KS3 Physics: 1. Energy & Thermodynamics

What is Science?

Welcome to your first science lesson. You have almost certainly learned about science before, but the adventure you are about to embark upon will take you far beyond anything you learned at primary school. Before we set off on our journey through the study of nature and the universe, we must ask ourselves a very simple question with a very complicated answer: what is science?

Imagine you are given a box. It is closed, and there is no way you can ever open it. There is something inside, but you have no way of seeing what it is. How might you try to work out what’s in there? First, you would probably shake the box around. You would listen to the sound it makes and try to feel the movement of the unknown object with your hand. Next, you would make a guess. Maybe it’s a ping pong ball; maybe sand; maybe a Lego brick. These guesses wouldn’t be completely random — you’d be a fool to suggest there was an elephant or an aeroplane inside. You would base it on what you already know: objects small enough to fit in a box, that might move or make the sounds you felt and heard in the first step. After you’d made a guess, you’d think: if it was a ping pong ball inside the box, what would I expect to happen if I tilted the box slightly? Well, you’d expect it to roll, which you’d be able to hear and feel. The next step would be to check whether that happened: you would tilt the box slightly. If you could hear it rolling and bouncing against the walls of the box, you would still think your guess was a good one. If it didn’t move, or if you could hear it sliding rather than rolling, you would probably decide your guess was wrong. You would need a new guess.

This thought experiment shows us how scientific knowledge is created. Nature and the universe are like a box, the inside of which we cannot see. Volcanoes erupt, the Earth moves around the Sun, chemicals react and explode, animals live and become extinct. All of these things happen, but we cannot see how or why. Scientists are like detectives; they try to work out what’s going on. Firstly, they guess what might have happened. Next they deduce (work out) the consequences of their guess being true: they ask themselves, if this was true, what would I expect to happen? Finally, they check whether these deductions do happen by making observations and measurements of the natural world. If their observations agree with what they expected to happen, they carry on with the same guess. If their observations do not agree with what they expected, their guess is wrong. In this case, it is time for scientists to come up with a new guess.

Matter & Motion

Scottish physicist James Clerk Maxwell was one of the greatest scientists who ever lived. In one famous survey, he was ranked at #3, behind Albert Einstein and Sir Isaac Newton. He discovered many important laws of physics, from the behaviour of gases to the nature of light. When he gave his first lecture as a professor, at the University of Aberdeen, he told his students that they must forget everything they thought they knew about the world. Studying science meant entering into a new world, in which one must learn to recognise the laws that govern matter and motion. Matter is the scientific word for stuff — every object is made up of matter, whether it is your arm, the water you drink from the tap, the air that you breathe or the burning fireball that we call the Sun. The only place where there is no matter is a vacuum. Space is an example of a vacuum: it is empty; there is quite literally nothing there. (For many years, scientists did not think it was possible that there could be a space without anything to occupy it. Showing this was possible was a key moment in understanding the universe). Motion is the scientific word for movement. In this unit, we will study matter and motion: what stuff is made of and how it moves.

When scientists make guesses, as we learned about earlier, they use sentences containing the words ‘as if’. As we said about the mystery box, a guess must always be based on what you already know, or at least something you can imagine. The first guess we are going to look at is an old one. It was first suggested by a Greek thinker from 2,500 years ago, Democritus. He suggested that all matter must be made up of tiny pieces which cannot be broken down further. This was not a scientific guess, however, as it wasn’t based on any observations or experience. Democritus was simply making a logical suggestion. Imagine folding a piece of paper in half, then folding it in half again, and again, and again, and again. If you kept doing this, you would reach a point at which you could no longer fold the paper in half. This was Democritus’ idea: if you kept cutting matter up into smaller and smaller pieces, you would eventually get to a point at which you could no longer cut any further. He called these pieces the Greek word that meant ‘uncuttable’: atomos. This is the origin of a word we still use today: atoms.

The first scientist to suggest matter was made up of atoms was John Dalton. He studied chemical reactions, and suggested that all matter behaves as if it is made up of tiny, hard spheres of stuff, like golf balls. Scientists call tiny balls of matter like these particles. Later experiments showed us that atoms are actually more complicated than golf balls, but only if you look very closely. Everything we will look at in this unit will use Dalton’s simple picture: matter behaves as if it is made up of tiny particles. This is the first key idea we need to understand.

