Energy and Thermal Physics

Light and sound as pathways

Physics Narrative for 11-14 Supporting Physics Teaching

Thinking about light and energy

Light is often seen as a place where energy can be found. Here we show a better way to think about light and energy.

In episode 02 we suggested that it makes more sense to think of lighting as a pathway rather than as an energy store. What does this mean when it comes to describing a simple system from an energy perspective?

Let's take the example of a bulb being lit from a battery. Suppose the bulb is switched on, left on for a few minutes and then switched off. At the point of switching off, the bulb and surroundings are warmer and we can easily describe this process in terms of energy stores.

Energy is shifted from the chemical store to the thermal stores of the bulb and surroundings. Notice that light is not referred to in this energy store description.

Circuits in terms of pathways

We can also describe the process in terms of pathways. When the bulb is connected to the battery, it warms up as energy is being shifted by electrical working (as the current passes through the resistance of the filament).

As the bulb warms up it gives out light and warms up the surroundings, mainly though conduction and convection. Try putting your hand close to a domestic light bulb. You can't feel any heating until very close, which suggests that there is relatively little heating by radiation.

The process in terms of pathways

We can describe this physical process in terms of pathways.

In words, the bulb:

  • Is continuously supplied by the electrical working pathway (as current passes through the resistance of the filament).
  • Continuously supplies the heating by radiation pathway (as the bulb gives out light and other frequencies of electromagnetic radiation) along with the heating by particles pathway (as conduction and convection processes take place around the bulb).

Lighting appears in this description simply because it provides a pathway for shifting energy around. A good light bulb needs to do a lot of lighting and not too much heating. The design of the bulb needs to ensure that the best part of the electromagnetic spectrum accounts for most of the heating by radiation pathway. On the other hand, when designing an electric fire, there needs to be lots of heating and not too much lighting. This time it is the heating by particles pathway that needs to be as big as possible.

Sound as a pathway

Loudspeakers function much like bulbs, enabling you to hear by turning the to-and-fro movements of electrical currents, which encode the sound, into the to-and-fro movements of air. Your ear is affected by these to-and-fro movements, once they arrive after traveling through the medium.

The loudspeaker is working remotely on your ear and only while that is happening can you hear the sound. So hearing a sound is a process. So it cannot be sensibly thought of as an energy in a store, which is as a result something that has happened: sound is happening, or it is not sound.

In everyday life this makes perfect sense: You buy a sound system rated by watts (must be measured over a duration), not joules(atemporal).

Making connections between sound and energy descriptions is best done through power. The sounding is a pathway that empties or fills stores of energy. Sound is not a store of energy.

The power of a sound we hear is tiny, so not very significant in energy descriptions, but it best thought of as filling or emptying stores through the mechanical working pathway.

Teacher Tip: Hearing sounds is about power in pathways—best thought of as the mechanical working pathway, because of the mechanisms of creation and destruction, and the closer link between pathways and mechanism as opposed to the abstract description posed by energy.

appears in the relation ΔEΔt>ℏ/2 ΔQ=mcΔθ E=hf E ∝ A^2
has the special case Photon Energy
is used in analyses relating to Emission/Absorption Spectra Phase Change
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