Ugo says: 'A force that does 12 J of work on a cart transfers 12 J of energy to the cart.' Is Ugo correct?
Ivo caps the end of a bike pump with his thumb and presses the handle in. The trapped air squashes smaller, and the pump barrel warms.
Energy entered the trapped air, yet no flame and no hot plate came anywhere near it. This lesson sorts energy transfers into two kinds you already have the pieces for: heat and work.
Earlier in the course, you saw that work is energy transferred by a force acting through a distance.
That is what the pump did: Ivo's force moves the piston in through a distance, so the energy entered the trapped air as work.
Now a different entry. A sealed can of air stands in a pan of hot water, and the canned air warms. The water is hotter than the can, and heat is energy transferred between objects because of a temperature difference.
No force moved the can's boundary through a distance. The energy entered the canned air as heat.
The system warming up does not decide the sort. The pumped air warmed with nothing hot anywhere near it. The driver decides: a temperature difference makes the transfer heat, and a force acting through a distance makes it work.
Delia spins a paddle inside a sealed barrel of water, and the water warms. Did the energy enter the water as heat or as work?
The paddle's force acts on the water through a distance as it turns.
No temperature difference drove the entry: the paddle was no hotter than the water.
The energy entered the water as work.
A sealed jar of salsa chills in a tub of ice water. Energy is leaving the salsa. Is the transfer heat or work?
The ice water is colder than the salsa: a temperature difference drives energy out.
No force moves any part of the jar's boundary through a distance.
The energy leaves the salsa as heat.
Emrys presses the plunger of a capped, sealed syringe of air halfway in. Did the energy enter the trapped air as heat or as work?
A sealed foil pouch of cooked rice sits on a warm counter, and the rice warms. Did the energy enter the rice as heat or as work?
A cold, sealed water bottle lies against the hot dashboard of a parked car. Did the energy enter the water as heat or as work?
Birta squeezes a sealed plastic bag of air between two boards, shrinking it. Did the energy enter the bag's air as heat or as work?
Rollers press a sealed, flexible juice pouch flatter as it rides a conveyor belt. Did the energy enter the juice as heat or as work?
A sealed glass jar of soup stands in a refrigerator, and energy is leaving the soup into the colder fridge air. Is the transfer heat or work?
Four sealed systems each gain energy. In which case does the energy enter as work?
A sealed system gains energy as work. Which feature of the transfer makes it work rather than heat?
A motor-driven paddle stirs the water in a sealed tub, and the water warms. Nothing touching the tub is hotter than the water. How did the energy enter the water?