The circumference (or perimeter) of a circle is made of many points that are all the same distance (equidistant) from the centre of the circle. An arc is part of the circumference of a circle. If the arc is over half of the circumference then it is called a major arc. If it is less than half of the circumference it is called a minor arc. The diameter cuts the circle exactly in half and goes through the centre. The radius is half the length of the diameter and is joined to the centre at one end. A chord is a straight line inside the circle that touches the circumference at 2 points but does not touch the centre of a circle. A tangent is a straight line outside the circle that touches the circumference at one point only. A segment is the area enclosed by a chord and an arc (it looks similar to the segment of an orange or grapefruit). A sector is the area enclosed by 2 radii (radius) and an arc (It looks like a slice of cake or pizza).
Here we will learn about the different parts of a circle including how to identify the key parts of a circle from a diagram, how to identify the key parts of a circle from a definition and how to draw a circle with the different parts labelled e.g. the diameter of a circle. There are also parts of a circle worksheets based on Edexcel, AQA and OCR exam questions, along with further guidance on where to go next if you’re still stuck.
The parts of a circle are the radius, diameter, circumference, arc, chord, secant, tangent, sector and segment. Circle terms  Naming circle parts: Circle A round plane figure whose boundary consists of points equidistant from a fixed point. Centre The centre of the circle is the fixed point from which all points on the boundary of the circle are equidistant. Often noted on diagrams as ‘O’. Centre/center are the same. Centre is the UK spelling whilst Center is the US spelling. Radius The distance from the centre of a circle to the outside. The radius of the circle is half the diameter of the circle. The plural of radius is radii. Diameter The distance across the circle going through the centre. The diameter is twice the radius. Circumference The distance once around the circle. Arc A part of the circumference. Major arc – A major arc is greater than half the circumference. Minor arc – A minor arc is less than half the circumference. Area The space inside a 2D shape. Chord A line segment going from one point of the circumference to another but does not go through the centre. Secant A line that goes through the circle at two points. Note: Secant is not a term you are required to know at GCSE, however it is important to note the difference between a chord and a secant. Tangent A straight line that touches the circle at a single point only. Sector A section of the circle created by two radii. Major sector – A major sector has a central angle which is more than 180^o . Minor sector – A minor sector has a central angle which is less than 180^o . Semi circle Half of a circle. Could be considered a sector where the circle has be split by the diameter. Quadrant A quarter of a circle, created by two perpendicular radii. Segment A section of the circle created by a chord. Major segment – a segment where the arc is greater than half the circumference. Minor segment – a segment where the arc is less than half the circumference.
Other keys terms: Equidistance A point is equidistant from 2 other points when it is always the same distance away from those points. Angles around a point Angles around a point are equal to 360^o .
In order to identify/label parts of a circle: 1 Identify the key aspects of the part of the circle.
2 Clearly state your answer, consider whether the part of a circle you have identified has a specific name e.g. semicircle, major segment.
Get your free parts of a circle worksheet of 20+ questions and answers. Includes reasoning and applied questions. DOWNLOAD FREE
Get your free parts of a circle worksheet of 20+ questions and answers. Includes reasoning and applied questions. DOWNLOAD FREE
Name the part of the circle shown in red in the below diagram.
2Clearly state your answer, consider whether the part of a circle you have identified has a specific name e.g. major segment. Radius
Name the part of the circle shown in red in the below diagram.
Identify the key aspects of the part of the circle. Here are some key questions you can ask yourself?
Clearly state your answer, consider whether the part of a circle you have identified has a specific name e.g. major segment.
Sector – specifically the minor sector.
Identify the key aspects of the part of the circle. Here are some key questions you can ask yourself?
Clearly state your answer, consider whether the part of a circle you have identified has a specific name e.g. major segment.
Name the part of the circle shown in red in the below diagram.
Identify the key aspects of the part of the circle. Here are some key questions you can ask yourself?
Clearly state your answer, consider whether the part of a circle you have identified has a specific name e.g. major segment.
Segment – specifically the minor segment.
Identify the key aspects of the part of the circle. Here are some key questions you can ask yourself?
Clearly state your answer, consider whether the part of a circle you have identified has a specific name e.g. major segment.
On the circle below: Draw a diameter. Draw a tangent. Label the circumference.
Identify the key aspects of the part of the circle.
Here you are being asked to draw the parts on a the given circle so you needs to consider each key term.
Clearly state your answer by labeling the diagram given.
The radius is from the centre of the circle to the circumference whilst the diameter goes across the whole circle whilst going through the origin.
A segment is made from a chord whilst a sector will have lines (radii) coming from the origin. HINT: Some students like to consider a sector like a slice of pizza.
A chord will not go through the origin of the circle whilst the diameter will.
A tangent only touches the circumference at a single point, it does not cross the line. A secant will cross the circumference twice. Practice parts of a circle questions
Middle of the circle (origin)
The ‘o’ refers to the centre of the circle which is called the origin of the circle
The circumference is the distance around the edge of the circle, this will always be longer
The radius is twice the length of the diameter
The diameter is twice the length of the radius
The radius is half the length of the circumference
The diameter is half the length of the circumference
The distance from the centre of the circle to the circumference is called the radius. The distance across the circle passing through the centre is called diameter. This means that the diameter is twice as long as the radius.
Part of a circle’s circumference
A line that goes through a circle
A sector consists of the area created by an arc and two radii Parts of a circle GCSE questions
1. Name the part of the circle shown in the diagram below: (1 mark)
2. Name the part of the circle shown in the diagram below: (1 mark)
3. Name the part of the circle shown in the diagram below: (1 mark)
4. Name the part of the circle shown in the diagram below: (1 mark)
5. Name the part of the circle shown in the diagram below: (1 mark)
6. Name the part of the circle shown in the diagram below: ‘A line that goes across the circle but does not go through the origin’ (1 mark)
7. Molly says ‘A chord is the same as a radius but shorter’. Is Molly correct? Explain your answer (1 mark)
No, the radius goes from the origin to the circumference. A chord does not touch the origin of the circle (1)
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What is part of a circle called?
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