Magnetic Hysteresis: Loop, loss, importance, Factors Affecting

Magnetic Hysteresis

When we take a piece of magnetic material—like iron—and magnetise it first in one direction, and then in the opposite direction, something interesting happens. The magnetic strength inside the material does not follow the applied force exactly. Instead, it lags behind. This delay is called Magnetic Hysteresis.

🔄 What is Magnetic Hysteresis?

Magnetic hysteresis is the lagging of magnetic flux density (B) behind the magnetising force (H) when a material goes through repeated cycles of magnetisation.

In even simpler terms:
When you turn a magnet on and off in a certain direction, and then reverse it, the material doesn’t immediately forget the previous magnetisation. It holds on to some of it for a while. This memory or lag is what we call hysteresis.

The word “hysteresis” comes from a Greek word that means “to be behind“—and that’s exactly what the magnetic flux does. It stays behind the applied force.

Hysteresis Loop – Explained in Simple Words

Let’s imagine a plain iron bar named AB. We wrap a wire coil around it. When we pass an electric current through this coil, it creates a magnetic field inside the iron bar. This magnetic force is called magnetising force (H). By increasing or decreasing the current, we can control how strong this force is.

What is a Hysteresis Loop?

The full path traced by B as H goes through one complete cycle of change — increasing, decreasing, reversing, and then returning — forms a loop on the graph. This loop is called the hysteresis loop.

It shows how magnetism in a material lags behind the magnetic force applied. That’s why it’s called “hysteresis”, which means delay or lag.

Now, what happens inside the iron?

As we slowly increase the current, the iron starts to become magnetic. The magnetic strength inside it is called magnetic flux density (B). At first, B increases as H increases. The more current we pass, the more magnetic the iron becomes — up to a certain point. This is called magnetic saturation. After this point, even if we increase the current, the iron won’t become more magnetic.

This first part of the journey is shown as a rising curve from point O to A on the graph.

What Happens When We Reduce the Current?

If we now slowly reduce the current, something interesting happens. The magnetism inside the iron doesn’t drop in the same way it rose. It follows a new path from A to B. At point B, we stop the current completely — so H becomes zero. But surprisingly, the iron still keeps some magnetism! This leftover magnetism is called residual magnetism or retentivity. It means the iron has a “memory” of being magnetized.

The point B shows that even without any current, B is not zero.

Reversing the Current

Next, we reverse the direction of current. This creates an opposite magnetic force. As we increase this reversed force, the magnetism in the iron starts to cancel out. At a certain value of H, the total magnetism becomes zero. This point is called the coercive force (point C on the graph). But we can keep going and reach a new saturation point in the opposite direction (point D).

Then, if we reduce the current again and return back to zero, the iron still keeps some magnetism — but in the opposite direction (point E). Finally, by reversing the current once again, we bring the iron back to its original state (point F), and the loop closes at point A.

Removing Residual Magnetism (Demagnetisation)

To remove the remaining magnetism from an iron piece, we reverse the current flowing through the coil around it. This also reverses the magnetising force (H). As we slowly increase this force in the opposite direction, the magnetic effect (called flux density or B) decreases along a new path on the graph. Eventually, it reaches a point where all the magnetism is gone. This exact value of force needed to make the magnetism zero is called the coercive force (Hc).

2. What Happens If We Keep Increasing the Force?

If we keep increasing the magnetising force in the reverse direction, the iron gets magnetised again — but in the opposite way. This continues until the iron becomes fully magnetised in the reverse direction. At this point, the material cannot hold any more magnetism in that direction. We call this the saturation point in the reverse direction.

3. Bringing the Force Back to Zero

Now, if we reduce the force slowly back to zero, the magnetism doesn’t become zero immediately. Even when the force is gone, the iron still holds some magnetism — but now in the opposite direction. This is the new residual magnetism, and it’s shown on the graph as point ‘e’.

4. Re-Magnetising in the Original Direction

To cancel out this reverse magnetism, we apply the force again — this time in the original (positive) direction. When this positive force reaches the value of Hc again, the magnetic effect becomes zero. This point is shown on the graph as ‘f’. If we keep increasing the force beyond this, the material again reaches its original magnetic strength. The entire process draws a loop on the graph, known as the hysteresis loop.

Hysteresis Loss – Simple Explanation

What is Hysteresis Loss?

When we take a magnetic material, like iron, and keep changing its magnetism—first in one direction and then in the opposite—it doesn’t like it! Inside the material, there are tiny magnetic parts called domains. These domains don’t easily flip back and forth. They resist the change.

Just like it takes energy to push a swing back and forth, it takes energy to force these domains to turn again and again. This energy is used up inside the material and turns into heat. This heating effect is called hysteresis loss.

