Energy Concept

Energy Is Identified through Change

When a lamp turns on, water boils, a bicycle moves, or a phone charges, energy is being transferred or transformed. In physics, energy is the ability to do work or cause change.

Energy is identified through its effects: an object moves, temperature changes, light appears, sound is produced, or electric charge flows.

Mechanical Work Transfers Energy

When a constant force acts parallel to an object's displacement, the energy transferred by mechanical work is:

W = F \Delta s

From here on, we use the International System of Units (SI) so force, displacement, and energy use one consistent unit system.

Symbol guide:

Symbol	Meaning	SI unit
$W$	work or transferred energy	$\text{J}$
$F$	force	$\text{N}$
$\Delta s$	displacement	$\text{m}$

Suppose you push a box with a force of $15 \text{ N}$ until it moves $2 \text{ m}$ . The energy transferred to the box through work is:

\begin{aligned} W &= F \Delta s \\ &= 15 \text{ N} \times 2 \text{ m} \\ &= 30 \text{ J} \end{aligned}

So, the box receives $30 \text{ J}$ of energy through that push.

If a force is applied but the object does not move, the mechanical work on the object is $0 \text{ J}$ . The person still uses metabolic energy and may feel tired, but the stationary object receives no energy through displacement.

Energy Is Measured in Joules

In SI, the unit for energy is the joule, with symbol $\text{J}$ . Because energy can be calculated from force times displacement, a joule can be read as:

\begin{aligned} 1 \text{ J} &= 1 \text{ N} \cdot 1 \text{ m} \\ &= 1 \text{ kg m}^2\text{ s}^{-2} \end{aligned}

The joule is the SI derived unit of energy. One joule equals one newton metre, or one kilogram metre squared per second squared.

Energy Transfer and Conversion

Energy can be transferred from a source to a device. The device can then convert it into a useful output such as light, motion, heating, or electricity.

Energy Transfer from Source to Use

In a solar panel, light energy changes into electrical energy. In hydropower, gravitational potential energy of water changes into turbine motion, then into electrical energy. In the body, chemical energy from food helps muscles do work.

Power Measures the Rate of Energy Transfer

Energy answers the question how much energy is transferred or transformed. Power answers the question how quickly that transfer or transformation happens.

P = \frac{E}{t}

From that equation, energy can be written as:

E = P t

The unit of power is the watt, with symbol $\text{W}$ . One watt means $1 \text{ J}$ per $1 \text{ s}$ .

1 \text{ W} = 1 \text{ J/s}

Watch the symbol context: italic $W$ usually means work, while upright $\text{W}$ means the unit watt.

A watt measures power, which is energy per unit time. The multiplication of power by time makes $\text{kWh}$ a unit of energy, and that is why electricity bills use kilowatt-hour.

\begin{aligned} 1 \text{ kWh} &= 1000 \text{ W} \times 1 \text{ h} \\ &= 1000 \text{ J/s} \times 3600 \text{ s} \\ &= 3.6 \times 10^6 \text{ J} \end{aligned}

A $10 \text{ W}$ lamp that stays on for $5 \text{ h}$ uses:

\begin{aligned} E &= P t \\ &= 10 \text{ W} \times 5 \text{ h} \\ &= 50 \text{ Wh} \\ &= 0.05 \text{ kWh} \end{aligned}

If the same lamp stays on longer, its energy use increases. If the power is higher, the energy use also increases for the same duration.

Energy Analysis for Renewable Sources

To analyze a renewable-energy system, identify the energy source, track each conversion, and compare the useful output energy with the input energy.

Situation	Physics question
Solar panels are installed on a roof	How much light energy becomes electrical energy?
Flowing water turns a turbine	How much of the water's kinetic energy can be transferred to a generator?
Home lamps turn on every night	How much electrical energy is used in $\text{kWh}$ ?
Fuel burns inside an engine	How much chemical energy becomes motion and heat?

An energy source is renewable when natural processes replenish it on a human timescale. Examples include sunlight, wind, flowing water, geothermal heat, and properly managed biomass. Non-renewable sources form much more slowly than people use them. Examples include coal, oil, and natural gas.

Global renewable capacity additions reached nearly $510 \text{ GW}$ in $2023$ . Gigawatts measure the rated power of the newly added equipment. The energy generated depends on the power used and the operating time.

Energy is an amount measured in joules. Power is the rate at which energy is transferred, and an energy source supplies the energy that a device converts.