1. 7.3 Efficiency

2. Efficiency Every process that is done can be simplified in terms of work Work input: the work or energy supplied to the process Work output: the work or energy that comes out of the process

3. Efficiency A rope and pulley machine illustrates a rule that is true for all processes that transform energy. A rope and pulley machine illustrates a rule that is true for all processes that transform energy. The total energy or work output can never be greater than the total energy or work input. The total energy or work output can never be greater than the total energy or work input.

4. Efficiency 65% of the energy in gasoline is converted to heat. 65% of the energy in gasoline is converted to heat. As far as moving the car goes, this heat energy is “lost”. As far as moving the car goes, this heat energy is “lost”. The energy doesn’t vanish, it just does not appear as useful output work. The energy doesn’t vanish, it just does not appear as useful output work.

5. Efficiency % Efficiency = work output work input work input *Smaller # for work will always go on top!! x 100

6. Calculating efficiency What is the efficiency of this rope and pulley machine? Input = 11 J Output = 10 J

7. Example Problem A car uses 30,000 J of energy to make 20,000 J of kinetic energy. What is the efficiency? A car uses 30,000 J of energy to make 20,000 J of kinetic energy. What is the efficiency?

8. Example Problem A cell phone charger uses 2.36 J of energy but the battery only gets 0.8 J of this energy. What is the efficiency of the charger? A cell phone charger uses 2.36 J of energy but the battery only gets 0.8 J of this energy. What is the efficiency of the charger?

9. Reminder… A “perfect” machine would have an efficiency of 100%. *Since all REAL machines have friction, perfect machines are technically impossible.

10. 7.3 Power

11. Power The rate at which work is done is called power. The rate at which work is done is called power. When we talk about power, it makes a difference how fast you do work. When we talk about power, it makes a difference how fast you do work. 1 watt = 1 J/s 1 watt = 1 J/s

12. Example Problems A motor does 2,000 J of work in 10 seconds. What is the power? A motor does 2,000 J of work in 10 seconds. What is the power? A microwave does 15,500 J of work in 20 seconds. What is the power? A microwave does 15,500 J of work in 20 seconds. What is the power?

13. Work and Power Example: 1. Who does more work – Jim or Michael? 2. Who has more power – Jim or Michael?

14. Work and Power Exit Question There is a 5m set of stairs down the hall. Sally (weight of 50N) runs up the stairs in 8 seconds, and Henry (weight of 50N) runs up the stairs in 4 seconds. There is a 5m set of stairs down the hall. Sally (weight of 50N) runs up the stairs in 8 seconds, and Henry (weight of 50N) runs up the stairs in 4 seconds. –Who does more work? –Who has/uses more power?