🔥 Why Does Hysteresis Loss Happen?

Hysteresis loss happens because of molecular friction—a kind of invisible resistance inside the material. When the magnetic direction keeps changing, the material has to keep adjusting, and this makes it heat up.

⚙️ Where Do We See Hysteresis Loss?

1. In Transformers and Motors

Transformers and most electric motors run on AC current. That means the magnetic flow (called flux) is always changing direction. So, these machines always have hysteresis loss.
The result? Heat is produced, and the machine gets hot if not controlled.

2. In DC Machines

Even in some DC machines, if a part made of iron spins inside a magnetic field, it faces hysteresis loss too—even if the field is steady.

💡 Why Is Hysteresis Loss Important?

• It causes heat, which can damage machines.
• It wastes energy, which makes machines less efficient.
• That’s why we try to reduce hysteresis loss by using special materials like silicon steel in machines.

Calculation of Hysteresis Loss – Explained Simply

When we magnetise a piece of iron again and again, a little energy is lost in every cycle. This energy turns into heat inside the material. This lost energy is called hysteresis loss.

Let’s understand this step by step.

🌟 What is Hysteresis Loss?

Imagine a soft iron bar. When you pass current through a coil wrapped around it, the bar becomes magnetised. If you increase and decrease the current again and again, the magnetism also goes up and down.

But — here’s the interesting part — the magnetism doesn’t follow the current perfectly. It lags behind. This lag creates a loop in the graph of magnetic field strength (H) vs magnetic flux density (B). That loop is called the hysteresis loop.

The area inside this loop tells us how much energy is lost as heat in one full cycle of magnetising and demagnetising the material.

📐 Let’s Do a Simple Calculation

Let’s say:

• l = length of the iron bar
• A = area of cross-section of the bar
• N = number of turns in the coil
• i = current at any time
• H = magnetising force = (N × i) / l
• B = magnetic flux density
• V = volume of the iron bar = A × l

When the current changes a little bit, say by di in a tiny time dt, the magnetic field also changes by dB.

This change creates a small voltage (called induced e.m.f.) in the coil. This e.m.f. tries to oppose the change in current — that’s nature’s way of resisting change (Lenz’s Law).

To keep the current flowing, we have to work against this e.m.f., which uses up energy. That small energy is:

dW = e × i × dt

After some simple steps, this becomes:

dW = V × H × dB

This means — in a small step — energy is used up in the material.

When we go through one full cycle, the total energy lost is:

🔻 Energy lost per cycle = Volume × Area of the hysteresis loop

And if the cycle is repeated f times every second (frequency):

🔥 Power lost due to hysteresis = Volume × Area of loop × Frequency

This lost power is converted into heat inside the material — that’s why machines with magnetic parts get hot.

📏 What If the Graph is on Paper?

When you draw the hysteresis loop on graph paper, you use scales.

Let’s say:

• 1 cm on horizontal axis = x units of H (magnetising force)
• 1 cm on vertical axis = y units of B (flux density)

Then:

📊 Energy lost per cycle = x × y × (Area of loop in cm²) × Volume

Make sure to use the right scale values when calculating!

Factors Affecting the Shape and Size of Hysteresis Loop

The hysteresis loop is a very important concept in magnetism. It shows how a magnetic material behaves when it is magnetised and demagnetised. But have you ever wondered why the shape and size of this loop can look different? There are three main factors that affect the shape and size of a hysteresis loop. Let’s understand them one by one in a very simple way.

1. The Type of Material

The first and most important factor is the material used.

• Some materials are easily magnetised. That means they allow the magnetic field to pass through them quickly. In this case, the loop is narrow and thin.
• Other materials are hard to magnetise. They resist the magnetic field and need more effort to get magnetised. For these, the loop becomes wider and bigger.

Also, different materials reach saturation (the point where they can’t be magnetised more) at different levels. So, the height of the loop also changes based on the material.

2. The Maximum Magnetic Strength (Flux Density)

The second factor is how strong the magnetic field is that we apply to the material.

• If the magnetic field is strong, the loop becomes bigger.
• If the field is weak, the loop stays small.

This means that the more powerfully we magnetise a material, the larger the loop area becomes. It’s like stretching a rubber band — the more you pull, the bigger it gets.

3. The Starting Condition of the Material

The third factor is the initial state of the material — how the material was before we started magnetising it.

• If the material was already fully magnetised or saturated before starting, the loop will be different.
• If it started in a neutral or zero-magnetism state, the loop will take a different shape.

This means the history of the material also matters. A magnetised material will behave differently from one that was not magnetised at all.

Importance of the Hysteresis Loop

The hysteresis loop tells us how a magnetic material behaves when it is magnetized and demagnetized again and again. The shape and size of this loop are very important. They help us choose the right magnetic material for different electrical devices.

