Efficiency When energy transfers and transforms, some of that energy dissipates into forms that were not intended. For example, in a car, the chemical energy of the burning fuel is eventually transformed into the mechanical energy of the moving car. However, not all of the energy transforms into movement—a large portion of the energy is transformed into thermal and sound energy released into the environment.
How effectively a device, such as the car, transfers and transforms the total energy into useful energy is called efficiency. When people talk about how many miles per gallon their cars get, they are talking about efficiency.
You can modify the efficiency of an entire system by changing its individual components. For instance, tires alone can increase or decrease the efficiency of the car system by changing the amount of energy transferred between the tires and the road.
No energy transfer or transformation is 100 percent efficient. Although you can improve the design and operation of a device or action, there will always be energy transferred to the surroundings, or transformed to a form we are not going to use.
We are really lucky that the energy transfers and transformations from the ocean bottom that produce tsunamis are very inefficient. Less than 10 percent of the energy from moving tectonic plates is actually transferred to the water above. Most of the energy is transferred and transformed into Earth’s crust and interior as thermal energy and earthquake waves, sometimes affecting locations hundreds of miles away.
Friction
There are many forces in nature that will decrease the efficiency of a device, and one of those forces is friction. Friction refers to any force that resists the relative motion of an object.
Friction is not always a bad thing. The friction between a car’s tires and the road allows the tires to grip the road and provides the means for the car to accelerate. Another use of friction occurs when you apply the brakes on a bicycle. Applying the brakes increases the friction on the wheels. This increase in friction causes the bike to quickly transform its mechanical energy into thermal energy, resulting in a decrease in speed of the bicycle.
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| Cars are generally not very efficient. Gasoline contains a significant amount of potential energy. When it is burned within a car’s engine, this stored energy is transformed into thermal energy. This released thermal energy expands air, which moves a piston, which turns a crank, which eventually is responsible for the spin of the car’s wheels.
During each step of this process, some energy is transformed into unwanted thermal energy. A bit is even transformed into the sound of a revving engine! In many car engines, only about 15 percent of the energy stored in gasoline is transformed into the movement of the vehicle. The remaining 85 percent is transformed mostly into thermal energy that does not contribute to the movement of the car. |
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Electrical Efficiency
Energy losses also accompany the flow of electrical current. You can sometimes detect this unwanted transformation to thermal energy by touching the side of your television set after it has been on for a few minutes; it may feel warm to the touch. At higher voltages, however, less electrical energy is transformed into thermal energy. That is why power transmission lines that distribute huge amounts of electricity are often energized to over 100,000 volts!
A lit incandescent bulb is extremely hot. The electricity that energizes the bulb is not transformed only into light. When objects such as the bulb’s filament heat up, they emit electromagnetic energy. Unwanted thermal energy makes this a very inefficient use of electrical energy. These days, however, there are alternative types of lighting devices. Compact fluorescent lamps (CFLs) and light-emitting diodes (LEDs) are more efficient options that transform a greater percentage of electrical energy into electromagnetic energy. Therefore, these lamps remain much cooler as they generate an equivalent amount of light.
| Math Connection |
| Efficiency is normally calculated as a percentage. Efficiency = useful energy transferred / total energy used x 100 For example, a 60-watt incandescent light bulb uses 60 watts of total energy and gives off 9 watts of useful light energy. Efficiency = 9 watts / 60 watts x 100 = 15% A compact fluorescent light bulb only uses 11 watts of total energy to produce 9 watts of useful energy, making it more than 5 times more efficient. Efficiency = 9 watts / 11 watts x 100 = 82% |  |