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.
Below are four ideas from the collection. I hope that you will find at least one of them new, challenging, informative and fun, and that the ideas go some way towards popularising the subject and making people realise that physics can be interesting and fun.
Age range: 13-15
This simple experiment demonstrates that the saturated vapour pressure of water depends on the temperature. It is best performed as a teacher demonstration, with a safety screen between the apparatus and the students.
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)!
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.
The explanation is that the saturated vapour pressure of water depends on the temperature: the lower the temperature, the less water vapour the air can hold (see Table 1). When the water condenses, it lowers the pressure in the flask – and this, of course, allows water to boil at less than 100 °C.
Saturated vapour pressure
|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|
A simpler method is to partly fill (about 20%) a syringe with 50-60°C warm water. Then pull on the plunger of the syringe. This lowers the pressure in the syringe, causing the water to boil at well below 100 °C.
Age range: 14-18
This is a simple demonstration of centripetal force.
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.
Age range: 16-18
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.
The AC electromagnet induces eddy currents within the aluminium scraps. These turn the scraps into tiny electromagnets that are then repelled by the large electromagnet and so fly off the card. With non-metallic scraps there are no induced currents and so these scraps remain on the card.
In a moving-belt version of this experiment, mixed metal and non-metal scraps are passed along a belt over an AC electromagnet. This induces eddy currents in the metal scraps, which are then repelled by the field and fly off sideways while the remaining non-metal scraps continue along the belt. Schools might be able to construct such a version for demonstration use, using a mixture of paper and aluminium.
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.
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.
This is a very useful demonstration of the equivalence of gravitational and inertial fields.
The editors of Science in School would like to thank Catherine Cutajar and Gerd Vogt for their help in selecting the experiments to include in this article.