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Another example of energy conversion occurs in a solar cell. Sunlight impinging on a solar cell (see [link] ) produces electricity, which in turn can be used to run an electric motor. Energy is converted from the primary source of solar energy into electrical energy and then into mechanical energy.
| Object/phenomenon | Energy in joules |
|---|---|
| Big Bang | |
| Energy released in a supernova | |
| Fusion of all the hydrogen in Earth’s oceans | |
| Annual world energy use | |
| Large fusion bomb (9 megaton) | |
| 1 kg hydrogen (fusion to helium) | |
| 1 kg uranium (nuclear fission) | |
| Hiroshima-size fission bomb (10 kiloton) | |
| 90,000-ton aircraft carrier at 30 knots | |
| 1 barrel crude oil | |
| 1 ton TNT | |
| 1 gallon of gasoline | |
| Daily home electricity use (developed countries) | |
| Daily adult food intake (recommended) | |
| 1000-kg car at 90 km/h | |
| 1 g fat (9.3 kcal) | |
| ATP hydrolysis reaction | |
| 1 g carbohydrate (4.1 kcal) | |
| 1 g protein (4.1 kcal) | |
| Tennis ball at 100 km/h | |
| Mosquito | |
| Single electron in a TV tube beam | |
| Energy to break one DNA strand |
Even though energy is conserved in an energy conversion process, the output of useful energy or work will be less than the energy input. The efficiency of an energy conversion process is defined as
[link] lists some efficiencies of mechanical devices and human activities. In a coal-fired power plant, for example, about 40% of the chemical energy in the coal becomes useful electrical energy. The other 60% transforms into other (perhaps less useful) energy forms, such as thermal energy, which is then released to the environment through combustion gases and cooling towers.
| Activity/device | Efficiency (%) Representative values |
|---|---|
| Cycling and climbing | 20 |
| Swimming, surface | 2 |
| Swimming, submerged | 4 |
| Shoveling | 3 |
| Weightlifting | 9 |
| Steam engine | 17 |
| Gasoline engine | 30 |
| Diesel engine | 35 |
| Nuclear power plant | 35 |
| Coal power plant | 42 |
| Electric motor | 98 |
| Compact fluorescent light | 20 |
| Gas heater (residential) | 90 |
| Solar cell | 10 |
A realistic mass and spring laboratory. Hang masses from springs and adjust the spring stiffness and damping. You can even slow time. Transport the lab to different planets. A chart shows the kinetic, potential, and thermal energies for each spring.
You do 30 J of work to load a toy dart gun. However, the dart is 10 cm long and feels a frictional force of 10 N while going through the dart gun’s barrel. What is the kinetic energy of the fired dart?
(c)
When an object is lifted by a crane, it begins and ends its motion at rest. The same is true of an object pushed across a rough surface. Explain why this happens. What are the differences between these systems?
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