Which feature of electromagnets makes them more useful than permanent magnets in many modern technologies?
A. Electromagnets are not dependent on a power supply.
B. The strength of the magnetic field is more consistent in
electromagnets.
C. Electromagnets cannot be turned on and off.
D. The current can be adjusted to control the strength of the
magnetic field.

Acommercial refrigerator with refrigerant-134a as the working fluid is used to keep the refrigerated space at -35°c by rejecting waste heat to cooling water that enters the condenser at 18°c at a rate of 0.25 kg/s and leaves at 26°c. the refrigerant enters the condenser at 1.2 mpa and 50°c. if the compressor consumes 3.3 kw of power, determine (a) the mass flow rate of the refrigerant, (b) the refrigeration load, (c) the cop, and (d) the minimum power input to the compressor for the same refrigeration load.