Using Units (Edexcel GCSE Combined Science): Revision Note

Exam code: 1SC0

Alistair Marjot

Last updated

Converting Units

  • 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 Units AQA, IGCSE & GCSE Biology revision notes

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:

Worked Example

Example extended magnification question, IGCSE & GCSE Biology revision notes

Answer:

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


👀 You've read 1 of your 5 free revision notes this week
An illustration of students holding their exam resultsUnlock more revision notes. It's free!

By signing up you agree to our Terms and Privacy Policy.

Already have an account? Log in

Did this page help you?

Alistair Marjot

Author: Alistair Marjot

Expertise: Environmental Systems and Societies & Biology Content Creator

Alistair graduated from Oxford University with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems & Societies. Alistair has continued to pursue his interests in ecology and environmental science, recently gaining an MSc in Wildlife Biology & Conservation with Edinburgh Napier University.

Download notes on Using Units