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Teachers’ notes

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SCIENCE MUSEUM LIVEPRESENTS

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Teachers’ notes

These teachers’ notes outline the science behind the demonstrations and experiments in The Energy Show. We hope you’ll find the information helpful in answering your pupils’ questions or when following up in class after watching the show.

We’ve also provided some ideas for classroom activities that you can use to further explore the science from the show.

The science behind the demonstrations

Methane bubbles

Methane is a colourless, odourless gas that is lighter than air. It’s found naturally in the atmosphere and also below the Earth’s surface. The exciting part, though, is that methane is a flammable gas, which means it can easily catch on fire. It can also explode when mixed with the right amount of air.

It is thought that 15% of all the methane gas in the atmosphere is produced by cows. Cows are ruminants – mammals that break down plant-based food in their stomachs instead of in their intestines like humans do. They eat food, then regurgitate it and then eat it again! Their stomachs are filled with bacteria that help them break the food down, but they also produce lots of methane.

For anything to catch fire you need three things: a fuel, oxygen and a source of heat. In The Energy Show the fuel was the methane gas, the oxygen was in the air and the source of heat was the small flame Phil was using. Methane is lighter than air, so when it is bubbled into soapy water the bubbles rise upwards. The fireball was made when the methane came into contact with the air and the flame. If you are close enough you can feel the heat from this experiment as well as seeing the light that is produced. This is heat energy and light energy.

Image © lifeonwhite.com

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Calcium carbide and oxygen

In The Energy Show calcium carbide and water are mixed to produce a flammable gas called acetylene which explodes if you combine it with oxygen and ignite it.

Calcium carbide is made by heating lime and coke. Lime is a mineral and coke is a fuel made from coal. When the calcium carbide touches the water a chemical reaction immediately begins which produces acetylene gas.

Acetylene can be used for lighthouse beacons, headlights on cars and in welding. It’s also found on a moon of Saturn called Enceladus, but we don’t think much welding goes on there.

Acetylene reacts with oxygen if it’s ignited, to form water and carbon dioxide. The reaction also makes a loud bang and lots of heat. This is sound energy and heat energy.

Hydrogen balloons

When hydrogen is mixed with oxygen in the right amounts and is ignited you get an explosion. Hydrogen is the most common element in the universe. It is found in most stars, including our Sun, where it is converted into helium, producing heat and light energy.

Combustion is burning. It’s where you rapidly combine a substance with oxygen, and heat and light are given off as a consequence. Under the right conditions, everything will burn, but some things take less heat to make this happen. This is because it takes less energy to break the bonds in the material. The burning of hydrogen is quite simple. Hydrogen burns with oxygen to produce water. Oxygen on its own is not combustible, but it’s what other things react with when they combust. Hydrogen needs to mix with oxygen in the right amounts to give the best reaction and the loudest bang.

When a hydrogen-filled balloon is ignited on its own the explosion isn’t so big. This is because it takes a little time for the hydrogen in the balloon to mix with the oxygen in the air, so the reaction is slightly slower. However, if you have the right amount of oxygen mixed with hydrogen before the balloon is ignited, you have a very fast release of heat energy. This causes the surrounding air to expand suddenly, resulting in a loud explosion and a burst balloon.

Image © Science Museum/ Science & Society Picture Library

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Plasma ball and fluorescent tube

The plasma ball was invented by a man named Nikola Tesla. He was an inventor who did a lot of work with electricity.

A plasma ball is a clear glass orb that contains a variety of different gases. At its centre is an electrode. This is the smaller coloured ball shape you see inside the plasma ball. Electricity passes into the gases at the electrode. The electricity has a high voltage, which means it has a strong driving force. The high-voltage electricity turns the gases into plasma.

Plasma is a mix of charged particles. It’s found a lot in our universe – in lightning, stars and even in plasma TVs in our homes. The plasma has the appearance of beams of coloured light reaching from the centre electrode to the edges of the glass ball. It looks a lot light lightning because it’s essentially the same thing. The thin beams of coloured light you can see are called filaments. The filaments become much brighter when any material that can carry electric current touches the outer glass ball. This happens when the fluorescent tube is touched against the glass. The filament gets brighter because there is more current flowing through it than before. Electric current is just the flow of electricity through a material. The electrical energy from the plasma flows into the fluorescent tube when it touches the glass of the plasma ball.

