# What is a circle? Diameter, Radius and Circumference of circle

Circle is a set of points equidistant from the fixed central point

For Example

Given above is the example of circle.
Note that every point on the circle is equidistant from the central point O

## Structure of Circle

All circle contain following components:
(a) Center
(c) Diameter
(d) Chord
(e) Arc

(a) Center of Circle
It is the fixed point located inside the circle.
All the points located on the circle are equidistant from the center.

In the above figure O is the center of the circle, which is located inside.
All the points on the circle (point A, B, C, D) are equidistant from point O

The line from center to any point on the circle is called Radius
For a circle, the length of radius is constant

Given above is the circle with center O with point A & B on the circle.
The line OA & AB are the radius of circle.
Note the length of radius OA & OB is same because the value of radius is constant for any circle

(c) Diameter of circle
Its a straight line meeting two ends of circle and passing through point O

Given above is the image of circle with center O
Here line AO is the radius of the circle which is extended to reach other side of circle at point B.

Hence AB is diameter of circle as its a straight line meeting two ends of circle at point A & B and passes through center O.

Note that its twice the length of radius

(i) It’s a straight line
(ii) It passes through center O
(iii) touch the two opposite ends of circle
(iv) its length is twice the radius

You can draw multiple diameter in any given circle
Given below is the circle with center O
The line AB, CD and EF are the diameter of the circle

(d) Chord
Line segment joining any two points of the circle is called Chord
Diameter of the circle is the longest chord

Given above is the circle with center O
Here AB, CD and EF are the chords joining two different points on the circle.
The line CD is also called diameter as it joins two opposite points on circle while passing through center O

(e) Arc
Any selected boundary of circle is known as Arc of circle

Given above is the circle with center O
Here the arc AB is the part of the circle between point A & B

(f) Circumference of circle
The length of the boundary of the circle is called circumference of circle

Finding circumference using ruler
Imagine a black wire in the form of circle.
Now if you twist the wire into straight line and find the length using ruler, you will get the circumference of the circle

## Questions on Circle

(01) Study the image below and find the name of the diameter

Here AB is the diameter of circle as it passes through center O
Rest of the lines EF & CD are the chords

(02) Study the figure and find the length of radius

Given line AB is the diameter as it passes through center O of circle
Length of diameter AB = 8 cm

We know that diameter is twice the radius

(03) Study the figure below and find the length of line OD

Solution
Here both OA & OD are the radius of circle.
We know that for a given circle the length of the radius is the same
OA = OD = 2 cm

(04) A wire of 8 cm is folded to form a circle
What is the circumference of circle

Since the wire is folded to form a circle, the circumference of the circle will also be 8 cm

(05) Study the circle below and find the length of its diameter

In the above image:
AB is the chord and OD is the radius of circle

We know that:
Diameter = 2 x 4
Diameter = 8 cm

(06) Observe the below image of circle and mention the name of radius, diameter and chord

Diameter = EF
Chord = AB & EF

(07) What is the line EF

(a) Diameter
(b) Chord

EF is a line touching two ends of the circle but doesn’t pass through center.
Hence its a chord

Option (b) is correct

(08) What is the line AB in below circle

(a) Diameter
(c) Chord

AB is a line that touches circle at end point and also passes through center.
Hence AB is a diameter
Option (a) is the right answer

(09) What is the line OE

(a) Chord
(c) Diameter

The line OE is a line from center O to point E on the triangle.
Option (b) is the right answer

(10) What is the length of line OF

Solution
AB is the diameter of circle length 10 cm