exam2 physics pt. 3

What happens when work is done on a system?

The temperature of the system increases

How can an object’s temperature be increased?

By heating or doing work on it

What is heat?

Another form of energy

What is the unit conversion for calories and joules?

1 calorie = 4.19 Joules = heat needed to raise 1g of water by 1°C

What is the formula for heat transfer related to temperature change?

Q=mcΔT

What is the formula for heat transfer related to phase change?

Q=mL

What are the two effects of heating a substance?

Increase in temperature or change in phase (solid, liquid, gas)

Which causes more severe burns, water at 100°C or steam at 100°C?

Steam, because it releases additional latent heat when condensing

What happens when a gas is compressed in a cylinder with a movable piston?

Work is done on the gas, increasing its energy

What is the equation for work done by a gas in a piston?

W=PΔV

What happens when a gas expands?

It does positive work on the surroundings, decreasing internal energy

What is the First Law of Thermodynamics?

ΔU=Q−W

If a gas gains 1200J of heat and does 800J of work, what is the change in internal energy?

ΔU=400J (1200J - 800J)

What is an ideal gas?

A gas where intermolecular forces are negligible

What is the relationship between temperature and kinetic energy in an ideal gas?

Absolute temperature is directly proportional to the average kinetic energy

What is the ideal gas law?

PV=NkB​T or PV=nRT

What is the value of Boltzmann’s constant (kB​)?

1.34×10−23 J/K

What is the value of the gas constant (R)?

8.31 J/(mol·K)

What does Boyle’s Law state?

PV= constant (at constant temperature)

What happens to gas volume when pressure increases, according to Boyle’s Law?

The volume decreases

How does doubling temperature and halving volume affect pressure in an ideal gas?

Pressure increases by a factor of 4

What is an isothermal process?

A process where temperature remains constant (ΔU=0, so Q=W)

What is an isobaric process?

A process at constant pressure

What is the energy equation for an isobaric process?

Q=ΔU+W

What is an adiabatic process?

A process with no heat exchange (Q=0)

What happens to temperature during adiabatic expansion?

Temperature decreases as work is done by the gas

What are the three main features of a heat engine?

1) Cyclic process, 2) Thermal energy input, 3) Partial conversion to work.

What is a steam engine?

A heat engine where steam pressure moves a piston

What is a gasoline engine?

A heat engine where fuel combustion expands gases to push a piston

What happens to waste heat in a heat engine?

It is released into the environment at a lower temperature

Isobaric process

Process at constant pressure

In an isobaric process, if volume increases what happens to temperature?

Temperature must increase

Adiabatic process

Process where no heat enters or leaves the system (Q = 0)

What happens when gas expands adiabatically?

Temperature decreases

What happens when gas is compressed adiabatically?

Temperature increases

Isothermal process

Process at constant temperature

For isothermal process, what's true about internal energy?

Internal energy doesn't change (ΔU = 0)

For isothermal process, relationship between heat and work

Q = W

Engine efficiency formula

e = W/Qh (Work output divided by heat input)

Carnot engine

Ideal theoretical engine with maximum possible efficiency

Carnot efficiency formula

e = (Th - Tc)/Th

What determines Carnot efficiency?

Only the temperature difference between hot and cold reservoirs

Maximum theoretical efficiency possible

100% (only if cold reservoir is at absolute zero)

First Law of Thermodynamics simplified

Energy cannot be created or destroyed

Second Law of Thermodynamics (version 1)

Heat engines can't convert all heat into work

Second Law of Thermodynamics (version 2)

Heat naturally flows from hot to cold, not the reverse

What are the four steps in Carnot cycle?

1) Isothermal expansion 2) Adiabatic expansion 3) Isothermal compression 4) Adiabatic compression

High-grade heat

Heat at high temperatures (500°C+), more useful for work

Low-grade heat

Heat at low temperatures, less useful for work

Perpetual motion machine of the first kind

Machine that creates energy (violates First Law)

Perpetual motion machine of the second kind

Machine that converts all heat to work (violates Second Law)

Why can't we achieve 100% efficiency?

Some heat must always be rejected to a cold reservoir

Why do brazil nuts rise to top when shaken?

Small nuts more easily fall beneath large nuts

How does sweating cool you down?

Hot water molecules evaporate, taking heat with them

Why doesn't sweating work in humid weather?

Water in air condenses on skin as fast as sweat evaporates

How does a dippy bird work?

Evaporation creates pressure difference that makes it move

What connects brazil nuts, sweating, and dippy birds?

Systems naturally move toward more probable states

Example: If hot reservoir is 400°C and cold is 120°C, what's the Carnot efficiency?

41.6%

For steam turbine with 41.6% efficiency taking in 500 kJ, max work output?

208 kJ

What happens as energy spreads more evenly?

Becomes less usable (entropy increases)

Why is temperature difference important for engines?

Bigger difference = higher possible efficiency

Can waste heat be fully recaptured and reused?

No, violates Second Law

What makes co-generation less efficient with each stage?

Each stage operates at lower temperature differences