The first function will be to calculate the circumference of a circle.

The formula to calculate the circumference of a circle is:

C = 2 * π * r

Where:
C = Circumference of the circle
π = Pi (approximately 3.14159)
r = Radius of the circle

We can define a function in Python to calculate the circumference of a circle as follows:

```python
import math

def calculate_circumference(radius):
circumference = 2 * math.pi * radius
return circumference

# Example Usage
radius = 5
print("The circumference of the circle with radius", radius, "is:", calculate_circumference(radius))
```

In this function, we first import the math module to use the value of Pi. We then define a function calculate_circumference that takes the radius of the circle as input and calculates the circumference using the formula mentioned above. Finally, we can call this function with a radius value to calculate the circumference of a circle.