Chapter Seven: Energy 7.1 Energy and Systems 7.2 Conservation of Energy 7.3 Energy Transformations

7.3 Conservation of Energy The idea that energy tranforms from one form into another without a change in the total amount is called the law of conservation of energy. The law of energy conservation says the total energy before the change equals the total energy after it.

7.3 Conservation of Energy When you throw a ball in the air, the energy transforms from kinetic to potential and then back to kinetic.

A 2 kg car moving with a speed of 2 m/sec starts up a hill. Solving Problems A 2 kg car moving with a speed of 2 m/sec starts up a hill. How high does the car roll before it stops?

Looking for: Given Relationships: Solving Problems …height of hill … mass = 2 kg, v = 2 m/s Relationships: Energy transformed from EK to EP EK = ½ mv2 EP =mgh

Solution Solving Problems Find beginning EK EK = ½ (2 kg) (2 m/s)2 = 4 Joules Assume energy before = energy after EK = EP EP =mgh 4 J = mgh h = (4 Nm)/(2 kg)(9.8 N/kg) = .2 m

7.3 Conservation of Energy Many people are concerned about “running out” of energy. What they worry about is running out of certain forms of energy that are easy to use, such as fossil fuels like oil and gas.

7.3 Conservation of Energy It took millions of years to accumulate these fuels because they are derived from decaying, ancient plants that obtained their energy from the Sun when they were alive. Because it took a long time for these plants to grow, decay, and become oil and gas, fossil fuels are a limited resource.

7.3 Conservation of Energy Regular (incandescent) light bulbs convert only 10% of electrical energy to light. That means 90% of the energy is released as wasted heat.

7.3 Conservation of Energy Other forms of energy, such as thermal energy, flowing water, wind, and solar energy are not as limited.