PHYSICS 1 HONORS CHAPTER 8 – ENERGY SECTIONS 8.8 AND 8.9

EFFICIENCY We have assumed so far that all machines are 100% efficient or ideal machines. In reality we can never reach 100% efficiency. In any machine at minimum, energy is transferred to the atoms of the machine, increasing their molecular kinetic energy. We see the increased molecular kinetic energy as heat, because when we measure temperature we are indirectly measuring the average kinetic energy of the atoms. 2

EFFICIENCY When we use a lever, as we move the lever across the fulcrum, some of the energy we apply to the lever becomes heat in the fulcrum. Think of using a branch as a lever and a rock as the fulcrum to lift a heavy object. As the branch is moved across the rock, some of the energy you are applying to the branch is becoming heat in the rock. The energy that becomes heat thus cannot be used to do work moving an object, so the machine loses efficiency. If 5% of the energy becomes heat, that leaves 95% of your energy to do work and the machine is 95% efficient. 3

EFFICIENCY The lower the percent efficiency, the more energy you lose to heat. 4

EFFICIENCY We have seen that using an inclined plane to move an object to a height is easier that just lifting it straight up. If the inclined plane is 5 m long and elevates the object 1 m, we need to exert 1/5 of the force to get the object up the plane. The inclined plane has a mechanical advantage of 5, assuming little or no friction. This is the theoretical mechanical advantage. 5

EFFICIENCY If the object is a block of ice with very little friction, the efficiency should be near 100%. However is you are moving a large, heavy wooden box up a wooden ramp, the efficiency will be significantly less. We can also express efficiency in terms of mechanical advantage: 6

EFFICIENCY Efficiency as calculated be the two formulas given so far, will always yield a value < 1. It is also common to express these values as percent efficiency by multiplying the value obtained by

EFFICIENCY If you use a screw jack to raise a car, to change a tire, you are using an inclined plane (the screw threads) which is wrapped around a cylinder. If you move the handle and the distance the handle moves is 200 times greater than the pitch (angle) of the screw, you have a mechanical advantage of 200. This makes it relatively easy to raise a car off the ground. In reality, due to the weight of the car, the efficiency of this type of jack is reduced by friction to about 20%. 8

EFFICIENCY A car is about 30% efficient in changing the energy of gasoline into engine output. Gasoline burns in the engine, which means it breaks down chemically and combines with oxygen. This releases the energy stored in the bonds and forms carbon dioxide and water if the combustion is complete. Energy is lost to the exhaust in the form of heat, to the cooling water as heat and to friction. 9

EFFICIENCY 10 From text: Fuel Energy In (100%) = Cooling Water Loss (35%) + Engine Output (30%) + Exhaust Heat (35%) Note – Only 30% of the energy is used by the engine.

EFFICIENCY Remember energy is never destroyed, it ultimately becomes thermal energy. Thermal energy is the graveyard of useful energy. 11

ENERGY FOR LIFE Every cell in your body is a tiny machine. You use the energy in the food you eat to power that machine. When you digest food, there is less total energy in the products that was stored in the food. You use this “missing” energy to sustain your life. In your body the reaction rate of metabolism is slower than the reaction in a car engine. You also combine carbon and oxygen to form carbon dioxide. 12

ENERGY FOR LIFE The reverse process of storing energy is done only by certain green plants and algae. They take carbon dioxide and the energy from the sun to make compounds like sugars, which store energy. In the process, plants release oxygen. Life results from this energy cycle. Without it, none of us would exist. 13