Making and keeping differences

To bring a pan of potatoes to the boil, set the hob to a high power. To keep them boiling for twenty minutes until they’re cooked, you can set the hob to a lower power. You can't just turn the hob off – the potatoes will stop boiling.

This is a common pattern around the house: you need a high power to make a change, but a lower power to maintain that change.

Examples of creating a change:

warming a room
cooling a drink in the fridge
getting an oven up to temperature

Examples of keeping a change:

keeping a room warm
keeping a drink cool in the fridge
keeping an oven at a temperature

It's not true of all changes: once you’ve lifted something onto a shelf or upstairs, that change stays all by itself. You don’t need to help the change stay (Unless there is some big jiggling about, like an earthquake).

Two kinds of actions with different kinds of costs

creating a difference`; that’s a fixed cost because you're shifting a fixed quantity of energy
same change, just about the same cost

maintaining that difference: that’s a changing cost because you're shifting a variable quantity of energy
the longer you maintain the difference, the more energy you've had to shift, so the higher the cost.

Examples of creating and maintaining differences;

Both changing and keeping a change in temperature — heating or cooling result in:

a fixed cost to make the change
a varying cost to keep the change

For example:

warming a room and then keeping it warm
chilling some milk and then keeping it cool

Just changing temperature results in a fixed cost only. For example bringing a kettle of water to the temperature where it boils.

Just changing the height results in a fixed cost only. For example, lifting a bag onto a shelf

Measurable changes that stay changed, needing no help

where something is (its position)
which way its pointing( its angle)
how much is there(mass)

Measurable changes that need help to stay changed

how fast something wizzes past you(its speed)
how warn or cold she thing feels (its temperature)
how much something glows (its brightness)
how noisy something is (its loudness)
how much electrical current there is a wire