Chapter 10 - Deformation Flashcards
Define tensile strain
Strain is the extension per unit original length
Define tensile stress
Stress is force per unit cross-sectional area
Define the Young modulus
Young modulus is the ratio of stress to strain
Explain what is meant by extend elastically
The object returns to its original shape when load is removed
Describe the meaning elastic limit
It is the point beyond which the spring will not return to its original length when load is removed
State what is meant by elastic deformation
- It is the change of shape of an object
2. When the force applied is removed, it returns to its original shape
State Hooke’s law
Force is proportions to its extension, provided the proportional limit is not exceeded
OR
Extension is proportional to force
Equation for strain energy
E(s) = 1/2 kx^2
Equation for stress
Stress = Force / Area
= F / A
Equation for strain
Strain = extension / original length
= ΔL / L
Equation for Young modulus
Young modulus = stress / strain
E = FL / Ae ; e = ΔL or extension
Equation for resistance of wire
R = ρL/A ; ρ = resistivity
Equation for Young modulus, relating to resistivity ρ and resistance R
E = FR / eρ
Equation for extension of spring
F = kx
Equation for work done, E
Work done, E = 1/2 kx^2
Springs in parallel & series
Parallel: k = k1 + k2
Series: 1/k = 1/k1 + 1/k2
Suggest why a thin rod can bend more than a thick rod without breaking
- When bent, the top and bottom edges have different extensions
- With thick rod, the difference between the extension lengths is greater than with a thin rod
- So, it breaks with less bending
Why the walls of tyres become warm, in accordance to the stress-strain graph of rubber?
- The area between the lines represents the lost energy when rubber is stretched and then released
- This energy is seen as thermal energy
Suggest how you could check that the elastic limit of the wire is not exceeded when the extra load is added
Remove the extra load and see if the wire returns to its original length
Describe apparatus for a Young modulus experiment
- Weights are used to straighten wire
- The horizontal wire is fixed to a clamped wooden
block over a pulley bearing the weights
Describe the method used to require the measurements
- Measure the original length of wire to the marker
with a metre rule - Measure diameter of wire with a micrometer
screw gauge to calculate the cross-sectional
area - Measure the diameter in several places and take
the average - Measure the initial and final reading for extension
of wire with a metre rule - Record the weights used for extension
- Take several readings with different load
Describe how the measurements taken can be used to determine the Young modulus
- Calculate the area from πd^2 / 4
- Determine extension from final and initial
readings of extension - Plot a graph of force against extension
- Calculate E from E = Fl / Ae
Why Young modulus is not used to describe liquids or gases?
Fluids have no fixed shape