The second key idea is that all matter behaves as if it has a certain amount of energy. We all know what energy is in everyday life: it’s what you use when you run around and do stuff. It’s something we might take for granted, but it is the most important concept in science. Pretty much everything you will learn about in science at Bobby Moore Academy happens because of energy moving from one place to another. We call the movement of energy a transfer. Energy can be transferred in four ways:

1. Mechanically, which involves forces.
2. Electrically, which involves a property called charge.
3. By radiation, which involves waves.
4. By heating, which involves collisions between particles.

You will study four physics units over the next two years, and each of them will focus on one of these ways of transferring energy. This unit — energy and thermal physics — focuses on heating. We will learn more about that shortly. Later this year, you will learn about sound and light, which are both examples of energy transfer by radiation. Next year, you will learn about forces and space, which involve mechanical transfers of energy. Later in Year 8, you will learn about electricity and magnetism, which are linked and involve electrical transfers of energy.

Energy Stores & Transfers

When objects have energy, we describe them in terms of energy stores. Energy can be thought of as something a bit like money. If you have money, you can buy stuff: a book, a new T shirt, food. Likewise, if objects have energy, they can do stuff: move around, heat things up, give out light or sound. There are different types of money depending on where you are in the world: for example, pounds, dollars and euros. If you have £100, you can change this into an equivalent amount of dollars or euros. Energy works in a similar way. Pounds, dollars and euros are like energy stores, different types of energy that allow objects to do different things. Changing money from one currency to another is like an energy transfer, in which one energy store is converted into another.

Examples of energy stores include:

• Kinetic — stored in moving objects
• Gravitational potential — stored when objects are raised upwards, away from Earth
• Elastic potential — stored when objects change shape (when they are stretched or squashed)
• Chemical potential — stored in bonds and transferred during chemical reactions
• Thermal — stored in hot objects

Just like when you change pounds into euros, energy can be transferred between these stores. Imagine you are holding a marble. If the marble is lifted up above the ground, it has a store of gravitational potential energy. If you let go, it falls towards the ground, losing its gravitational potential energy but gaining kinetic energy. (This is a mechanical transfer of energy. It happens because of the gravitational force acting on the object).

We can represent this simple energy transfer using an energy transfer diagram:

• The boxes show that the marble is an energy store.
• The words underneath the boxes tell us how the marble is storing energy.
• The arrow shows an energy transfer takes place.
• The word above the arrow tells us how energy is transferred.

Energy transfer diagrams show how energy is transferred usefully between stores. Sometimes, however, energy is lost to the surroundings. We call this loss of energy dissipation, or wasted energy. It is important to be able to identify useful and wasted energy in something like a lightbulb, car or kettle. The useful energy transfer from a lightbulb is the light given out by radiation. This is what you use a lightbulb for. You may have held your hand near a lightbulb and felt that it was hot, too. This is wasted energy, transferred by heating the surroundings. We call it wasted energy because we do not use lightbulbs to heat the room; we use them to light up the room. Most devices waste some energy by heating the surroundings. For example, cars transfer energy usefully in a mechanical transfer from chemical potential energy (in the fuel) to kinetic energy (in the moving car). As the engine runs, however, it gets hot, and dissipates some thermal energy to the surroundings.

The idea that all energy is either stored, transferred usefully, or wasted is fundamental. Another Scottish physicist, Lord Kelvin, suggested that all the matter in the universe behaves as if the energy associated with it can never be created or destroyed. Energy may be dissipated, but it does not disappear. It always comes from somewhere; it always goes somewhere. We might not be able to see where it goes (e.g. when a toaster heats the air in our kitchen), but it is still there, somewhere in the universe. This fundamental idea, perhaps the most important in the whole of science, is called the principle of conservation of energy. If something is conserved, it means it stays the same. Kelvin’s idea is that there is a fixed amount of energy in the universe; all the matter and motion we observe can be explained by how this energy moves between objects and places.

The principle of conservation of energy is also known as the first law of thermodynamics, a subject in which Kelvin and Maxwell were leading figures. The word thermodynamics is made up of two smaller words: thermo, which means involving heat, and dynamics, which means motion. The next section of this unit will focus on these two ideas: heat and motion.

