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The physical quantity is the property you measure, while the units are the reference, allowing others to know the value of that measurement.
Two simple examples show what we mean by measuring physical quantities:
The branch of a tree has a length of 2.3 metres. Length is a physical quantity of the branch; metres are the units that tell us how long it is against a reference. In this case, our reference is one metre, so the branch measures twice plus 0.3 times the length of a metre.
You need 200 g of flour to make pancakes in the morning. The weight of the flour is the property you are measuring, while grams are the units you use as a reference.
What is the advantage of measuring by standard units instead of using a reference?
Units provide a standardised reference that can be reproduced everywhere. If you were to use an arbitrary reference, the same value could not be accurately measured twice.
To explain further, we will use a simple example.
Let’s say that you have a piece of wood and you want to make a chair. You will need every leg to be the same length, or the chair will topple over.
In order to measure the same length every time, you need a pattern. Let’s say that you use a pencil as a reference, and you use it to measure the first leg length. The leg is four ‘pencils’ long. You can easily use your pencil to measure the wood for the other three legs.
But what if you want to give instructions to a friend about how you made your chair? The instructions say that you used a piece of wood that measured four ‘pencils’ long.
Your friend might have a pencil, but the pencil length is not standardised. Other pencils will be shorter or longer than yours, so the instructions for making the chair won’t work.
However, what if you used a fixed reference for your chair?
Let’s say that this time you use a ruler to measure the wood for the chair legs, which, you determine, are 45cm long. Now your friend can use their own ruler and reproduce the chair you made.
Units are used widely in industry and everyday life. Without them, modern life would not be possible.
What are the SI units?
The International System of Units (SI) is a unified measurement system. It is composed of seven units that measure the seven elemental physical quantities. The SI system is the only system of units that has official status in almost every country.
What are the original SI units and their symbols?
The original SI units measuring the basic physical quantities are as follows:
Metre: used to measure length; its symbol is m.
Second: used to measure time; its symbol is s.
Kilogram: used to measure mass; its symbol is kg.
Candela: used to measure luminosity; its symbol is cd.
Ampere: used to measure electrical current; its symbol is A.
Kelvin: used to measure temperature; its symbol is K.
Mole: used to measure the number of particles contained in a sample of a substance; its symbol is mol.
The standard units used by the SI have been changed in the last decades, from using standard weights and lengths, to now depend on constant quantities. An example of this is the kilogram, whose definition is the mass of one litre of water.
What are the derived SI units?
Derived units are those that have been created by a combination of the basic units. Derived units measure more complex physical quantities.
With basic units, we can only measure time, length, and other elemental physical properties. However, if we combine the basic units, we can measure more complex things. For instance, after combining units from two different measurements, we can determine how fast an object moves by measuring the distance it travelled, and the time it took to travel that distance. The list below features some SI derived units.
| Derived unit | Symbol | Measures | Units |
| pascal | Pa | Pressure | \(kg / m \cdot s ^ 2\) |
| joules | J | Energy | \(kg \cdot m ^ 2 / s ^ 2\) |
| newton | N | Force | \(kg \cdot m / s ^ 2\) |
| hertz | Hz | Frequency of a process | \(s^{-1}\) |
| volt | V | Electric potential | \(kg \cdot m ^ 2 / s ^ 3 \cdot A\) |
| lux | lx | Amount of luminosity | \(cd / m ^ 2\) |
| ohm | Ω | Resistance to the electrical flow | \(m ^ 2 \cdot kg / s ^ 3 \cdot A ^ 2\) |
| becquerel | Bq | Radiation by disintegration | \(s^{-1}\) |
| henry | H | Inductance | \(m ^ 2 \cdot kg / s ^ 2 \cdot A ^ 2\) |
| weber | Wb | The flux of a magnetic field | \(m ^ 2 \cdot kg / s ^ 2 \cdot A\) |
Units - key takeaways
- Units are the references we use to measure an object’s properties.
- Units are important because they allow us to reproduce measurements. Technology, manufacturing, and goods depend on the use of units.
- SI units are today’s standard system used for measuring.
- The basic SI units are kelvin, candela, metre, kilogram, mole, ampere, and second.
- Units can be combined, leading to derived units that are used to describe more complex properties. Examples of derived units are velocity and acceleration.
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Frequently Asked Questions about Units Physics
What are SI units?
SI units are the basic units used today. Examples of SI units include metre, second, mole, and kelvin.
What is a unit of measurement?
A unit of measurement is a fixed value that allows us to reproduce measurements using a fixed reference.
What are the different types of units of measurement?
In the SI system, there are seven basic units of measurement. They are the metre for length, the second for time, the kelvin for temperature, the mole for the number of molecules in an object, the kilogram for the mass of an object, the candela for the amount of light, and the ampere for the amount of electrical charge.
What does unit mean in science?
In science, a unit is a reference value set by convention or law that is used as a standard to measure the physical property of an object.
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