Expansion of a solid rod

Demonstration

An iron rod expands when it is heated.

Apparatus and Materials

• Iron rod, approximately 50 cm long, 5 mm in diameter
• Mass, ¹/ ₂ kg
• Mass, 1 kg
• Wooden blocks, 2
• Needle
• Drinking straw
• Bunsen burner
• Microscope slide
• Heat-proof mats to protect the bench
• Bimetallic strip (e.g. copper & iron), optional

Health & Safety and Technical Notes

This could be done as a class experiment if all members of the class can be trusted to place the burner flame on the rod, and not on other parts of the apparatus or each other.

Read our standard health & safety guidance

If the rod slips on the needle, hang a load on it near the needle. If the straw slips on the needle, run a short piece of fine copper wire through the eye of the needle, and wrap it round the straw.

Procedure

1. Rest the iron rod on the two supports as shown in the diagram. Prevent the rod from moving at one end by placing the kilogram mass on it. Let the other end roll on the needle resting on a small piece of glass (e.g. a microscope slide) placed on the block. Pierce the straw with the needle so that it acts as a pointer.
2. Heat the rod by moving a Bunsen flame along it. The straw will move with the rolling needle as the rod expands. The movement of the rod cannot be seen, but the magnification of this movement by the straw is impressive.

Teaching Notes

• Students can estimate, roughly, how much the rod has expanded by measuring the circumference of the needle. Note that the rod expands by twice the circumference of the needle when the straw rotates once, because the point of contact between the needle and the rod also moves across the glass. Do not complicate the explanation with students at this stage. An order of magnitude estimate is all that is needed.
• You can make an estimate of the rod's temperature by dropping a drop of water onto it. The ferocity of boiling gives some idea of how much higher than 100°C it is.
• Different metals expand by different amounts for the same temperature rise. You could show a bimetallic strip being heated. These are often made of copper and iron. Copper expands by about 1.5 times as much as iron and so the bimetallic strip must bend. Bimetallic strips are used in some heat-operated switches such as in an electric iron.

This experiment was safety-tested in March 2006