Specific heat capacity of a substance
The amount of energy required to change the
temperature of a 1 kg of a substance by 1 K
Why is the E required to increase the T of 1 mol of Al and Co by the same amount about the same when they have very different cs?
One mole of any substance contains the same number of molecules; to raise the
temperature by 1 K the internal energy will increase by the same amount and so
the same heat must be provided
But one kg of different substances contains different numbers of molecules and
so different amounts of energy are required to increase the temperature by 1 K
How can it be deduced that there must be air resistance F acting on the object? (from a graph)
The graph is a curve
- If there was no air resistance the acceleration would have been constant and
the velocity – time graph a straight line
What is meant by the internal energy of…
The sum of the random kinetic energy of the molecules and the total intermolecular potential energy of the molecules
Eint = Ek + Ep
Explain the origin of intermolecular potential E in a solid
There are forces between molecules in a solid
- Intermolecular potential energy is the (negative) work done by these forces as the separation of the molecules increases
Why is the intermolecular potential E of an ideal gas = 0
There are no forces between the molecules in an ideal gas and so no work is involved in increasing their separation
A student claims that the K temp. of a body is a measure of its internal E. Explain
why this statement is not correct by reference to a solid melting.
Energy must be provided to a solid for it to melt, thus increasing the internal energy
– But the temperature of the solid stays constant during melting
So statement is not correct.
Power-heat quantity-time relationship formulas
P = Q/t
meaning: P = mc*(T/t)
Thermal E (heat) vs temperature
- Heat is the total amount of thermal energy that is transferred between substances due to a difference in temperature; Q = heat (J); depends on mass
- Temperature is a measure of the average kinetic energy (motion) of the particles in a substance; does NOT depend on mass
- Temperature is proportional to the average kinetic energy of molecules.
How is thermal energy transfered
- conduction: transfer E without any bulk movement of particles (Transfer Ek): electrons transfer energy to colder (slower) parts
- convection: with bulk movement of particles (transfer due to variations of density); more common in liquids and gases
- radiation: transfer by electromagnetic radiation (light); works in a vacuum
Apparent brightness
- how a star appears from/on Earth
Luminosity
- the power emitted from star (W or J/sec)
The internal E of…(chocolate)… during the melting process
- The internal E is increasing since E is being provided. The added E goes into increasing the intermolecular potential E. None of the E gets transferred into the random Ek of the molecules since the temperature stays constant.
Why would a piece of chocolate at its melting point held in hand melt?
The hand is at a higher temp. than the chocolate and so heat will flow into the chocolate by the mechanism of conduction.
Black vs. white chocolate under a heating lamp
The dark chocolate absorbs more of the radiant E from the lamp because it is a better approximation to a black body than the white chocolate, so it will melt faster.
Average speed of molecules
- smaller molecules will have a greater average speed
When a liquid boils, its temperature does not increase. Why?
- Heat E is given to increase the gravitational potential E of the molecules but not their kinetic E
