A power transistor for which Tjmax=180degrees Celsius can dissipate 50Watts at a case temperature of 50degrees. If it is connected to a heat sink using an insulating washer for which the thermal resistance is .6degrees Celsius/Watt, what heat sink temperature is necessary to ensure safe operation at 30Watts? For an ambient temperature of 39degrees Celsius, what heat sink thermal resistance is required? If for a particular extruded aluminum finned heat sink, the thermal resistance in still are is 4.5degrees Celsius/Watt per centimeter of length, how long a heat sink is needed?