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Parts of Circle

1) Center

In a circle, every point on the circle is the same distance from the center point. The center point is not actually part of the circle, but it is a very important point to recognize. As you can probably guess from the name, it is at the center of the circle.

2) Diameter

The diameter of circle is a line segment that goes all the way across a circle through the center point.

A diameter looks like this:

A circle actually has many diameters since you can draw many different lines through the center of the circle.

Each diameter, however, has the same length.

For example: Can you think of examples of diameters that occur in our everyday lives? The first example that comes to my mind is a pizza. Every time the pizza is cut, it is a diameter right through the middle.

3) Radius

The radius of a circle is a line segment that goes from the center point to a point on the circle. Thus, it goes halfway through the circle.

Just like with a diameter, you can draw many radii (that’s the plural of radius) on each circle.

(See the picture given below) Each radius (MN, MO, MP) has the same length because the distance from the center point to the circle is always the same throughout the circle.

An example of a radius that you see all the time would be the spokes of a bicycle wheel (as shown in the picture given below). They each start at the center and go out towards the edge of the circle (the wheel). Although, the wheel has many radii, they are all the same length.

4) Circumference

The circumference of a circle is the distance around the outside of the circle. It’s really a fancy name for the perimeter of the circle.

For example: If you were to run around a circular track, that would be the circumference of the circular track.

5) Chord

A chord is any line segment that connects two points on the circle.         

In the example given below, line segment AB is a chord.

Since the diameter connects two points on the circle, it is also a chord. It is the largest chord in the circle because it goes all the way across through the center.

        

The diameter is always a chord, but that doesn’t mean every chord is a diameter. In the first example, line segment AB was a chord, but since it doesn’t go through the center, it is not a diameter.

6) Arc

Any connected part of the circle.

An arc is part of the circle itself. It is always curved since circles are curved.

7) Plane

A circle exists on a plane. A plane is a flat surface that extends forever in all directions

like a sheet of paper except that it continues forever.

A circle divides a plane into three sections:

1. The points inside the circle (such as A and B)

2. The points on the circle (such as C and D)

3. The points outside the circle (such as E and F)

For Example:

Identify the different parts of the circle.

 

1. What is BC?

It is a radius since it goes from the center to a point on the circle. You also know it is a radius because it goes halfway across the circle.

2. What is point B?

Point B is the center point since it is at the center of the circle.

3. What is DC?

DC is a diameter since it goes all the way across the circle through the center. It is also a chord since it connects two points on the circle.

4. What is DE?

DE is a chord since it connects two points on the circle. It is NOT a diameter because it does not go through the center.

5. What is AB?

AB is also a radius because it goes halfway through the circle, from the center point to a point on the circle.

6. What is the distance all the way around the circle?

The distance all the way around the circle is always the circumference.

7. What is the part of the circle that goes from A to C?

This is an arc because it is part of the circle itself.

Summary

Various parts of a circle:

1. Center point:  It is the center of the circle. Every point on the circle is the same distance from the center.

2. Diameter: A line segment that goes all the way across a circle through the center point.

3. Radius: A line segment that goes from the center point to a point on the circle. Thus, it goes halfway through the circle.

4. Circumference: The distance around the outside of the circle.

5. Chord: A line segment that connects two points on the circle.

6. Arc: Any connected part of the circle.

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