Using Units (Edexcel GCSE Biology: Combined Science)

• You may be given a question in your Biology exam where the measurements for a magnification calculation have different units
• You need to ensure that you convert them both into the same unit before proceeding with the calculation (usually to calculate the magnification)
• Remember the following to help you convert between mm (millimetres), µm (micrometres) and nm (nanometres):

Converting between mm (millimetres), µm (micrometres) and nm (nanometres)

• If you are given a question with two different units in it, make sure you make a conversion so that both measurements have the same unit before doing your calculation
• For example:

Step One:

• • Remember that 1 mm = 1000 µm
  • So to get from µm to mm you need to divide by 1000

Step Two: Calculate the thickness of the leaf in mm

• • 2000 ÷ 1000 = 2, so the actual thickness of the leaf is 2 mm and the drawing thickness is 50 mm

Step Three: Put these values into the equation for calculating magnification 

• • Magnification = image size ÷ actual size
    • = 50 ÷ 2
    • = 25

  • So the magnification is x 25

Standard form

• When doing calculations and unit conversions, it is common to come across very big or very small numbers
• Standard form can be useful when working with these numbers
• Standard form is a way of writing very big and very small numbers using powers of 10

How to use standard form

• Using standard form, numbers are always written as follows: a × 10ⁿ
• The rules:
  • 1 ≤ a < 10 (the number 'a' must always be between 1 and 10)
  • n > 0 for LARGE numbers ('n' = how many times 'a' is multiplied by 10)
  • n < 0 for SMALL numbers ('n' = how many times 'a' is divided by 10)

Using standard form to convert between units

• For example, you can write 1 metre in millimetres using standard form:
  • 1 m = 1000 mm
  • So, 1 m = 1 mm × 1000
  • So, 1 m = 1 mm × 10 × 10 × 10
  • So, as we had to multiply 1 mm by 10 three times to get 1 m, we write this as:
  • 1 m = 1 × 10³ mm

• Writing 1 millimetre in metres using standard form is also possible and is just the opposite:
  • 1 mm = 0.001 m
  • So, 1 mm = 1 m ÷ 1000
  • So, 1 mm = 1 m ÷ 10 ÷ 10 ÷ 10
  • So, as we had to divide 1 m by 10 three times to get 1 mm, we write this as:
  • 1 mm = 1 × 10^-3 m

• Exactly the same process can be used if you needed to convert micrometres into millimetres. For example:
  • 1 µm = 0.001 mm
  • So, 1 µm = 1 mm ÷ 1000
  • So, 1 µm = 1 mm ÷ 10 ÷ 10 ÷ 10
  • So, as we had to divide 1 mm by 10 three times to get 1 µm, we write this as:
  • 1 µm = 1 × 10^-3 mm

Examples of using standard form in conversion calculations

• You could be asked to state 45 centimetres in millimetres using standard form:
  • 1 cm = 10 mm
  • So, 45 cm = 450 mm
  • So, 45 cm = 4.5 mm × 10 × 10
  • So, as we had to multiply 4.5 mm by 10 two times to get 45 cm, we write this as:
  • 45 cm = 4.5 × 10² mm

• You could also be asked to state 250 micrometres in millimetres using standard form:
  • 1 µm = 0.001 mm
  • So, 250 µm = 0.25 mm
  • So, 25 µm = 2.5 mm ÷ 10
  • So, as we had to divide 4.5 mm by 10 just once to get 250 µm, we write this as:
  • 250 µm = 2.5 × 10^-1 mm