Teaching Resources

Demonstrations from Physics on Stage 2

Below is a summarized version of all the demonstrations. A complete description of the Physics on Stage 2 demonstrations (Word 1 MB) including where to get the materials, is available.

Readers are reminded that before they carry out any of the demonstrations they should be mindful of their local safety regulations. Whilst these demonstrations were carried out safely at Physics on Stage all experimental activities have an inherent risk which should not be underestimated, and the contributors cannot be held responsibe for any accidents. Be careful and ENJOY!

The blow out
Blowing through a funnel with a candle in front of it makes the candle blow towards the blower! Shows how low pressure is formed.

And, if you have access to a bell-jar connected to a vacuum pump:

• Put a partially inflated balloon in a vacuum chamber and it will inflate
• Try reducing the pressure around a marshmallow
• The famous Magdeburg hemispheres demonstration shows that the atmosphere exerts a great force. When the air between two perfectly fitting hemispheres is removed they cannot be pulled apart. To separate the hemispheres you need to let air back in between them, or you can put the sealed hemispheres into a bell jar and switch on the vacuum pump.
• Following on from this - a rubber sucker uses atmospheric pressure to hold on. Suckers inside a bell jar fall off when the pressure is reduced.

Propulsion Lab

Blow up a balloon, don't tie it, and let go - it's a rocket.

• Blow up a balloon and fasten to a hovercraft made from a cotton reel stuck on a CD.
• Hang two inflated balloons on strings close together but not touching and blow between them. They don't blow apart, as you would expect, they move together. This illustrates Bernoulli's principle.
• Connect 2 balloons to a T piece with taps on. Blow one up more than the other and then open the taps connecting the 2 balloons. What happens? (this can also be done with bubbles blown on pipes)

Exploding Trash

Blu-tak an alka seltzer or vitamin C tablet to the lid of a 35 mm film canister. Put a little water in the cannister, put the lid on and quickly place it lid-side down on the floor and stand back!

The flying teabag

Cut the top off a teabag, empty it and put it open side downwards. Light at the top and stand back.

Weightlessness simulator

Many toy shops sell tubes with whistles inside. When you hold the tube vertically, the whistle falls within the tube and makes a noise. If, while it is whistling, you drop the tube, the noise stops. When you catch it again the noise returns. Why? When you drop the tube the whistle and tube fall, together, in free fall, so the whistle does not push through the air in the tube. It is as 'weightless' as an orbiting astronaut.

The Egg and Sheet Airbag

Get two students to hold up a large bed sheet between them so it is vertical, and the bottom curls into a J-shape. Get another student to hurl an egg as hard as possible at the middle of the sheet. The sheet gives, the egg drops into the curl at the bottom and is unbroken no matter how hard they try.

Circular Jets

Bend the stalk of a sparkler through 90 degrees. Put the short end in the chuck of a hand drill and light the sparkler. Drop the lights and turn the drill - the sparks from the sparkler shoot off in straight lines at tangents to the circle.

More Balloons

• Use a large balloon bounced around the room by the audience to illustrate Brownian motion.
• Use a balloon with pictures of galaxies stuck on it. (Use a drop of flexible glue). As you inflate it the galaxies move apart - a bit like the expanding Universe.
• Balloons can replace water for a dry, life-size demonstration of Archimedes' bath.
• Have you ever noticed that wet sand becomes dry-looking when you walk on it? Put some sand and a little water in a balloon. Fasten a glass tube securely into the neck of the balloon and 1/2 fill it with coloured water. When you squeeze the balloon the water falls in the tube. As you increase pressure on sand particles they let more water between them.
• Seal a balloon on to the head of a soda siphon to show cooling by expansion. It can go well below freezing.

The 'reaction timer'

Get a volunteer and tell them that they have to catch the magnet when it drops out of the tube. Drop a powerful magnet down a copper tube- the magnet is massively slowed by the induced currents. The volunteer won't know this…!

Home Entertainment Centre

Making a sound to light show. Take both ends off a baked bean can, fasten a piece of stretched balloon over one end and stick a small piece of mirror to it. Fasten a laser pointer to one end of a fairly stiff strip of metal and fasten the other end to the end of the can with the balloon on and bend the metal until the laser is hitting the mirror. Sing a loud note into the other end of the can and the Lissajous figure will be displayed on the wall opposite you.

Making Sparks

A piezo-electric lighter generates sparks which can be heard on a radio tuned to any frequency.

Loud and Clear

Connect two loudspeakers together to make an intercom. Put some sand in one cone and tap the other - see the sand move.

Bar Physics!

• Use a fizzy drink (beer works well) poured into a pint glass. Measure the thickness of the head against time. It is a roughly exponential decay
• Atmospheric pressure pushed the beer back in your pint! Pour some drink in an ashtray or similar shallow dish. Pile some nuts (or a candle) in the middle and stick a lighted match on top. Quickly cover with your pint glass. The match goes out and the beer moves into the inverted glass.
• Peanuts in your pint. Drop some peanuts in your beer, or raisins in your lemonade. The CO2 will stick to the peanut surface, raising them to the top. When the bubbles burst, the peanuts plummet.
• Changeable cork Put a cork in a glass - it floats to the edge, being the highest point. Now fill the glass until the water bulges at the top, held in only by surface tension. The cork will float to the centre - again the highest point.
• The escaping ping-pong ball Put two round wine glasses next to each other. Put a ping-pong ball in the nearer one and blow over the top. If you get it right the ball will lift up and pop into the next glass.

KITCHEN

Remember that kitchens can be dangerous places! Don't touch hot objects, don't let things get slippy or sticky. In particular, remember that microwaves can deceive, so leave things in the microwave after you have switched off. Let them cool a little before you take them out. They could be hotter than you think.

Sinking satsumas

A satsuma with skin traps air and it floats. Remove the skin and it sinks.

Microwaving light bulbs

Put an incandescent bulb in a glass of water in a microwave oven. Now try a fluorescent tube - hold it under water by blu-tacking it to the bottom of a tall glass.

Measuring the Speed of light with Cheese

Take the turntable out of your microwave. Cover a dish in cheese slices (or marshmallows if you prefer). Pop it in the oven long enough to start melting the food. You'll see evidence of standing waves in the oven - melted hot spots, and solid cold spots. Measure the distance between the hot spots. Hot spot to hot spot is half the wavelength of the microwaves. The frequency of the waves is printed on the back of the oven. The speed of light is the frequency multiplied by the wavelength. A tastier improvement: do it with cheese on nachos and you can eat the evidence with less mess!

Cooking balloons

Put two balloons in a microwave: one with a bit of water in it, one with just air. See what happens when you switch on.

Scrambled eggs

Have you ever tried standing on a carton of eggs. If you are gentle and your shoes are flat your weight is spread over a big enough area to be supported by the eggs. If you get it wrong be prepared to clear up the mess!!

The impossible lift

Stand with your head against the wall. Lift a stool and try to stand up. It's impossible because your centre of gravity is displaced beyond your feet.

Water merge

Using a plastic bottle filled partly with water and with several small holes critically placed you can produce two streams that can be brought together to show how water molecules "hold on to" other water molecules.

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