Power
Define 'Power'
Power is the rate at which work is done.
Define 'Efficiency'
Efficiency is how well something uses energy - it tells us how much of the energy going into a system is actually used for its job, instead of being wasted.
What is thermal energy?
Thermal energy is the heat energy that comes from the movement of tiny particles in a substance.
According to the Principle of Heat Exchange how does thermal energy transfer between two substances?
From the hotter substance to the colder substance.
What is the formula for power?
P=W/t or P=E/t
What is the formula for efficiency?
efficiency=output power/input power x 100%
or
efficiency=output energy/input energy x 100%
What is the formula for thermal energy?
Eth=mcT
Cold noodles are added to hot pasta sauce, causing the sauce to lose 190,755 J of thermal energy. How much thermal energy is absorbed by the noodles? Assume an isolated system.
190755 J
A hair dryer does 18,000 J of work in 60 seconds. What is the power rating of the hair dryer?
300 W
Why can't a system be more than 100% efficient?
Because a system cannot output more energy than it takes in (you cannot create energy according to the Law of Conservation of Energy).
A 2.0 kg sample of water is heated from 18°C to 65°C. The specific heat capacity of water is 4186 J/kg°C. How much thermal energy is absorbed by the water?
393484 J
A piece of hot copper at 150°C is placed into 0.400 kg of oil at 30°C. The final temperature of the system is 40°C. If the specific heat capacity of copper is 385 J/kg°C, and the specific heat of oil is 2000 J/kg°C, what is the mass of the copper?
0.19 kg
A microwave uses 300,000 J of energy to heat food over a period of 5 minutes. What is the power of the microwave?
1000 W
A motor receives 5000 J of electrical energy and performs 4200 J of useful work. What is the efficiency of the motor?
84%
A substance absorbs 52,250 J of thermal energy as it warms from 22°C to 37°C. Its specific heat capacity is 900 J/kg°C. What is the mass of the substance?
3.9 kg
DAILY DOUBLE
A 0.3 kg block of unknown metal at 180°C is placed into 0.5 kg of ethanol at 25°C. The final temperature is 35°C. What is the specific heat capacity of the metal? (Specific heat capacity of ethanol = 2440 J/kg°C)
280.5 J/kg°C
A student lifts a 15 kg backpack from the ground to a shelf 2 meters high in 4 seconds. What is the power output of the student during this lift?
73.5 W
DAILY DOUBLE
A machine is 75% efficient and uses 2000 J of energy in 10 seconds. How much work does the machine do in this time?
15000 N
A 1.5 kg piece of metal absorbs 9,000 J of energy. Its temperature rises from 25°C to an unknown final temperature. The specific heat capacity of the metal is 500 J/kg°C. What is the final temperature of the metal?
37°C
A 0.25 kg block of lead (c = 130 J/kg°C) at 100°C is dropped into 0.5 kg of milk (c = 3900 J/kg°C) at 20°C. What is the final temperature of the system?
21.6°C