Wave Energies: Light, Sound, and Electrical
Waves transfer energy from one place to another without transferring matter: light carries energy as electromagnetic waves (in packets called photons, E = hf), sound carries energy as pressure vibrations in a medium, and electrical energy is transferred by moving charges in a circuit, E = Pt. In brief: light can travel through a vacuum, while sound needs a material medium. Electrical energy transferred is power multiplied by time, measured in joules or kilowatt-hours. All three are routinely converted from or into other stores, such as chemical or kinetic energy.
The sun warms the Earth by light, a speaker makes sound from electricity, and a battery lights a lamp. This guide describes light, sound and electrical energy, shows how to calculate the energy of a photon and electrical energy used by an appliance, compares them in a table, works through examples and corrects common mistakes.
What is light energy?
Light is part of the electromagnetic spectrum, with wavelengths between about 400 nm (violet) and 700 nm (red). It carries energy as electric and magnetic fields oscillating, and can travel through a vacuum at 3.00 × 108 m/s. Light is emitted by hot objects, by excited atoms and by LEDs. When it is absorbed, its energy is transferred to the absorber: a dark surface warms up, a solar cell produces a current, and a plant makes glucose by photosynthesis. In the particle model, light consists of photons with energy E = hf = hc ÷ λ, where h = 6.63 × 10−34 J·s is the Planck constant. The energy per photon increases with frequency, so blue light photons carry more energy than red ones.
What is sound energy?
Sound is a longitudinal wave of compressions and rarefactions that moves through a solid, liquid or gas, as particles vibrate and pass on the vibration. It cannot travel through a vacuum. The energy of the sound wave is shared as kinetic energy of the vibrating particles and the elastic potential energy of the compressed medium. The energy flows outward and spreads out, so the intensity (power per unit area) falls with distance. Sound waves carry very little energy compared with other forms: the total acoustic power of a shouting person is only about 1 milliwatt. The energy is eventually dissipated as a very small increase in thermal energy.
What is electrical energy?
Electrical energy is transferred when a current flows through a circuit: the source (battery or generator) gives energy to the charges, and components such as lamps, heaters and motors transfer it to other stores. The energy transferred is E = Pt = VIt, where P is the power in watts, V the voltage, I the current and t the time. For billing, the unit is the kilowatt-hour: 1 kWh = 1,000 W × 3,600 s = 3.6 × 106 J = 3.6 MJ.
Light, sound and electrical energy compared
| Feature | Light | Sound | Electrical |
|---|---|---|---|
| Carrier | Electromagnetic wave, photons | Pressure wave in matter | Moving charges |
| Needs a medium? | No | Yes | Conductor |
| Formula | E = hf | Power × time | E = VIt |
| Source example | Sun, lamp | Loudspeaker | Battery, mains |
Worked example: photon energy and appliance energy
- Frequency of green light (λ = 550 nm): f = c ÷ λ = 3.00 × 108 ÷ 550 × 10−9 = 5.45 × 1014 Hz.
- Photon energy: E = hf = 6.63 × 10−34 × 5.45 × 1014 = 3.6 × 10−19 J. In electronvolts (1 eV = 1.60 × 10−19 J): 3.6 ÷ 1.60 = 2.3 eV.
- Electrical energy of a lamp: a 60 W lamp switched on for 3.0 hours transfers E = 60 × 3.0 × 3,600 = 648,000 J = 648 kJ. In kWh: 0.060 kW × 3.0 h = 0.18 kWh.
- Cost: at 0.15 per kWh (an illustrative price), 0.18 kWh costs 0.18 × 0.15 = 0.027, about 2.7 cents.
- Heater: an electric kettle of 2,000 W running for 3.0 minutes: E = 2,000 × 180 = 360,000 J = 360 kJ = 0.10 kWh.
How were these energies understood?
Thomas Young’s double-slit experiment (1801) showed the wave nature of light, and James Clerk Maxwell predicted electromagnetic waves in 1865. Max Planck introduced the quantum of energy in 1900, and Einstein explained the photoelectric effect with photons in 1905. Alessandro Volta invented the battery in 1800, and Michael Faraday discovered electromagnetic induction in 1831, the basis of electrical generators.
Which mistakes are common?
Mistake 1: Thinking that sound can travel in a vacuum
Sound needs particles to vibrate. Light does not.
Mistake 2: Saying that electrons carry energy from the battery to the lamp
The charges in a circuit are not used up. They transfer energy to components as they move through them.
Mistake 3: Confusing kilowatts and kilowatt-hours
The kilowatt is a unit of power. The kilowatt-hour is a unit of energy.
Mistake 4: Thinking that waves move matter along with energy
The medium’s particles oscillate about their positions. The energy moves on.