It does this using the energy that is built up within the inductor to slow down and oppose changing current levels.

But, how does an inductor store energy? An Inductor stores magnetic energy in the form of a magnetic field. It converts electrical energy into magnetic energy which is stored within its magnetic field. It is composed of a wire that is coiled around a core and when current flows through the wire, a magnetic field is generated.

This article shall take a deeper look at the theory of how energy is stored in an inductor in the form of a magnetic field. 

Energy, Magnetic fields and Inductors  

Before looking at how an inductor stores energy, we will need to take a couple steps back and learn a little bit about energy.

You might be aware of the phrase; “Energy cannot be created or destroyed, only changed from one form to another.”

This is the first law of Thermodynamics.

The universe has a constant amount of energy that just changes from one form to another. 

Types of energy include;

• Mechanical
• Thermal
• Nuclear
• Chemical
• Electrical
• Magnetic
• Electromagnetic

The main type of energy that we shall be concerned about for this article is Magnetic energy which is found within a Magnetic Field.

How magnetic fields are produced

We have all witnessed the awesome powers of a magnet as it attracts metal objects without the need of physical touch.

But, what exactly gives magnets their awesome abilities?

This comes down to the fact that all matter is made of Atoms which have a nucleus that contains protons and neutrons. 

Orbiting the nucleus on the outside are electrons (which are tiny moving charges).

As these electrons move around the nucleus they create a magnet field around each atom. 

Each atom’s magnet field will have a certain orientation which is known as its Moment. If you were able to look at an object at a subatomic level you would see that every atom resembles a tiny magnet.

However, for an object to be magnetic, it needs to have a large magnetization which is created when all the moments (orientation of an atom) within an object are aligned. 

The majority of objects (like a piece of wood) have their moments misaligned so the net magnetization is zero. 

When the net magnetization of an object is large enough that it creates a substantial magnetic field around it and can be considered to be magnetic. 

How magnetic fields are generated in a wire

We just saw that all atoms have a magnetic field. 

But, there are other instances where a magnetic field can be generated. 

A magnetic field is generated around a wire when a current flows through that wire. A circular magnetic field will surround it.

The strength of the magnetic field is directly proportional to the current. So, increasing the current will increase the magnitude of the magnetic field.

Also, the strength of the magnetic field is strongest closest to the wire. An inductor makes great use of this as we shall soon learn.

What is an inductor?

Now let’s take a brief look at an Inductor which will further help us understand how it stores energy.

An inductor is a two terminal passive component which has the ability to store energy in the form of a magnetic field when current flows through it.

The main purpose of an Inductor is to oppose any sudden changes in current. 

They slow down current spikes and surges by storing this extra energy in their magnetic field and then slowly releasing it back into the circuit.

Its ability to resist this change can be shown by its Inductance which is the ratio of voltage and current change within the inductor. 

Inductance is given in the unit of Henry (H). 

Construction and working principle of an Inductor

If you were to look at a circuit schematic which had an inductor, you would see a symbol as seen below. 

The construction of a basic Inductor involves a wire that is coiled around a core material. 

This core material can vary depending on the needs of the application and can include either magnetic iron or ferrite core (amongst the most common).

An insulated copper wire is the choice of material for the wire wrapped around the core. 

There are many variables that can alter the inductance of an inductor which include; number of turns (of the wire), spacing between turns, number of layers of turns, material of core, magnetic permeability of core material, size, and shape. 

So how does an inductor work?

As current starts to pass through the inductor, a magnetic field is generated. The relationship between current and the strength of the magnetic field are directly proportional.

So, an increase in current will see an increase in the strength of the magnetic field.

How energy is stored in an inductor

Now that we have learnt about magnetic energy in magnetic fields, magnetic fields around a wire, and a little bit about inductors, we can take a look at how energy is stored in an inductor.

As mentioned earlier, energy is never created or destroyed, just changed from one form to another.

There are some components (such as a resistor), that just dissipate energy (in the form of heat) when current flows through them. 

However, an inductor is a type of passive electronic component that is capable of converting kinetic energy (flow of electrons) and storing it in its magnetic field which is generated. 

When current flows through a wire a magnetic field is generated around that wire. An energy is stored within that magnetic field in the form of magnetic energy.

An inductor utilises this concept. It consists of wire wrapped in a coil formation around a central core.

This means that when current flows through the inductor, a magnetic field is generated within the inductor.

So how does an inductor store energy?

An inductor stores magnetic energy in the form of a magnetic field. So it converts electrical energy (flow of electrons) into magnetic energy (stored in the magnetic field). 

How much energy is the inductor capable of storing?

As you can imagine, the amount of energy stored in the magnetic field of a straight wire is going to be far less compared to that of a wire that has been coiled.

This is due to the fact the magnetic field (and therefore magnetic energy) is increased as a straight wire is coiled.

The strength of a magnetic field around a straight and coiled wire is known as magnetic field strength or H.

Below are the two formulas for the H in a straight and coiled wire.

H – strength of magnetic field ampere/turn

N – number of turns of coil

I – current flowing through in Ampere (A)

L – length of coil (in metres)

By looking at the equation for a straight wire you can see that the only way to increase the magnetic strength is to increase the amount of current, or material of wire.

