Lecture 6 - 2nd Law Introduction Flashcards
The second law of thermodynamics helps us know …
the direction the energy is flowing in a process and the efficiency of the process
If you throw a ball, you are doing work by …
changing the potential energy to kinetic energy
If you throw a ball and the temperature of the ball doesn’t change, is there a change in internal energy?
No
Work done ON a system is positive/negative
Negative
Work done throwing a ball raises the kinetic energy of the ball. So, why does the ball slow down after throwing?
Due to air drag reducing the kinetic energy of the ball
In the first law of thermodynamics, Q is …
heat transfer
Energy equation for an incompressible stationary fluid
change in internal energy of the fluid (specific heat * density * change in temperature with respect to time) = thermal conduction within fluid + viscous dissipation (shear velocity)
Heat diffusion equation
(density*specific volume * change in temperature with respect to time)/k = d^2T/dx^2 + d^2T/dy^2 + d^2T/dz^2 + q/k
Drag force causes a fluid to churn, contributing to the viscous dissipation, which results in a temperature change from a change in thermal energy.
True
An increase in the random kinetic energy of a system does not ensure that work will be done.
True
Direction of energy transfer is determined by the equation of the first law.
FALSE ; the second law tells us the direction of energy flow
Q-W = ∆U + ∆KE + ∆PE does not influence the direction of energy flow in the real world
The first law shows us that work can easily be converted into other forms of energy (KE, PE, Q, ∆U).
True
What is a heat engine?
A cyclical machine that receives heat from a high temperature source and converts part of the heat into work and rejects the other half of the heat to a heat sink.
Describe a heat engine.
HEAT SOURCE (Temp. high)
↓ Q(in) = Q(high) ** Q(high) = high heat
HEAT ENGINE → W (net, out)
↓ Q(out) = Q(low) ** Q(low) = low heat
SINK (Temp. low)
Describe the Rankine Cycle.
↓Q(in)
BOILER (Caldera) → TURBINE (Turbina) → W(out)
↑ ↓
W(in) → PUMP (Bomba) ← CONDENSER (Condensador)
↓ Q(out; rejected)
Boiler
Turbine
Condenser
Pump
Caldera
Turbina
Condensador
Bomba
The Rankine Cycle is an example of a heat engine.
True
How do we determine the net work out of a heat engine?
W(net, out) = W(out) - W(in) = Q(in) - Q(out)
Symbol for thermal efficiency
ɳ
Thermal efficiency, ɳ =
ɳ = net work out/total heat in
ɳ = W(net, out)/Q(total, in)
ɳ = 1 - Q(out)/Q(in)
If net work out = Q(in) - Q(out), we can write the thermal efficiency, ɳ as …
ɳ = [Q(in) - Q(out)]/Q(in, total) = 1 - Q(out)/Q(in)
In terms of a heat engine, thermal efficiency, ɳ, can be written as …
ɳ = 1 - Q(low)/Q(high)
Thermal efficiency is bounded between …
0 & 1
0 < ɳ < 1
What is the Kelvin - Planck statement?
A statement that says that it is impossible for a device that operates on a cycle to receive heat from a single source (reservoir) and produce a net amount of work.
