# Energy

Energy is **the capacity (or capability) to do work. **

Energy is used or consumed when work is done.

The SI unit for energy is *joule - J*, where

1 J = 1 Nm

which is the same unit as for work.

#### Imperial Units of Energy

*1 J = 0.738 ft lb**1 ft lb = 1.356 Nm (J)*

### Energy forms

There can be several forms of energy, including

- mechanical energy
- heat or thermal energy
- electrical energy
- chemical energy
- nuclear energy
- light energy

### Energy Efficiency

Energy efficiency is the ratio between useful energy output and input energy, and can be expressed as

μ = E_{o }/ E_{i }(1)

where

μ = energy efficiency

E_{o }= useful energy output

E_{i }= energy input

It is common to state efficiency as a percentage by multiplying *(1)* with *100*.

### Example - Energy Efficiency

A lift moves a mass *10 m* up by exerting a force of *100 N*. The input energy to the lift is *1500 J*. The energy efficiency of the lift can be calculated as

μ = 100 (N) 10 (m) / 1500 (J)

= 0.67 or

= 67 %

## Related Topics

### • Mechanics

The relationships between forces, acceleration, displacement, vectors, motion, momentum, energy of objects and more.

### • Thermodynamics

Work, heat and energy systems.

## Related Documents

### 1st Law of Thermodynamics

The First Law of Thermodynamics simply states that energy can be neither created nor destroyed (conservation of energy). Thus power generation processes and energy sources actually involve conversion of energy from one form to another, rather than creation of energy from nothing.

### Bernoulli Equation

Conservation of energy in a non-viscous, incompressible fluid at steady flow.

### Energy and Hydraulic Grade Line

The hydraulic grade line and the energy line are graphical presentations of the Bernoulli equation.

### Energy Transfer Equation

Fluid energy transfer.

### Flywheels - Kinetic Energy

The kinetic energy stored in flywheels - the moment of inertia.

### Food - Energy Content

Energy in carbohydrates, fats and proteins.

### Heat, Work and Energy

Heat vs. work vs. energy.

### Mechanical Energy Equation vs. Bernoulli Equation

The Mechanical Energy Equation compared to the Extended Bernoulli Equation.

### Potential Energy - Hydropower

Elevation and potential energy in hydropower.

### Power

Power is the rate at which work is done or energy converted.

### Salt Hydrates - Melting points and Latent Melting Energy

Melting points and latent energy of salt hydrates.

### Work done by Force

Work done by a force acting on an object.