List of physical quantities facts for kids

Physical quantities are like the building blocks we use to describe and measure the world around us. Think of them as the different ways we can put a number to something in science, like how long something is, how much it weighs, or how hot it is. Scientists use these quantities to understand how things work and to make predictions.

There are two main types of physical quantities: base quantities and derived quantities. Base quantities are the fundamental ones, meaning they are independent and can't be described using other quantities. Derived quantities, on the other hand, are made by combining base quantities. For example, speed is a derived quantity because it combines length (distance) and time.

It's important to know that while the ideas behind these quantities are universal, the names and symbols for them can sometimes vary. For instance, what one person calls "magnetic flux density," another might simply call the "magnetic field." In these tables, we'll use the most common names and symbols.

We'll also look at some special features of these quantities. Some are extensive, meaning they depend on the amount of stuff you have (like mass), while others are intensive, meaning they don't (like temperature). Some are scalars, which only have a size (like time), and others are vectors, which have both a size and a direction (like force).

Understanding Base Quantities

Base quantities are the fundamental measurements that everything else in physics is built upon. The International System of Units (SI) uses seven of these basic quantities to define all other physical measurements. These are like the primary colors that you mix to get all other colors.

Exploring Derived Quantities

Derived quantities are created by combining the base quantities using multiplication or division. For example, if you combine length and time, you can get speed. These quantities help us describe more complex physical phenomena.

