How Is the Energy Source Used for Mechanical Energy?

Energy is the ability to do work. It can take many different forms.

Energy is the ability to do work. It can take numerous different forms. Energy of motion is called kinetic energy, while energy that an object has because of its location is called potential energy. The total mechanical energy of an object is the sum of its potential and kinetic energy. Energy stored in other forms like chemical energy can be transformed into mechanical energy.

Chemical Energy

Chemical potential energy is energy stored in bonds between atoms. When these atoms take part in chemical reactions the bonds are rearranged and energy can be released. Combustion is a common type of chemical reaction that releases heat and light. In automotive engines and other similar machines, burning gasoline inside cylinders releases a large amount of heat, dramatically changing the pressure inside the cylinder and thus driving the piston down the cylinder to turn a crankshaft, thereby converting chemical energy into mechanical energy. Fossil-fuel burning power plants also convert chemical energy into mechanical energy by using the heat from combustion to produce steam that can drive a turbine.

Motors

The simplest kind of motor is a loop of wire in a magnetic field, like the field between two bar magnets. As current runs through the loop of wire it creates a magnetic field. The opposing magnetic fields create forces on the two halves of the loop of wire that cause it to turn. While most motors are somewhat more complex, they all function through the same basic principle. Electric motors transform electrical energy into mechanical energy, since the electrical energy of the current in the wire is used to turn a shaft.

Thermodynamics

Thermodynamics is the branch of physics that studies how heat is converted into mechanical energy--and vice versa. There are three basic laws of thermodynamics: Energy can neither be created nor destroyed, entropy of a closed system always increases over time, and it's impossible to cool a system to absolute zero with a finite number of processes. From the first law, we know the change in the internal energy of a system--i.e. its temperature and pressure--must always equal the sum of the heat added to or removed from the system and the mechanical work performed by the system.

Heat Engines

Heat engines turn heat energy into mechanical energy. Power plants, diesel engines and steam engines are all common examples. Heat engines typically operate in a cycle. First heat from another process--combustion in cars, for example, or nuclear fission in nuclear power plants--is added to a gas. The gas expands in keeping with the formula PV=nRT, where P is pressure, V is volume, n represents the amount of gas, R is a constant and T is temperature. As it expands, the gas does mechanical work--for example, by driving a turbine. At this point part of the heat that cannot be used to do work is exhausted or expelled; in a power plant, for example, the steam from the turbine is condensed back into water. From here the cycle can repeat again.

Efficiency

Unfortunately, owing to the laws of thermodynamics, no heat engine can ever operate at 100 percent efficiency. Thanks to friction and other losses, typical engines operate at well below 100 percent efficiency. The same holds true for other energy conversion devices like motors; ultimately, due to friction and other losses, we are always losing a substantial amount of the energy we expend when we convert from other kinds of energy to mechanical energy.

References

• "Physics for Scientists and Engineers"; Wolfson, Richard and Pasachoff, Jay; 1999.

Resources

• Weber University: Physics Chapter 4
• Georgia State University: HyperPhysics, How does a Motor Work