States of Matter

If we accept Dalton’s guess that all matter is made up of particles, like tiny golf balls, then we can start to think about how those particles behave. There are three states of matter: solid, liquid and gas. Imagine taking an ice cube out of the freezer. If you left it out at room temperature, (which is usually around 20–25 degrees Celsius — we will return to the idea of temperature scales later), it would melt. Melting is when a substance changes from a solid to a liquid: the ice cube is a solid, the water is a liquid. (Substance can be thought of as another word for ‘some stuff’). If you were to then heat up the water in a saucepan, it would eventually evaporate. Evaporation is when a substance changes from a liquid to a gas. The name we give to water in gas form is steam. If the steam reached a cold surface like a mirror, we would see it condense: this is when a substance changes from a gas to a liquid. (The reason the mirror ‘steams up’ when you take a shower is because steam hits the cold mirror, condenses, and leaves water droplets behind. It would better be described as ‘unsteaming’. More on this later.) Finally, we could put liquid water in the freezer to get ice: freezing is the change of state from a liquid to a solid.

Ice, water and steam are all made up of the same stuff: tiny particles. When we heat or cool substances, we change their temperature. We will examine what this means in more detail later on. When we change the temperature of a substance, the relationship between the particles in the substance changes. In solids, they are close together; arranged in a regular pattern (we call this a lattice); they do not move freely (they can only vibrate — we call them fixed); and there are strong bonds between them. In a liquid, the particles are still close together, but they are randomly arranged and can move relatively freely. The bonds between particles in a liquid are weaker than in solids. In a gas, particles are far apart and completely free to move. They move randomly, in all directions, and relatively fast (particles in a gas have the most energy, as we will see). There are no bonds between the particles in a gas.

Temperature & Energy

There are a couple of important points to note about the particles in solids, liquids and gases. Firstly, we must consider how a change in temperature affects the motion of the particles. Particles in a solid vibrate slightly more as the temperature increases; particles in a liquid or gas will move faster. This is because temperature is a measure of the average kinetic energy of the particles in a substance. The hotter something gets, the more kinetic energy its particles have. This is a very important definition to remember. Secondly, we must consider what happens during a change of state. Melting, freezing, evaporating and condensing are all changes of state, i.e. changes from one state of matter to another. These are examples of physical changes. The crucial point is that the particles themselves stay the same during a physical change. All that changes is the strength of the bonds between the particles. For this reason, physical changes can be reversed. If you take ice out of the freezer, you get water. If you put that water back in the freezer, you get ice again. Likewise, because a change of state only affects the strength of the bonds between the particles, not their kinetic energy, the temperature does not change during a change of state. We will study this idea further in Year 9.

The temperatures at which changes of state occur have specific names. The melting point of a substance is the temperature at which it changes from a solid to a liquid (melting) or from a liquid to a solid (freezing). The boiling point of a substance is the temperature at which it changes from a liquid to a gas (evaporation) or from a gas to a liquid (condensation). (Boiling and evaporating are effectively the same: boiling is the process of heating a substance until it evaporates.) The melting point of water is 0 degrees Celsius (oC). The boiling point of water is 100oC. This means that at room temperature (around 20–25oC), water is between its melting point and boiling point — it is a liquid. Metals are examples of materials that have very high melting points: they are solids even at very high temperatures (this is why you would use a metal tray to cook something in a 200oC oven). The air we breathe contains substances like oxygen and nitrogen. These have very low melting and boiling points: you have to cool these gases way below 0oC to change them to their liquid form (you may have heard of liquid nitrogen being very cold). Because changes of state are physical changes, the melting and boiling points of different materials depends on the bonds between the particles. We will learn more about this in Year 10.

In the examples we have looked at, we have referred to temperatures using the Celsius scale. This is probably the scale you are most familiar with: when you watch the weather forecast on TV, temperatures are quoted in degrees Celsius. There are two other common temperature scales: Farhenheit and Kelvin. The Faranheit scale is the oldest temperature scale, and is still used in America. The Celsius scale, or centigrade scale, was developed a few years later: it defines the melting point of water as 0oC and the boiling point of water as 100oC. This is useful because water is such a common substance. The Kelvin scale is most useful to scientists, however. It tells us how much kinetic energy particles have on average. Zero Kelvin (0 K) is the lowest temperature that any substance could ever reach: it is the temperature at which the kinetic energy of the particles is zero. This means particles in a substance at absolute zero do not move at all. Outer space is close to absolute zero — it’s chilly out there! It’s easy to move between the Celsius and Kelvin scales: 0 K is equivalent to -273oC, and intervals are exactly the same. Diagrams like the one below are helpful in understanding this difference.

