## Quantifying pathways

Physics Narrative for 14-16

#### Calculating power in all four pathways

Here is how to calculate the power in each pathway directly – at least in principle. In practice, some of the required measurements might be hard to do.

Electrical working: joulecoulomb × coulombsecond

Mechanical working: joulemetre × metresecond

Heating by particles: jouleparticle × particlesecond

Heating by radiation: joulephoton × photonsecond

You might be more familiar with the first two as:

Electrical working: p.d. × current

Mechanical working: force × velocity

#### Checking the switching from pathway to pathway

If you can calculate the power in each pathway in a particular case, then this can give another route to finding energy efficiency. Remember that power is just energy per second.

Because the pathways description is often closer to the mechanism, this may be a better practical route to improving the efficiency. Find the leaky pathways, decide which can be fixed and then plug the leaks. More of the input power will be directed to the desired output pathway.

Imagine running the system for exactly 1 second. Calculate the percentage of the input energy ending up in the desired output and you have the efficiency: efficiency = energy_{out}energy_{in} × 100 %.