Fluorescent tubes are the long thin tubes you see in some lighting fixtures. They normally contain a small amount of a liquid called mercury and a gas, such as argon. In this experiment the electric current is flowing from the plasma ball into the fluorescent tube. Inside the tube the electricity excites the atoms of the gas and mercury so they have lots of energy. As the atoms return to their original energy levels they release photons – the basic units of light. So that’s why the fluorescent tube illuminates like a light sabre when touched against the plasma ball.

Liquid nitrogen

Liquid nitrogen is very cold. It’s not the coldest thing that we can make, but it’s pretty close. In science we like to know exactly how hot or cold things are. So we have three common temperature scales: Fahrenheit, Celsius and Kelvin. They were all named after the people who came up with them.

The most useful to scientists is the Kelvin scale. It begins at the coldest theoretically possible temperature, absolute zero. At this temperature all the atoms that make up materials are motionless. Absolute zero is at 0 K on the Kelvin scale and at −273.15 °C on the Celsius scale. From there Kelvin goes up in the same intervals as Celsius. Liquid nitrogen is at 77 K (–196 °C).

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States of matter are the forms in which a material can exist. Water for instance is a solid when it is ice, a liquid in rain and a gas in the air around us. There are quite a few more states of matter, but in everyday life we normally talk about solids, liquids and gases. What state a material is in depends on the type of material and on the temperature and pressure on it. On Earth nitrogen is normally a gas and makes up about 80% of the air we breathe. Nitrogen can be made into a liquid using very low temperatures and pressures.

When liquid nitrogen comes into contact with the air, it absorbs enough heat energy to reach its boiling point and turn into a gas. The nitrogen gas is still really cold. It is cold enough to condense water vapour from the air it comes into contact with, so this water vapour turns into a liquid. Although it looks as though steam is coming off the nitrogen, in fact it’s condensed water vapour from the air – the same thing as clouds.

Liquid nitrogen has many uses. It can be used to preserve blood or cells, to remove warts or even to make ice cream.

When flowers are put into liquid nitrogen in The Energy Show, all the water in the flowers freezes very quickly. Frozen water takes up more space than liquid water in the cells of the flowers. So the cells burst open and make the flowers very brittle and rigid. A gentle tap and they smash to pieces.

Gases take up a lot more room than liquids. This is because the particles of gas are free to spread out. When liquid nitrogen is placed in a sealed plastic bottle at room temperature it boils inside the bottle, i.e. turns from a liquid into a gas. The gas spreads out more than the liquid did and fills the plastic bottle. Putting the plastic bottle in hot water speeds up this process. The pressure in the bottle increases because the bottle is sealed and the gas has nowhere to escape. Eventually the bottle can’t handle the pressure any longer and it explodes.

Van de Graaff generator

Van de Graaff generators weren’t invented just to make people’s hair stand on end. They were first made to create very high voltages. A Van de Graaff generator builds up lots of electrical charge on the metal dome on its top and produces something called static electricity.

Static electricity is where charges build up in one place and you get a spark when they move. To understand static electricity we have to think about atoms. We know that everything is made up of atoms. Atoms themselves are made up of charged particles: protons, which are positive, and electrons, which are negative. Electrons can be exchanged between materials. Some materials hold onto their electrons more tightly than others. Some give up electrons easily, some receive electrons easily.

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When two materials are touching, electrons can move from one material to the other. When they separate, one material is left positively charged and one negatively charged. This build-up of charge is known as static electricity. The materials will stay charged until they can be neutralised by an electric current. This is the spark you feel when you’ve been walking over carpet and you touch a metal door handle. It’s the flow of charge that causes the spark.

When the belt inside a Van de Graaff generator moves it creates a build-up of charge on the metal dome. In the show, when someone touches the dome, his or her hair stands on end. He or she is insulated from the ground by standing on something that won’t let electricity pass through it. The electric charge from the Van de Graaf generator transfers to this person and his or her hair gets electrically charged. Things having the same electric charge repel or push away from each other so the strands of hair repel and start standing up. If the person carrying the charge touches something that conducts electricity, such as a metal table, then the charge will be neutralised by a spark.

Magnets

A magnet is anything that produces a magnetic field. The magnetic field is invisible but is the area where magnetic materials experience a force. The Earth is a big magnet. It has one end near the North Pole and one end near the South Pole. Compasses point to the North Pole because they contain a magnetic material and so experience a force from the Earth’s magnetic field.

