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# Units Physics Save Print Edit
Units Physics
• Astrophysics • Atoms and Radioactivity • Electricity • Energy Physics • Engineering Physics • Fields in Physics • Force • Further Mechanics and Thermal Physics • Magnetism • Measurements • Mechanics and Materials • Medical Physics • Nuclear Physics • Particle Model of Matter • Physical Quantities and Units • Physics of Motion • Radiation • Space Physics • Turning Points in Physics • Waves Physics Units are used to measure a physical quantity, such as mass or length. In science, units are an established reference allowing you to define the magnitude of a quantity. Figure 1. When weighing a product, you measure the physical property weight using the unit of mass, with the standard units being kilograms or pounds. Source: Seika, Flickr (CC BY 2.0).

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.

Lets 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. Lets 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 wont work.

However, what if you used a fixed reference for your chair?

Lets 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. Figure 2. Measuring units is important when manufacturing or building objects, as it allows us to reproduce the work. Source: Limarie Cabrera, Flickr (CC BY 2.0).

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. Figure 3. The last of the original metres, installed in Paris in 1796. Source: Groume, Flickr (CC BY-SA 2.0).

### 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 * s ^ 2 joules J Energy kg * m ^ 2 / s ^ 2 newton N Force kg * m / s ^ 2 hertz Hz Frequency of a process 1 / s volt V Electric potential kg * m ^ 2 / s ^ 3 * A ^ 1 lux lx Amount of luminosity cd / m ^ 2 ohm Ω Resistance to the electrical flow m ^ 2 * kg / s ^ 3 * A ^ 2 becquerel Bq Radiation by disintegration 1 / s henry H Inductance m ^ 2 * kg / s ^ 2 * A ^ 2 weber Wb The flux of a magnetic field m ^ 2 * kg / s ^ 2 * A ^ 1

## Units - key takeaways

• Units are the references we use to measure an objects 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.

SI units are the basic units used today. Examples of SI units include metre, second, mole, and kelvin.

A unit of measurement is a fixed value that allows us to reproduce measurements using a fixed reference.

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.

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.

## Final Units Physics Quiz

Question

What is a unit?

A unit is a standard reference used for measuring.

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Why are units important?

Units are important because they allow us to reproduce measurements.

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What is the SI system?

The SI system is a standard system of units used for measurements.

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How many basic units does the SI have?

The SI has seven basic units.

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How are derived units composed?

They are composed of the SI basic units.

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What are the seven basic units of the SI?

The second, mole, kilogram, kelvin, metre, candela, and ampere.

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What is hertz the derived unit of?

Hertz is the derived unit of the second.

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Does the SI system use Celsius or Kelvin to measure temperature?

Kelvin.

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If you use the length of an object to measure another object, are you measuring by reference or using units?

You are measuring by reference.

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What is the symbol for a metre?

m.

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What is the symbol for an ampere?

A.

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The speed of an object is measured using which units?

Speed is measured in metres and seconds.

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Which measurements do you need to combine in order to measure speed?

Distance and time.

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What is the purpose of the kilogram as a unit of measurement?

The kilogram is used to measure the mass of an object.

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What is the symbol of the kilogram?

kg.

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What is the symbol for a candela?

Cd.

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What is measured in candelas?

Candelas measure the amount of light.

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Is the newton a derived unit?

Yes, it is derived from kilograms, metres, and seconds.

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What is the symbol for a mole?

mol.

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Is Kelvin a derived unit or a basic unit?

It is a basic unit.

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