Shapes and Space
Circles: centre, radius and diameter
Name the centre, radius and diameter of a circle, and use the fact that the diameter is twice the radius.
A circle is drawn by keeping a pencil the same distance from one fixed point the whole way round. That fixed point is the centre, and that fixed distance is the radius. Everything else about a circle follows from those two things — which is why a pair of compasses is set to a radius and pinned at a centre, and nothing else needs deciding.
Across is twice as far as out
A straight line from the centre to the edge is a radius. A straight line right across, passing through the centre, is a diameter. Since crossing the circle means going out to the edge, back to the centre and out again, the diameter is always exactly twice the radius — and the radius is always half the diameter.
Worked example
A circle has a radius of 6 cm. What is its diameter?
The radius reaches from the centre to the edge: 6.
That is the distance the compasses were set to.
Try it together
Now let us work out a round table together.
The table top has a diameter of 90 cm, and a cloth has to reach from the middle to the edge.
1.Is the distance from the middle to the edge the radius or the diameter? Write the word.
Have a play
Three parts are marked on these two equal circles. Find the point in the middle of the first, the line that reaches from its middle to the edge, and the line that crosses the whole of the second, and see what each one is called.
Tap any part of the picture.
Have a go
Have a go on your own: a circle has a diameter of 26 cm. What is its radius, in centimetres? Give just the number.
Hint: Half of twenty-six.
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.