Let’s understand this with simple examples:

1. Small Loop Means Less Energy Loss

If a magnetic material has a small hysteresis loop, it means it loses very little energy when the magnetism changes direction again and again. This is called hysteresis loss, and we always want it to be low in machines like transformers and motors.

One such material is silicon steel. Its loop is very narrow. That’s why silicon steel is commonly used to make transformer cores and rotating machines. These machines need to work fast and often, so low energy loss is important.

2. Big Loop Means Strong Permanent Magnets

Some materials have a big hysteresis loop. This means they can hold magnetism for a long time. We call this property high retentivity. They also need a strong reverse force to remove the magnetism, which is called high coercivity.

Hard steel is one such material. It keeps its magnetism well, so it is perfect for making permanent magnets. However, because the loop is large, the energy loss is also high. So, it is not good for transformers or motors.

3. Medium Loop for Electromagnets

Wrought iron has a hysteresis loop that is not too big and not too small. It has good residual magnetism and medium coercivity, which means it can become magnetised easily and lose its magnetism when needed.

This makes wrought iron a good choice for electromagnet cores, where we turn magnetism on and off again and again.

Applications of Ferromagnetic Materials

(Iron, Steel, Nickel, Cobalt & More)

Ferromagnetic materials are special kinds of materials that get strongly attracted to magnets. Some common examples are iron, steel, nickel, and cobalt. These materials play a very important role in our daily life and in many machines.

But did you know? Not all ferromagnetic materials are the same. Some are soft, like soft iron, and some are hard, like steel. Their use depends on their magnetic properties, such as:

• Retentivity – how well the material holds magnetism
• Coercivity – how hard it is to remove the magnetism
• Hysteresis loop area – how much energy is lost when magnetism is turned on and off

Why Do We Use Ferromagnetic Materials?

Ferromagnetic materials are chosen based on how they behave with a magnet. Some of them can get magnetized easily and also lose their magnetism quickly — these are called soft magnetic materials (like soft iron). Others can stay magnetized for a long time — these are called hard magnetic materials (like steel).

The choice between soft and hard ferromagnetic materials depends on three important magnetic properties:

1. Retentivity – How well a material can keep its magnetism
2. Coercivity – How much effort is needed to remove magnetism
3. Hysteresis Loop Area – This tells us how much energy is lost when the material is magnetized again and again

We also look at:

• Maximum magnetic strength the material can hold (flux density)
• The starting magnetic condition of the material (whether it’s already magnetized or not)

Where Are Ferromagnetic Materials Used?

🔹 Electric Motors and Generators

Soft iron is often used in the core of motors and generators because it gets magnetized quickly and loses magnetism easily — this helps the machine work smoothly.

🔹 Transformers

Transformers use soft magnetic materials so they can transfer energy without wasting too much in the form of heat.

🔹 Permanent Magnets

Hard magnetic materials like steel or special alloys are used to make permanent magnets. These magnets are found in speakers, toys, and fridge doors.

🔹 Magnetic Storage Devices

Ferromagnetic materials are used in computers and credit cards to store information using tiny magnets.

🔹 Relays and Switches

Soft iron is used in electrical switches and relays because it responds quickly to electric current and stops as soon as the current is turned off.

Soft vs Hard Ferromagnetic Materials

1. Permanent Magnets – Always Magnetic

Permanent magnets are made from hard ferromagnetic materials like steel, cobalt steel, or carbon steel. These materials are called “hard” not because they are difficult to touch, but because they keep their magnetism for a long time.

• They have high retentivity, which means once they become magnets, they stay strong.
• They also have high coercivity, which means it is hard for them to lose their magnetism even if other magnetic fields are nearby.

That’s why permanent magnets are great for things like fridge magnets, speakers, and motors. They stay magnetic all the time.

2. Electromagnets – Magnetic Only When Needed

Electromagnets, also called temporary magnets, are made from soft ferromagnetic materials like soft iron.

• These materials have low coercivity, which means they can lose magnetism easily when the electric current is turned off.
• But they also have high saturation, which means they can become very strong magnets when current is passed through them.

Electromagnets are used in cranes, electric bells, and many machines where we want the magnet to work only when needed.

3. Transformer Cores – Smart Use of Soft Materials

Transformers are used to increase or decrease voltage in electric circuits. Inside a transformer, there is a core made from soft ferromagnetic materials. But why soft?

• When a transformer works, its core is magnetised and demagnetised again and again.
• Each time this happens, some energy is lost in the form of heat. This is called hysteresis loss.
• Soft magnetic materials have a small hysteresis loop, which means less energy is lost.

That’s why soft materials are perfect for transformer cores. They save energy and keep the transformer efficient.

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