Not all metals are magnetic materials. Magnetic materials contain iron, nickel, cobalt or some rare-earth metals. The magnets in The Energy Show are made with a rare-earth metal called neodymium. They are small but very powerful. Rare-earth metals were named because they originally were thought to be uncommon on Earth and hard to obtain.

In an atom the central nucleus is surrounded by electrons. Electrons are the negatively charged particles we met earlier. They spin as they circle the nucleus, a bit like tiny planets orbiting a tiny sun. When enough electrons are spinning in the same direction they turn the atom into a tiny magnet. When these tiny magnets line up inside a metal such as iron, they create a powerful magnetic field. This is the invisible field that can attract or repel other objects.

Magnets can be used to attract or pick up things made of magnetic materials, such as steel. That’s why some cutlery attracts to magnets or you can make a steel object move with a magnet under a table.

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Trebuchet

A trebuchet was a weapon used for sieges in the Middle Ages. It was useful because it could fling objects large distances with lots of power.

It has two arms, one long arm for throwing and one short arm acting as a counterweight. The counterweight must be heavier than the object being thrown. When the machine is set off, the counterweight falls to the ground, pulled by gravity. This rotates the arms. When the throwing arm accelerates to the right speed and gets to the right angle, the projectile gets released. So the trebuchet is really just powered by gravity.

The reason it can throw things long distances is also down to the fact that the two arms are different lengths. The counterweight arm travels a much shorter distance than the throwing arm, giving a greater turning effect. (It’s the same as using a spanner with a longer handle to undo a nut. The long handle means that you use the same amount of force to undo a tighter nut.) In the trebuchet the greater turning effect of the longer arm throws the projectile whichever way the trebuchet is pointing.

Dynamo

In The Energy Show you may have seen a dynamo on the bike in the lab. It is used to generate electricity, and you can think of it as the opposite of an electric motor. With a dynamo you generate electrical energy from kinetic energy. Kinetic energy is the energy of motion. This means that electricity is generated from movement. You see dynamos being used in everyday life in clockwork radios or hand-powered torches.

Dynamos work by coils of metal wire being turned within a magnetic field. The magnetic field is the area around a magnet where some materials experience a force. The magnetic field and motion of the wire make the negative electrons in the metal move – so there’s an electric current in the wire. Magnetism and electricity are very closely linked.

A British scientist called Michael Faraday made the first electric generator. Although it wasn’t exactly the same as a dynamo, it was quite similar and the dynamos we have today are based on his initial work.

Image © iStock

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Classroom activities

On the Science Museum website you’ll find a number of free-to-download classroom activities that you can use with pupils to further explore the science portrayed in The Energy Show.

Try the following:

Magnetic Pennies (www.sciencemuseum.org.uk/magneticpennies)

Blow-Up Balloon (www.sciencemuseum.org.uk/kitchenscience)

Alka Seltzer Rocket (www.sciencemuseum.org.uk/kitchenscience)

Mousetrap Drag Racer (www.sciencemuseum.org.uk/mousetrapdragracer)

Burning Money (www.sciencemuseum.org.uk/burningmoney)

For other easy-to-do activities visit: www.sciencemuseum.org.uk/classroomresources

Rocket launcher

In the show, plastic bottles are fired across the room using hydrogen. For this to work the plastic bottles are half filled with water and half filled with hydrogen gas. Remember from earlier that the hydrogen gas is flammable? When the bottle is tipped up and the cap opened the water flows out of the bottle because of gravity. In its place air rushes into the bottle. This gives a mixture of hydrogen and air inside the bottle. The hydrogen doesn’t come out of the bottle because it is lighter than air and so rises to the top of the bottle.

We saw from the experiment with the hydrogen balloons that if you have the right amount of oxygen mixed with hydrogen and you ignite it then you have a very fast release of heat energy. In this case the oxygen is provided by the air that has rushed into the bottle. When the hydrogen is ignited the fast release of heat energy causes the surrounding air to expand or spread out suddenly. The expanding air has nowhere to go except out of the bottle.

A British scientist called Isaac Newton told us that if a force pushes in one direction you get an equal force in the opposite direction. You see this when you are swimming – you push the water backwards, which drives you forwards. In our experiment the direction of the force is backwards away from the bottle. The result is that the bottle is projected forwards into the audience.