Bulk properties of solids Density Hooke’s law, elastic limit, experimental investigations. F = kΔL Tensile strain and tensile stress. Elastic strain energy, breaking stress. Derivation of energy stored =1/2FΔL Description of plastic behaviour, fracture and brittleness; interpretation of simple stress-strain curves. The Young modulus = tensile stress tensile strain One simple method of measurement. Use of stress-strain graphs to find the Young modulus.

Density ρ Density is a measure of mass is in a given volume. A kilogram of wood occupies more volume than a ton of gold. The density of a material = its mass ÷ the volume it occupies

Measuring density Finding the mass is generally easily done using a balance. Finding the volume of a regular solid is easy: This is the equation you need to find the density of a wire V = l x w x h r h

Volume of an irregular solid Often this is done by displacement Take care with units: Sometimes density is quoted in gcm-3 More correctly the SI unit of kgm-3 is used The rise in volume due to the displaced water is the volume of the irregular object

Finding The Density Of A Wire Measure the diameter of the wire with a micrometer Check the wire. The cross section may be elliptical. Record Your results and take the average value. If the difference in values for the cross section is greater than the uncertainty of the micrometer use this difference as your estimated uncertainty. Estimated uncertainty x 100 Measured Value Calculate your percentage uncertainty = Your percentage uncertainty on the radius of the wire is the same as your percentage uncertainty on the diameter of the wire.

Finding The Density Of A Wire Measure the length of the wire with a meter rule Measure it in at least two places on the rule and take the average value. Record your results. What is your percentage uncertainty? Apply your formula When quantities are multiplied together the percentage uncertainties for each term are added. Here the total uncertainty on the volume is 2 x % uncertainty of r + % uncertainty of h

Finding The Density Of A Wire Measure the mass of the wire with a top pan balance Here the estimated uncertainty is +or - 0.01g 0.56 Again calculate the percentage uncertainty Now apply the density formula The total percentage uncertainty is given by adding the percentage uncertainty in the mass and in the volume

Questions What is the density of a rock if 200g occupies a volume of 40cm3? A wire of regular cross section has a mass of 3.2g per metre and a diameter of 0.6 mm. Calculate the density of the metal. The density of gold is 19.3 gcm-3. What is it in kgm-3?

1. 2 The wire forms a cylinder of radius 0.3mm and length 1 metre 2.8 x 10-7m3 3 19300 kgm-3