Circle

A circle is a round, closed 2D shape
with no sides and no corners.
What is a Circle?
  • A circle is a round shape.
  • It is closed and curved.
  • It has no sides.
  • It has no corners (vertices).
  • All the points on the boundary are at the same distance from the centre.
Examples in Real Life
₹ Coin
Clock
Wheel
Plate
Pizza
CD
Properties of a Circle
  • Sides: 0
  • Vertices (Corners): 0
  • Angles: 0
  • One continuous curved boundary.
  • Infinite lines of symmetry.
Key Terms with Diagrams
CENTRE
O
The fixed point in the middle of the circle.
RADIUS (r)
O A
Line segment OA is the radius.
DIAMETER (d)
O A B
Line segment AB is the diameter.
d = 2r
Quick Facts
  • A circle is a 2D shape.
  • It has no sides and no corners.
  • It has a curved boundary.
  • All points on the boundary are at the same distance from the centre.
  • A circle has infinite lines of symmetry.
  • It is not a polygon.
Important Terms
Centre (O) : The fixed point in the middle of the circle.
Radius (r) : The distance from the centre to any point on the circle.
Diameter (d) : A line passing through the centre and joining two points on the circle. It is twice the radius.
Different Positions of Points
Inside
Point lies inside the circle.
O
On the Circle
Point lies on the circle.
O
Outside
Point lies outside the circle.
Lines Related to a Circle
Secant
A B
A line that cuts the circle at two points.
Tangent
P
A line that touches the circle at exactly one point.
Chord
A B
A line segment whose endpoints are on the circle.
Comparing Circles
r₁ r₂
If r₁ < r₂, then the left circle is smaller than the right circle.
r r
If r₁ = r₂, then both circles are equal in size.
Real-Life Examples
Bicycle wheel
Wall clock
Bangle
Round table
Hoop
Summary
  • A circle is a special 2D shape.
  • It has no sides and no corners.
  • It has one curved boundary.
  • All points on the boundary are at the same distance from the centre.
  • Diameter = 2 × Radius
Think and Answer
  1. How many sides does a circle have?
  2. What is the relation between diameter and radius?
  3. If the radius of a circle is 5 cm, what is its diameter?
  4. Name three real-life objects that are in the shape of a circle.