M4.5.8 For the adult
Rates of flow and rate graphs
A tap fills a tank at 12 litres a minute. The tank holds 120 litres. How long does it take?
What this chapter teaches
A rate of flow is a volume divided by a time, in litres per minute. Volume = rate × time, and time = volume ÷ rate. On a graph of volume against time, a straight line has a gradient equal to the rate. A flat part means no flow, and a falling line means water is leaving.
Three questions to ask
There is no one right wording. Listen for the idea.
- How would you find how long a tap takes to fill a tank, if you know its size?
- What does the steepness of a volume time graph tell you?
- What does a flat part of a volume time graph mean, and what does it not mean?
After the chapter, the learner can
- Find how long a tank takes to fill at 12 litres a minute
- Read the gradient of a straight line graph as a rate
What they copy into their notebook
Rate = volume ÷ time. Volume = rate × time. Time = volume ÷ rate. On a volume time graph the gradient is the rate. Flat means no flow. Down means water leaves.