Over one afternoon, a system releases more energy than it takes in. By the first law of thermodynamics, what happens to the system's energy?
Delphine's flashlight radiates light for an hour. Nothing drips out, nothing evaporates, the case stays sealed. Yet by the end, the flashlight's mass is very slightly smaller than when she switched it on.
You have run energy ledgers before. In the work and energy topic, the full ledger balanced: kinetic, potential and thermal energy, totalled before and after, came out equal. In the thermal physics topic, the first law ran the same ledger as energy in and energy out. One column was missing.
In any process, the total of all the energy forms, including the energy in mass, is the same before and after.
The energy an object carries in its mass is called its mass-energy.
Mass-energy is the ledger's last column; with it written in, the books always balance.
A system's mass can fall only if the equivalent energy leaves the system.
Delphine's flashlight radiated light away for an hour; the light carried energy out, so the flashlight's mass fell by exactly the equivalent amount.
The fall is real but tiny: the flashlight ends the hour very slightly less massive, and the ledger balances exactly.
No energy out means no mass lost: a perfectly sealed, perfectly insulated box keeps its mass exactly.
Delphine's flashlight radiates light away for an hour. What happens to the flashlight's mass?
Light carries energy out of the flashlight.
A system's mass can fall only if the equivalent energy leaves it, and here energy does leave.
So the flashlight's mass falls by the mass equivalent of the energy the light carried away.
The flashlight's mass is very slightly smaller, and the ledger balances: the mass-energy lost equals the energy radiated away.
A sealed, perfectly insulated flask holds hot coffee, and the coffee warms the flask walls. No energy leaves the flask. Does the flask's total mass change?
The coffee warming the walls only moves energy between columns inside the flask.
No energy leaves the sealed, insulated flask.
A system's mass falls only if the equivalent energy leaves, and none leaves.
No. Energy only changed column inside the flask, so the flask's total mass is unchanged.
A flashlight radiates light away for an hour. What happens to its mass?
Caspian says: 'With mass counted as a form of energy, conservation of energy holds with no exceptions.' Is Caspian correct?
What must happen for a sealed system's total mass to fall?
A camping lantern radiates light and heat to the night air until morning. Nothing else enters or leaves it. Which statement is true by morning?
A kettle of hot water cools on a counter, radiating energy to the kitchen. What happens to the kettle's total mass as it cools?
Delphine lists the ledger columns for a radiating beacon: kinetic energy, gravitational potential energy, thermal energy, and one more column that makes conservation of energy exact, with no exceptions. Which column is it?