Converting between Celsius and Kelvin.

Heating & Cooling

So far we have looked at two key ideas:

1. Matter behaves as if it is made up of tiny particles.
2. The temperature of a substance is a measure of the average kinetic energy of its particles.

These two ideas can help us explain many everyday events.

• Why does a hot cup of tea cool down when you leave it out in the kitchen? First we must consider the temperature of the two substances. The tea is at a high temperature, probably close to 100oC, the boiling point of water, whereas the air in the room is at room temperature around 20oC. This means particles in the tea have a higher kinetic energy (on average) than the particles in the surrounding air. When particles from the air collide with the particles near the surface of the tea, they gain kinetic energy from the particles in the tea. Because energy must always be conserved, this means particles in the tea must lose kinetic energy at the same time. In other words, energy is transferred from the tea to the surroundings. The average kinetic energy of the particles in the tea falls, i.e. its temperature decreases. It cools down. The average kinetic energy of the particles in the surrounding air increases, meaning the temperature of the room increases. The room is so large compared to the cup of tea, however, that we wouldn’t notice the change in temperature of the room.
• Why does a cold drink warm up when you leave it out in the kitchen? If you put your hand near a cold can of coke, it feels as though the coke is giving out ‘coldness’, but in reality, there is no such thing as coldness. What is actually happening is exactly the same as the previous example. The temperature of the room is around 20oC; the temperature of the cold drink is close to the freezing point of water: 0oC. This means the particles in the surrounding air have a higher kinetic energy (on average) than the particles in the cold drink. When particles from the air collide with particles from the cold drink, this time the particles from the air lose kinetic energy and the particles in the drink gain kinetic energy. The average kinetic energy of the particles in the room decreases and the average kinetic energy of the particles in the drink increases. The room cools down as it heats the drink, but again we wouldn’t notice this change in temperature as the room is so large compared to the drink.

The two examples above are simple illustrations of the second law of thermodynamics: energy is always transferred from hot objects to cold objects. We call this type of energy transfer heating. The critical point to remember is that there is no such thing as coldness. What we experience as ‘coldness’ is actually a form of heat transfer in the opposite direction. When you hold your hand close to the cold can of coke, or reach into the fridge, or stand near a window on a cold day, it feels as though coldness is arriving at your body. This is not what happens. In fact, your body is at around 36oC. This is a higher temperature than the coke, fridge, or outside world. This means your body is transferring energy to the cold object. The can of coke does not cool you down; you are heating the can of coke. This is a difficult concept to get your head around, but is fundamental to understanding many physical processes.

There are three main ways in which heating can happen: conduction, convection and radiation. Here, we will study the first two of these; we will look at radiation in the sound and light unit. Conduction is the simplest method of heating. Solids tend to transfer energy this way. It happens when objects collide with one another and pass on their kinetic energy directly. Materials which transfer energy very quickly by this method are called conductors. Materials which transfer energy slowly by this method are called insulators, or poor conductors. Metals are very good conductors, because they contain extra particles called free electrons. (We will look at electrons in more detail in Years 8 and 9). These can move through the fixed lattice of particles very quickly, colliding with lots of other particles and therefore transferring energy very quickly. This is why a teaspoon gets hot if you leave it in a cup of tea. Other materials, like wood, glass, water, air and plastic, are very poor conductors, or good insulators. This is why saucepans often have plastic handles. If you had a solid metal handle, it would get hot very quickly (because of the conduction described above), and you would be likely to burn your skin.

The plastic handle of a saucepan is an example of where insulators are useful. Insulators are also useful if we want to keep things warm, for example when transporting hot drinks or takeaway food, or keeping our house warm during the winter. To keep things warm, we must surround them with a material that only transfers energy by heating slowly. We can test the quality of different insulators by wrapping different materials around a beaker of hot water and measuring how the temperature changes over time. We measure temperature using a thermometer. To make sure that the insulating material is the only thing affecting how quickly heat is transferred, we must make sure we use the same volume (amount) of water for each material and cover the water with a lid each time. We must also compare the temperature changes of the water after the same amount of time, e.g. ten minutes. We can plot our results on a graph, which we call a cooling curve.

