly suited to the thermal job the are required to do based on the properties of the material.  The denser the material, the closer the particles are together, the more likely the particles are to collide, the faster the heat transfer by conduction.  Free, or delocalised, electrons in metals are not fixed to their atom, they respond quickly to the presence of heat, vibrating more vigorously and collide with other electrons and the positive ions in the metallic bond.  This is the reason good electrical conductors are also good heat conductors.  More about this in the Electricity topic.

Convection

Convection is heat transfer in fluids.  Convection is when hot fluids rise.  Convection is due to less dense fluids floating on top of more dense fluids in a gravitational field.  Convection is heat spreading through a closed system in currents.  Convection is how a radiator warms a room, and how we can cool a room using an open window and a fan.  Convection is also the cause of seismic activity on Earth, as magma heats from the Earth's cause then cools near the surface.  When students talk of convection they often are quite secure in the cycle heat source causes expansion and so a reduction in density, then that thing rises and displaces the more dense, which sinks, and this forms the current.  However students tend to imagine the particle pictures of matter that they are so comfortable and familiar with, so they talk about the particles themselves expanding.  Be aware of this when you are writing, and make statements like, the fluid expands, the fluid becomes less dense, the fluid rises.

Radiation is the name given to anything which travels in all directions from a central point, it stems from the word radius.  In this case we mean heat radiation, it is heat that travels in waves.   Heat that does not need particles to transfer energy.  But we can be more specific than this, it is the infra red portion of the Electromagnetic Spectrum, which we will visit later, but for now you can see this as the extension of a rainbow, which you can't see, the redder than red bit, the infra red bit.  All objects emit infra red, (emit means to give out,) the hotter an object is the more it emits.  You will also need to know that the colour and the lustre (shininess) of a surface has an effect on what an object emits or absorbs, we say that light shiny objects are poor emitters and absorbers of infra red and that dark matt surfaces are good emitters and absorbers of infra red.  If you learn these facts then you should find questions about this pretty straightforward you'll need to make sure that you don't use words like attract when you mean absorb and bounce when you mean reflect and you'll be fine.  Also remember to consider which way the heat is travelling, Thermodynamics I.

SPECIFIC HEAT CAPACITY

Heat vs Temperature

Remember I said that energy was absolute?  And that temperature was an average?  You'll need to be absolutely sure on that one whilst we move on to this, which is the first reasonably difficult mini section of the book.  To help you visualise what this means I'd like you to shut your eyes and imagine this... oh wait, you'll need to open your eyes to read... unless you've got the spoken word version of the book, or I guess you could have someone read this to you, whilst you imagine, but you'll need to imagine in any case.  Get a cup, fill it with hot water, make it  80°C, now get a bath tub and fill that with hot water, also 80°C.  I don't want you to hurt yourself, so just imagine, but which one would you rather spilled on you?  Well obviously the one with the least energy.  The bathtub of hot water has far more energy, because it has far more mass.

Meaning

This is one of the cases in Physics where unpicking the meaning of the quantity is very useful.  Specific refers to the fact that this quantity is different for different materials, heat refers to the type of energy we are talking about, capacity indicates we are talking about how much something can hold.   So it is how much heat different materials can store.  I think this is a good way to imagine it, and you'll need to be aware that the value is for a temperature change of 1°C and a 1kg mass of the material, so it's also a good idea to picture specific capacity as how much energy is needed for a 1°C temperature rise for a 1kg mass of something.  When you are asked questions about it then you'll need to remember that the energy in question could be going in to the object, or that it could be coming out of the object.

Experiment

Ever bitten into a pastry, hot from the oven, and burnt your tongue?  It's probably because the pastry had jam, or chocolate sauce in it right?  Did you wonder why does the jam burn me but not the pastry? Well, it's because the jam has a higher amount of energy in it even though they are at the same temperature.  The jam has a higher specific heat capacity.  We can measure specific heat capacity by heating up different materials and measuring the energy we put in, their mass and the temperature change.  This is always difficult because the material is rarely pure, or uniformly dispersed within any given sample, and that as soon as the material is hotter than the surroundings it starts transferring some of the heat to the surroundings.  We can make it as accurate as possible by using insulation to try and reduce this heat loss.

Calculation – key point 1L of water is, is 1kg, (1000ml is 1000g)

Energy = mass x specific heat capacity x temperature change

So let's, for example say we took 500ml of water, that's a 0.5kg mass of water, heated from 20°C from the tap to 50°C, and we measured that it took 63 000J to do this.  We rearrange our formula as we know all the values except specific heat capacity:

specific heat capacity = energy / mass x temperature change

Input the numbers:

= 63 000 / 0.5 x 30

Calculate:

= 4 200

And put a unit on the end, we've done Joules divided by kilograms divided by degrees Celsius so that's:

= 4 200 J/kg°C

Use

Understanding specific heat capacity helps scientists select materials for their jobs based on their thermal properties.  For example, if they want a material to store a lot of heat energy but not reach very high temperatures then they would choose a material that has a very high specific heat capacity.  A good example of this would be when making a passive heating system, which the sun heats during the day and then radiates into the building at night.  Another example might be in building a building to stay cool, a large amount of thermal mass enables the building to absorb the suns energy whilst not reaching high temperatures.  Whereas if you wanted something to reach a high temperature very quickly then you would need to use something with a low specific heat capacity, this might be a good property for a cooking oil, but not so much for an engine oil!  Lastly let me mention water, water is such a good habitat for life, because it has a very high specific heat capacity.  That means that the oceans do not change much in temperature, so maybe that is the reason that life started in the seas?  I don't know, we'd have to do some experiments.  But it's also bad news, for a similar reason it takes a lot of energy to melt ice, and our ice caps are definitely melting... what does that mean for the heat energy of our atmosphere?

SPECIFIC LATENT HEAT AND KINETIC THEORY

Water

Water is the strangest of materials, it is one of the few materials that actually expand when they freeze.  Why is that?  Well it's hydrogen bonding.  All most other materials are more dense when in their solid form, they sink in their liquid form, but ice floats on water.  So what sank the Titanic?  Hydrogen bonding did.  Water is often the first example of the idea of change of state.  That is change between solids, liquids and gasses.  It is a good example as, it is linked to a familiar temperature scale, degrees Celsius.  You can very easily calibrate a thermometer using some water, as you know that some ice which is melting is at 0°C and some water which is boiling is at 100°C.

Kinetic Theory – annotated particle diagrams

You remember kinetic energy, energy of things that are moving.  Kinetic Theory states that all materials are made of particles that are moving randomly with more or less energy, and that they are bound together by intermolecular forces of attraction.  The states of matter are simply put balances between these two things, particles in a solid have low kinetic energy but high force binding them together, particles in a gas have high energy and low force binding them together.  This accounts for our particle pictures of solid liquid and gas that you will have learned to recognise from a young age.  Remember, whatever else you know about the states of matter, it is always a balance between forces and energy.

Change of State

Changing state takes energy.  It takes energy for the particles to break free of the forces holding them together.  We call the energy used to change the state of a material the latent heat.  This energy doesn't increase the temperature of the material.   You get this really interesting graph and you can clearly see the boiling and melting points as horizontal sections on the graph.