Physics teacher Keith Gibbs shares some of his many demonstrations and experiments for the physics classroom.

During more than 30 years of teaching physics, I have come across many interesting demonstrations and teaching ideas – often suggested by relatives, friends, colleagues and past students. In 2000, I began to gather these ideas together – this was the basis of the Schoolphysics website and CD-ROM collection. Over time, I added more explanation and background for teachers whose specialism was not physics.

1. Fit the rubber tube onto the end of the glass tube.


2. Fit the thermometer and glass tube into the holes in the bung, pour cold water into the flask until it is just less than half full. Then seal the flask with the bung.


3. Fit the clamp onto the rubber tube but do not close the clamp.


4. Use the Bunsen burner to heat the water to boiling.


5. Close the clamp and turn off the Bunsen burner.


6. Invert the flask and pour cold water over it.

Steam will condense inside the flask, reducing the pressure and allowing the water to start boiling again. When the water stops boiling, pour more water over the flask. How low can you get the temperature and still observe the water boiling? You should be able to get the water to boil at 40 °C – I once observed the water boiling at body temperature (37 °C)!

Safety note: wear safely goggles. Although unlikely, it is possible that the glass flask could shatter, so keep a safety screen between the experiments and the students. If possible, stand behind the screen yourself.

See also the general safety note.

37 °C

0.06 x 105 Pa

60 °C

0.19 x 105 Pa

75 °C

0.38 x 105 Pa

85 °C

0.57 x 105 Pa

100 °C

105 Pa

1. Bend the wire coat hanger until it forms a square.


2. File the tip of the hook flat and then bend the hook until it points towards the opposite corner of the square (see diagram).


3. Balance a small coin (try a UK one-penny piece or a 5 or 10 Euro-cent coin) on the hook.


4. Place one finger in the corner of the square opposite the hook and spin the coat hanger in a vertical circle. The coin should remain in place.

Theory

The force of the hook on the coin provides the centripetal force, and this always acts towards the centre of rotation.

How many coins can you balance on the swinging coat hanger? My record is five one-penny pieces. With only one penny and with great care, I have once even been able to bring the coat hanger to rest without the coin falling off.

This is a small-scale simulation of the type of electromagnetic separator that is used industrially to separate non-ferrous metals from other non-metallic scrap, and is suitable as a teacher demonstration.

Materials

• A U-shaped electromagnet with an iron core to give a high field intensity
• An AC (alternating current) power supply
• Aluminium scraps (e.g. kitchen foil)
• A piece of thin card
• Some scraps of paper

Procedure

1. Place the card on top of one arm of the electromagnet and put a few scraps of aluminium and paper onto the card.


2. Connect the electromagnet to an AC supply and turn on the current. The aluminium scraps will be ejected from the magnetic field.

A floating block in a falling jar

Age range: 11-18, depending on the treatment of the theory.

This is a very useful demonstration of one of the ideas of general relativity, using a wooden block floating in a jar of water that is suspended from a spring.

Materials

• A helical spring tied with string to a plastic jar
• A wooden block or straw loaded with modelling clay (e.g. Plasticine®)
• Water

Procedure

1. Half fill the jar with water, add the wooden block or straw, and suspend the apparatus from the spring.


2. Support the jar, then let it fall, suspended from the spring. The jar and contents will then oscillate in a vertical plane (up and down) but the water level will stay at the same position in the jar, and the block or straw will float at the same level in the water as it falls and rises.

The depth at which the wooden block or straw floats depends on both its weight (not its mass) and the upthrust on it. The upthrust depends on the weight of water displaced. Thus, as the acceleration of the jar and the block changes, the weight of the block and the upthrust on it change in direct proportion to each other; as a result, the depth at which the block floats remains unchanged as the apparatus oscillates.

Objects undergoing acceleration behave in the same way as they would in a gravitational field. As the jar and its contents oscillate, they have an acceleration that is due to both the constant gravitational field of Earth and the simple harmonic motion of the oscillation.

As the jar moves upwards, its net acceleration is greater than that of Earth’s gravitational field and as it falls, its acceleration is less than that of Earth’s field. On the downward part of the motion, it is as if the jar were on the Moon, where the gravitational acceleration is less than on Earth.