Convection is slightly more complicated than conduction. Liquids and gases tend to transfer energy this way. It happens when particles in a liquid or gas gain energy by colliding with a hotter object; for example, air in the kitchen coming into contact with a hot cup of tea. When the air particles collide with the particles in the tea, the air particles gain kinetic energy. This causes them to move faster and spread out. As the particles spread out, they become less densely packed. In other words, the air becomes less dense (we will learn more about density later), and begins to rise. You may have heard the expression ‘heat rises’. This is not quite true. It’s actually warm air that rises, because its particles have more energy and are therefore less densely packed. It’s easy to see convection by placing dyes into liquids and heating them at the bottom. We can also experience it a tall building: it often feels warmer on the fifth floor than on the ground floor, because the warm, less dense air tends to rise upwards.

We saw earlier that cup of tea cools down because its particles transfer energy to lower energy particles in the air around it. We can now give a more sophisticated explanation than this in terms of evaporation. Evaporation, as we saw earlier, is the change of state from a liquid to a gas. The tea is close to the boiling point of water (100oC), which means some of it is likely to be evaporating. (If you take a hot drink outside on a cold day, you can see steam rising from the cup. This is evaporation.) We know that the temperature of the tea is determined by the average kinetic energy of its particles. This means that some particles have more energy than this, some have less. The highest energy particles escape from the surface of the tea, colliding with air particles around them and causing warm air to rise by convection. This means the average kinetic energy of the remaining particles is lower, which means the temperature of the remaining liquid is lower. Hence, the tea cools down.

This process of cooling by evaporation also explains why humans sweat. When we’re hot, we release a layer of liquid onto our skin. The highest energy particles escape the surface of this liquid, reducing the average kinetic energy of the remaining particles. This cools the liquid down, which in turn cools our skin down. The opposite happens when you take a shower. Because the water in the shower is hot, steam is released into the bathroom. Particles in the steam (a gas) have lots of kinetic energy. When the steam reaches a cold mirror, its particles collide with those in the mirror and lose energy: they transfer their energy to the particles in the mirror. When they lose their kinetic energy, the steam condenses, i.e. turns into a liquid. The ‘steam’ we see on a bathroom mirror is therefore not steam at all, it is water. We often call such water condensation. One way to avoid the formation of condensation is to warm up the surface so that particles in the steam do not lose their energy when they collide with it. Many cars have thin lines across the rear windscreen: these are tiny heating elements to warm up the glass in order to remove any condensation that forms when you get in the car.

Density & Pressure

Earlier on, we described how particles could be densely packed. This meant they were less spaced out; that there were smaller gaps between each one. In simple terms, then, density is a measure of how many particles there are in a given space. If we zoom out from the particle level and look at objects as a whole, we can calculate their density by making two measurements. We cannot count the number of particles, but we can measure how much stuff they’re made up of. This measurement is called mass: we measure it using a balance. (In everyday language, we would call a balance a set of weighing scales, but weight and mass are very different, as we will find out in Year 8.) We then need to measure how much space this stuff takes up. Looking at the object as a whole, we call the amount of space it takes up its volume. We can measure the volume of regular solids (e.g. a cube) using a ruler and some maths. If we have an irregular solid (e.g. a stone or a piece of plasticine), we must use a displacement method: drop the object into some water and measure how much the volume of the water changes using a measuring cylinder. If we have a liquid, measuring the volume is easy: we simply use a measuring cylinder.

Once we know the mass and the volume of an object or substance, we can calculate the density by dividing the mass by the volume. So a heavy object (large mass) that doesn’t take up much space (small volume) has a very high density, whereas a large object (large volume) that is also light (small mass) has a low density. Imagine having two objects the same size next to each other. One is made of chocolate, one is made of gold. Which one would be easier to pick up? If you guessed the chocolate, you’d be correct. This is because the two objects have the same volume, but gold has a much greater mass. This means it is much more dense. Now imagine you had two bars, one of chocolate and one of gold, both with the same mass e.g. one kilogram. Which would be larger? This is trickier to think about, but the answer is that the chocolate bar would be larger, effectively because its particles are more spread out, so the same mass would take up more space.

