Physical Quantities

What are the different physical quantities?

There is a range of physical properties that we can measure. All these properties are related to an object’s dimensions or its constitution. The seven elemental physical quantities are:

• Mass: this is the property that tells us how much matter is contained in the object. An object with a larger quantity of matter has a larger mass. Weight is the force exerted over an object’s mass. Mass and weight are often confused. The equation for weight is: \(weight = mass \cdot 9.81m / s ^ 2\).
• Length: this is the property that tells us how long an object is. This property is related to the properties of area and volume.
• Time: this property is related to the flow of events, and it always increases. Like mass, time is one of the properties that cannot be negative. Time tells us the flow of things in the universe.
• Electrical charge: this is a physical quantity that can be positive or negative, only affecting polarity. It causes a force to act upon the matter when placed in an electric field.
• Temperature: this is the property that measures the quantity of heat in a substance or object. Heat is related to the movement of the particles in the object.
• Mole: this is a fixed physical quantity that measures the number of molecules in a substance. The property represents an exact number of particles or molecules equal to \(6.02214076 \cdot 10 ^ {23}\) molecules of the substance.
• Luminosity: this is an energy measure, just like temperature. Luminosity measures the quantity of electromagnetic energy emitted by an object as light per unit time.

The difference between weight and mass

People confuse weight and mass all the time. The best way to explain the difference is by using an example featuring a ball.

What are extensive and intensive quantities?

Physical quantities have two categories: extensive quantities and intensive quantities. This classification is related to an object’s mass. Extensive quantities depend on an object’s mass or size, while intensive quantities do not.

Examples of extensive physical quantities

Mass and electrical charge are examples of extensive physical quantities.

Examples of intensive physical quantities

Intensive physical quantities do not depend on the object’s mass or size. Simple examples of this are time and temperature.

What are derived physical quantities?

Derived physical quantities are the properties of an object that result from two elemental physical quantities. Derived quantities can result from a relationship of the same physical quantity (e.g. area) or by relating two different ones (e.g. velocity). See below for some examples of derived physical quantities.

Area and volume: related to length:

\[Area = length \cdot width; \space Volume = length \cdot width \cdot height\]

Velocity and acceleration: related to length and time:

\[Velocity = \frac{length}{time}; \space Acceleration = \frac{length}{time^2}\]

Density: related to length and mass:

\[Density = \frac{mass}{length^3}\]

Weight: related to acceleration and mass (in a planet, acceleration is its gravitational acceleration):

\[Weight = gravity \cdot mass\]

Pressure: related to force and length (for pressure, the force can be the weight exerted by an object, and the area over which this force acts is related to length):

\[Pressure = \frac{force}{length^2}\]

What are some characteristics of physical quantities?

Physical quantities have several characteristics related to their properties, some of which are listed below.

• No physical quantity can be less than zero, except for electrical charge and temperature values.
• Some physical quantities can have a value of zero, such as electrical charge or mass. In these cases, the object is electrically neutral (has no charge) or is massless (light).
• Some physical quantities are scalar, which means that they have only a value but no direction. Examples of these quantities are volume, mass, and mole.
• Other physical quantities are vectorial, in which case you need the direction to understand what is happening. Examples of vectorial quantities are velocity and acceleration.

Temperatures below zero are the result of taking the temperature at which water freezes as a zero (0) value. In Celsius, every temperature below the freezing point of water is negative.

How are units and physical quantities linked?

Physical quantities are important because they allow us to describe an object. Objects have a certain mass, a certain length, and a certain amount of atoms. The units are the values of reference we use to measure the properties of objects.