# Making and keeping differences

### 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