Parts of a circle · Usually met around P5
Radius and diameter
Diameter = 2 × radius; open the compasses to the radius.
How to do it
Method
Diameter = 2 × radius; open the compasses to the radius
Radius 4 cm → diameter 8 cm
Another way
Fold it
Fold a circle in half: the crease is a diameter
In full
The centre point of a circle is its centre. A line from the centre to any point on the circle is a radius, and every radius of a circle is the same length. A line through the centre with both ends on the circle is a diameter: diameter = 2 × radius, and it is the longest line inside the circle (a line with both ends on the circle is a chord; the diameter is the longest chord). The distance once round the circle is its circumference. To draw a circle with compasses, put the point on the centre and open the legs to the radius. To tell a diameter, check two things: it goes through the centre and both its ends are on the circle.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
A circle's radius is 8 cm. How many centimetres is its diameter?
Show the answer and working
Answer: 16 cm
Diameter = 2 × radius = 2 × 8 = 16.
Example 2
To draw a circle 20 cm across with compasses, how far apart should the legs be opened?
- A. 5
- B. 10
- C. 40
- D. 20
Choose the right answer
Show the answer and working
Answer: B. 10
The legs are opened to the radius: 20 ÷ 2 = 10.
Example 3
A line is drawn inside a circle of radius 17 cm with both ends on the circle. What is the longest it can be, in cm?
Show the answer and working
Answer: 34 cm
Of all lines with both ends on the circle, the diameter through the centre is longest: 2 × 17 = 34.
Practise it in the game →Parts of a circle · Free to start, unlimited questions, no download