Derived Quantity	Symbol	What it Measures	SI derived unit	Dimension	Special Features
Acceleration	a→	How quickly an object's velocity changes	m/s2	L T−2	Vector (has direction)
Angular momentum	L	How much an object is spinning or orbiting	kg⋅m2/s	L2 M T−1	Conserved (stays the same), bivector
Angular velocity	ω	How fast an object is rotating or turning	rad/s	T−1	Bivector
Area	A	The size of a surface	m2	L2	Depends on amount, bivector or scalar
Area density	ρA	Mass spread over a certain area	kg⋅m−2	L−2 M	Doesn't depend on amount
Capacitance	C	How much electric charge a device can store	farad (F = C/V)	L−2 M−1 T4 I2	Scalar (just a number)
Chemical potential	μ	The energy related to adding or removing a substance from a system	J/mol	L2 M T−2 N−1	Doesn't depend on amount
Current density	J →	How much electric current flows through a specific area	A/m2	L−2 I	Conserved (stays the same), doesn't depend on amount, vector (has direction)
Dynamic viscosity	v	How much a fluid resists flowing (like thick syrup vs. water)	Pa⋅s	L−1 M T−1	Doesn't depend on amount
Electric charge	Q	A basic property of matter that causes it to experience a force in an electric field	coulomb (C = A⋅s)	T I	Depends on amount, conserved (stays the same)
Electric charge density	ρQ	How much electric charge is packed into a certain volume	C/m3	L−3 T I	Doesn't depend on amount
Electric field strength	E→	How strong an electric field is at a point	V/m, N/C	L M T−3 I−1	Vector field (has direction at every point)
Electrical conductance	G	How easily electric current can flow through a material	siemens (S = Ω−1)	L−2 M−1 T3 I2	Scalar (just a number)
Electrical conductivity	σ	A material's ability to conduct electricity	S/m	L−3 M−1 T3 I2	Scalar (just a number)
Electric potential	φ	The "push" or "pull" that makes electric charges move	volt (V = J/C)	L2 M T−3 I−1	Depends on amount, scalar (just a number)
Electrical resistance	R	How much a material resists the flow of electric current	ohm (Ω = V/A)	L2 M T−3 I−2	Depends on amount, scalar (just a number)
Energy	E	The ability to do work or cause change	joule (J)	L2 M T−2
Energy density	?	How much energy is packed into a certain volume	J⋅m−3	L−1 M T−2	Doesn't depend on amount
Entropy	S	A measure of disorder or randomness in a system	J/K	L2 M T−2 Θ−1	Depends on amount, scalar (just a number)
Force	F→	A push or a pull that can change an object's motion	newton (N = kg⋅m⋅s−2)	L M T−2	Depends on amount, vector (has direction)
Frequency	f	How many times something happens in a certain period (like waves passing a point)	hertz (Hz = s−1)	T−1	Scalar (just a number)
Heat	Q	Energy transferred due to a temperature difference	joule (J)	L2 M T−2
Heat capacity	Cp	How much energy is needed to raise an object's temperature by a certain amount	J/K	L2 M T−2 Θ−1	Depends on amount
Illuminance	Ev	How much light falls on a surface	lux (lx = cd⋅sr/m2)	L−2 J
Impulse	J	The change in momentum of an object	newton-second (N⋅s = kg⋅m/s)	L M T−1	Vector (has direction)
Inductance	L	How much a circuit resists changes in electric current	henry (H)	L2 M T−2 I−2	Scalar (just a number)
Intensity	I	The power of something (like light or sound) spread over an area	W/m2	M T−3	Doesn't depend on amount
Jerk	j→	How quickly acceleration changes	m/s3	L T−3	Vector (has direction)
Linear density	ρl	Mass spread over a certain length	kg⋅m−1	L−1 M
Luminous flux (or luminous power)	F	The total amount of light seen by the human eye from a source	lumen (lm = cd⋅sr)	J
Mach number (or mach)	M	How fast an object is moving compared to the speed of sound	unitless	1
Magnetic field strength	H	How strong a magnetic field is	A/m	L−1 I	Vector field (has direction at every point)
Magnetic flux	Φ	The total amount of magnetic field passing through an area	weber (Wb)	L2 M T−2 I−1	Scalar (just a number)
Magnetic flux density	B	The strength of a magnetic field at a specific point	tesla (T = Wb/m2)	M T−2 I−1	Pseudovector field
Magnetic moment (or magnetic dipole moment)	m	A measure of an object's magnetic strength and orientation	N⋅m/T	L2 I	Vector (has direction)
(Mass) Density (or volume density)	ρ	How much mass is packed into a certain volume	kg/m3	L−3 M	Doesn't depend on amount
Molar concentration	C	How much of a substance is dissolved in a certain volume of liquid	mol⋅m−3	L−3 N	Doesn't depend on amount
Moment of inertia	I	How much an object resists changes in its rotation	kg⋅m2	L2 M	Depends on amount, tensor, scalar
Momentum	p→	The product of an object's mass and its velocity	kg⋅m/s	L M T−1	Vector (has direction), depends on amount
Optical power	P	How strongly a lens or mirror bends light	dioptre (dpt = m−1)	L−1
Permeability	μs	How easily a material can be magnetized	H/m	L M T−2 I−2	Doesn't depend on amount
Permittivity	εs	How easily a material can store electric energy in an electric field	F/m	L−3 M−1 T4 I2	Doesn't depend on amount
Plane angle	θ	The ratio of an arc's length to its radius	radian (rad)	1
Power	P	The rate at which energy is transferred or work is done	watt (W)	L2 M1 T−3	Depends on amount, scalar (just a number)
Pressure	p	Force applied over a certain area	pascal (Pa = N/m2)	L−1 M T−2	Doesn't depend on amount, scalar (just a number)
(Radioactivity) Activity	A	How many radioactive particles decay per second	becquerel (Bq = Hz)	T−1	Depends on amount, scalar (just a number)
(Radiation) Dose	D	The amount of radiation energy absorbed by a certain mass	gray (Gy = J/kg)	L2 T−2
Reaction rate	r	How fast a chemical reaction happens	mol/(m3⋅s)	L−3 T−1 N	Doesn't depend on amount, scalar (just a number)
Refractive index	n	How much light slows down when it passes through a material	unitless	1	Doesn't depend on amount, scalar (just a number)
Solid angle	Ω	The 3D angle that an object takes up from a certain point (like a cone)	steradian (sr)	∠2
Specific energy		Energy per unit of mass	J⋅kg−1	L2 T−2	Doesn't depend on amount
Specific heat capacity	c	How much heat energy is needed to raise the temperature of a unit mass of a substance	J/(K⋅kg)	L2 T−2 Θ−1	Doesn't depend on amount
Specific volume	v	The volume occupied by a unit mass of a substance (opposite of density)	m3⋅kg−1	L3 M−1	Doesn't depend on amount
Strain	ε	How much a material deforms or stretches when a force is applied	unitless	1
Stress	σ	The internal forces within a material caused by external forces	Pa	L−1 M T−2	Order 2 tensor
Surface tension	γ	The force that makes the surface of a liquid act like a stretched elastic skin	N/m or J/m2	M T−2
Temperature gradient		How quickly temperature changes over a distance	K/m	L−1 Θ	Vector (has direction)
Thermal conductivity	λ	How well a material conducts heat	W/(m⋅K)	L M T−3 Θ−1	Doesn't depend on amount
Torque	τ	A twisting force that causes rotation	newton-metre (N⋅m)	L2 M T−2	Bivector (or pseudovector in 3D)
Velocity	v→	How fast an object is moving and in what direction	m/s	L T−1	Vector (has direction)
Volume	V	The amount of 3D space an object occupies	m3	L3	Depends on amount, scalar (just a number)
Volumetric flow rate	Q	How much volume of a fluid passes a point per unit time	m3⋅s−1	L3 T−1	Depends on amount, scalar (just a number)
Wavelength	λ	The distance between two matching points on a wave (like from crest to crest)	m	L
Wavenumber	k	The number of waves that fit into a certain distance	m−1	L−1	Scalar (just a number)
Weight	w	The force of gravity pulling on an object	newton (N = kg⋅m/s2)	L M T−2	Vector (has direction)
Work	W	Energy transferred when a force moves an object over a distance	joule (J)	L2 M T−2	Scalar (just a number)
Young's modulus	E	How stiff a material is, or how much it resists being stretched or compressed	pascal (Pa = N/m2)	L−1 M T−2	Scalar (just a number)
spring constant	k	How stiff a spring is, or how much force it takes to stretch or compress it	N/m	M T−2	Scalar (just a number)

See also

• List of photometric quantities
• List of radiometric quantities