Now consider a different situation. You have a balloon, which isn’t blown up. What happens when you blow air into it? Why does it inflate (get larger)? Or consider a bicycle or car tyre. When you pump it up, what’s happening? Why does the tyre get firmer? We can first think about this in terms of density. As you blow air into a balloon or pump air into a tyre, you are putting more particles into the space surrounded by the walls of the object. In scientific language, we call this an increase in density, as we saw above. If the particles in the balloon or tyre are more densely packed, then they collide with the walls of the object more frequently. Every time a particle bumps into the wall, it pushes it outwards slightly, so if there are more collisions every second the wall is pushed outwards with more force. We call this increase in force, spread out over the area of the walls, an increase in pressure. What’s important to understand is that liquids and gases exert a pressure on the walls that contain them because particles collide with the walls. As we will learn in Year 8, this is why ships experience an upwards force called upthrust: particles are more densely packed lower down in the ship than near the surface of the water, which means the collisions between the particles and the walls of the ship are more frequent, which means the pressure is greater. This pushes the ship upwards, and therefore allows it to float.

The reason particles lower down in the water are more densely packed than particles near the surface is because the ones up above push down on the ones below. The same happens in air. High up in the atmosphere, the layer of gas that surrounds the Earth, particles are relatively spread out. The air is not dense at all. Down at sea level (e.g. in London), particles are very densely packed, because all of the particles above are pushing down on them. (This is due to their weight, the force of gravity, which we will learn more about in Year 8). This difference in density leads to some strange effects. If you take a bottle of water on an aeroplane and have a drink when you’re up in the sky, you capture some of this low density air in your bottle. Because there are fewer particles within the space of your bottle, there are less frequent collisions between the particles and the inside of the bottle. This means the force outwards is lower, or the pressure inside the bottle is lower. If you look at your bottle when the plane lands again, you will see it looks like it has been crushed. This is because you are now down in more dense, higher pressure air. Particles outside the bottle are more densely packed and therefore colliding with outside of the bottle more frequently. There is a greater force from the outside in than from the inside out, so the particles inside the bottle are forced to take up less volume or space.

This relationship between pressure and volume was discovered by the British physicist Robert Boyle. Through experiments that we can recreate in school today, Boyle discovered that the greater the pressure on a container full of gas, the smaller its volume. We can also investigate the relationship between temperature and pressure. What would you expect to happen if you heat up a container full of gas? You might suggest that as you transfer energy to the particles inside the container, they move around with more energy, or faster. The faster they move, the more frequently they collide with the walls of the container. The more frequent the collisions, the greater the force exerted outwards. The greater the force, the higher the pressure. We would expect then, based on these reasons, that the higher the temperature, the higher the pressure of a gas. This turns out to be exactly what happens. Finally, if you let the gas expand freely (i.e. put the gas in a container without fixed, rigid walls), then you would expect the gas to take up more space as it was heated. This is exactly what happens too: the higher the temperature, the greater the volume. These three relationships, between temperature, pressure and volume, are known as the gas laws.

The Scientific Method

The gas laws are an excellent example of the scientific method in action. Look what we did above. First, we made a guess about what the gas inside the container was made up of. We have said all the way through this unit that matter behaves as if it is made of tiny particles. Next we made some deductions. If this picture was true, we said to ourselves, and we heated up the gas inside the container, what would we expect to happen? Finally, we would check this by doing an experiment. We would make measurements of the temperature and pressure in order to work out whether our deductions were correct. If they were, then our guess is OK. If they were not, then we would have needed a new guess. This highlights the most important point about scientific study, one you will come back to again and again during your time at secondary school: we must always check our guesses and deductions by experiment.

For the next two and a half years, you will be learning about the guesses and deductions scientists have made about nature and the universe. You will learn a whole new, scientific language; you will explore a whole new world of concepts. Mid-way through Year 9, you will find out how scientists go about checking their deductions. You will learn how they make measurements and observations, and how this allows them to understand the world in terms of mathematics. After that, you will have the opportunity to apply these new methods to understand ever more complex scenarios. This is the adventure you are about to embark upon, an adventure that will help you see beyond the everyday world and discover the hidden structure of the universe. The study of science gives you a superpower, something similar to X-ray vision: a glimpse of what’s inside the box.