However, when it comes to a coil, you have many more options to increase magnetic strength which include;

• Number of turns contained within the coil
• How much current is flowing 
• Type of core material used for the inductor
• Cross sectional area of wire

As you can imagine, inductors make use of this fact in how they are constructed. When you wrap a wire in a coil formation, you increase the strength of the magnetic and therefore increase the amount of energy it can store as well. 
To know the exact strength of an inductor’s magnetic field (and how much energy it stores), you will need to use the formula above and know the values of the variables N, I and L.

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Of the extensive list of components, the Capacitor is one of the most utilised.

It is an essential part of many circuits, and I would be surprised if I were to stumble upon one that does not contain a capacitor. 

The capacitor is very versatile and has many different applications which include Power Conditioning, Power factor correction, Signal coupling, Decoupling and much more.

Another common application of a capacitor is Energy storage.

But, does a capacitor store energy in the form of a magnetic field?

No, a capacitor does not store energy in the form of a magnetic field. Energy storage in a capacitor is in the form of an Electric Field which is contained between the two conducting plates within the housing of the capacitor.

How a capacitor stores energy in the form of an electric field

So, we now know that energy in a capacitor is not stored in the form of a magnetic field, but rather an electric one.

Let’s dive deeper into how a capacitor is constructed and how it stores its energy. 

Capacitors come in a variety of shapes, sizes, material used, etc. But the basic construction and operation from one capacitor to the next stays the same. 

It typically contains two parallel conducting plates which are separated by a dielectric.

When power is applied to the capacitor, an electric charge is generated between the two plates. This process causes one plate to be positive and the other negative. 

Fundamentals of how electric fields are formed

But, why does an electric field form?

To understand why, let’s take a closer look at the Electric Field. 

If we had the ability to view an electric field, you would see that it is a physical field that surrounds an electrically charged particle ( both positive and negative)

Just like a magnet creates a Magnetic Field that cannot be seen, an electrically charged particle produces an Electric Field that cannot be seen.  

Also, just like magnets, charges of the same polarity repel each other, while opposite charges attract each other. 

In the diagram you can see that electric fields radiate away from positive charges, and towards negative charges.

The electric field inside a capacitor

The simplest form of a capacitor is two metal plates separated by a dielectric as we saw earlier.

When a voltage is applied to a capacitor, an electron is added to one plate making it negatively charged.

This electron has an electric field which repels other electrons. It travels through the space between the plates and bumps the electrons off the other plate making it positively charged.

As the process increases, the amount of electrons added to the first plate increases thus increasing its negative charge, which in turn increases the induced positive charge on the second plate. 

Eventually, the amount of negative charge on the first plate is going to reach a maximum value which will prevent the battery from adding more electrons.

The capacitor is said to be fully charged at this point, and its electric field will be at its strongest. 

Since electric fields radiate away from positive charges and towards negative ones, an electric field will point across the gap between the two metal plates in straight lines. 

How much energy is stored within the electric field of a capacitor?

When a capacitor is connected to a power source (like a battery), it stores the received energy in the form of the electric field which we have just discussed. 

The amount of energy stored in a capacitor’s electric field comes down to a singular formula and a couple of variables. 

Without going into too much detail of its derivation, below is the formula used to calculate the amount of energy stored within a capacitor’s electric field. 

W represents energy, C is capacitance and V is voltage. 

Let’s take a look at how capacitance affects the magnitude of a capacitor’s electric field. 

Capacitance is a crucial part of a capacitor which determines its ability to store electrical energy in an electric field.

As you just saw before, when a voltage is applied to a capacitor, a fixed amount of positive (q+) and negative (q-) charges build up on either plate of the capacitor. 

Using the value of voltage (V) and total amount of charge (q), we can calculate the total capacitance (C). 

Different types of capacitor

In this article we have been discussing the most basic type of capacitor, known as a parallel-plate capacitor whose dielectric is just  a vacuum (air). 

But, there are many different types of capacitor available. Below is a list of the most common;

• Ceramic 
• Film and Paper
• Aluminum, Tantalum and Niobium 
• Polymer
• Silver mica, glass

Capacitors are usually named after the material used for their dielectric.

Each of these capacitors will vary in either size, dielectric, material used for anode (positive plate), and construction of cathode (negative plate). 

Things like the area of the plates, distance between the plates, type of dielectric used, dielectric constant, etc, all play a role in determining a capacitor’s total capacitance. 

Changing the capacitance will directly impact the amount of energy that will be stored in the capacitor’s electric field. 

From the equation above we can deduce that the higher the capacitance, the higher the energy storage capability for that capacitor and vice versa.

Is the energy stored in the capacitor permanent?

No, the energy stored in a capacitor’s electric field is not permanent. 

When the capacitor is connected to a battery, charges build up on both plates until it reaches a maximum value.

This does not happen instantaneously but takes a certain amount of time which is known as the Rate of Charge. 

Now, if we disconnect the voltage source that charged the capacitor, the capacitor is going to discharge the charge (or energy).

This does not happen instantaneously either, but over time. 

This is known as Discharge rate, which defines the time it takes for a capacitor to release all its charge and voltage.

So, the energy in a capacitor’s electric field stays at a constant level as long as there is a voltage present. When voltage is removed, the energy will drain over time. 

What component stores energy in the form of a magnetic field?

Ok, so a capacitor stores energy in the form of an electric field. 

But, is there a component that stores energy in the form of a magnetic field?

Yes, an Inductor is a passive electrical component that stores its energy in the form of a magnetic field when subject to a current. 

They are also referred to as a Coil, Choke, or Reactor.

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