Circle Math

In circle math the terms related to the circle are discussed here.

We have learned to draw a circle, by tracing the outlines of objects like a bangle or a bottle cap.

Circle using a Bangle

We shall learn now how to draw a circle using a compass.

Compass is a handy drawing tool available in the geometry box. A compass has two arms which can be pulled apart easily to adjust to the size of the circle we want to draw.

Fix a sharp pencil into the holder on the compass.

Fix the needle of the compass on the point where the center of the circle would be.

Open and adjust the other arm which is holding the pencil and rotate the compass to draw a circle.

Circle using a Compass

The point O where the needle of the compass is placed in the centre of the circle. The distance of any point on the circle from the centre O is the radius of the circle. In the given figure OX is the radius.

Parts of a Circle

A line segment which joins any two points on the circle is called a chord. MN is a chord in the given figure.

The line passing through the centre which joins two points on the circle is the diameter. Diameter of the circle is twice its radius. YZ is the diameter of the circle in the given figure.

The length of the circle is called its circumference.


A circle is such a closed curve whose every point is equidistant from a fixed point called its center.

The symbol of circle is O.

Symbol of Circle

What are the terms related to the circle?

The terms related to the circle math are:

(i) Center:

The center of a circle is a fixed point within the circle from which all the points of the closing curve are equidistant. 

Centre of a Circle


O is the centre.


(ii) Circumference:

The curve which closes a circle is called its circumference. The length of the circumference is called the length of the circle.

Circumference of a Circle


(iii) Radius:

The distance from the centre to any point on the circumference of a circle is called the radius of the circle. The symbol of the radius is r.

Radius of a Circle


(iv) Diameter:

The line-segment passing through the centre and meeting the points on the circumference is called the diameter of the circle. Diameter is denoted by 'D'.

Diameter of a Circle



AB is a diameter of the circle.

Diameter is twice the length of the radius concerned.

Thus, D = 2r [Diameter = 2radius]

or

r = D/2 [Radius = Diameter/2]



(v) Arc:

Any part of the circumference is called an arc of the circle.

Arc of a Circle


(vi) Chord:

The line-segment joining the two ends of an arc is known as a chord. A diameter is the longest chord of a circle.

Chord of a Circle


Here,

O is the centre.

OP is one radius.

AB is a diameter.

MN is a chord. (line-segment)

OA and OB are also radii.

Related Concepts on Geometry - Simple Shapes & Circle

Simple Closed Curves

Polygon

Different Types of Polygons

Angle

Triangle

Quadrilateral

Circle Math

Line Symmetry

Symmetrical Shapes






4th Grade Math Activities

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