Parts of a Circle
What You’ll Learn
Section titled “What You’ll Learn”In this lesson you’ll learn the names and definitions of the main parts of a circle and how they relate to one another.
The Concept
Section titled “The Concept”A circle is a closed curve where every point on the curve is the same distance from a fixed point inside called the center.
Key parts:
- Center: The fixed point in the middle of the circle (usually labeled O).
- Radius: The distance from the center to any point on the circle. All radii in the same circle are equal.
- Diameter: A straight line that passes through the center and connects two points on the circle. Diameter = 2 × radius.
- Chord: Any straight line connecting two points on the circle (a diameter is a special chord that passes through the center).
- Arc: A portion of the circle’s circumference (the curved part between two points).
- Sector: The region bounded by two radii and the arc between them (like a slice of pizza).
- Circumference: The total distance around the circle.
Important relationships:
- Diameter = 2 × radius
- Radius = diameter ÷ 2
Worked Example
Section titled “Worked Example”-
A circle has a radius of 7 cm. What is the diameter?
Diameter = 2 × 7 = 14 cm
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A circle has a diameter of 20 inches. What is the radius?
Radius = 20 ÷ 2 = 10 inches
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Name the longest chord in any circle.
The diameter (it passes through the center).
Real-World Application
Section titled “Real-World Application”Circles and their parts appear everywhere:
- Wheels, tires, and gears (radius and diameter determine size and fit).
- Clocks and watches (the hands move along arcs).
- Pizza or pies (each slice is a sector).
- Round tables, plates, and manhole covers.
- Navigation: Circles are used in maps for distance from a central point.
- Construction: Arches, domes, and circular windows or pipes.
Understanding these parts helps with measuring, designing, and working with anything round, from baking pans to car tires.