A centrifuge rotor spins inside a casing. The power supply is disconnected and friction in the bearings causes the rotor to stop.
The rotor, bearings, air and casing are chosen as the system. Energy in all other stores of the system remains unchanged.
| Energy store | Energy when the power supply is disconnected / kJ\text{kJ}kJ | Energy when the rotor has stopped / kJ\text{kJ}kJ |
|---|---|---|
| kinetic energy store of the rotor | 12 | 0 |
| thermal energy stores of the rotor, bearings, air and casing | 188 | 200 |
Which statement is correct?
The system can be treated as closed; 12 kJ12\ \text{kJ}12 kJ is transferred from the kinetic energy store to the thermal energy stores.
The system cannot be treated as closed; its total energy increases by 12 kJ12\ \text{kJ}12 kJ because the thermal energy stores rise.
The system cannot be treated as closed; its total energy decreases by 12 kJ12\ \text{kJ}12 kJ because the rotor stops.
The system can be treated as closed; 12 kJ12\ \text{kJ}12 kJ is transferred from the thermal energy stores to the kinetic energy store.