Magnetism

Electromagnetic Induction: The relationship between electricity and magnetism

What is electromagnetic induction?

Previously, we have learnt about the relation between magnetism and electric current. We have learnt that when a current carrying conductor is placed in a magnetic field, it experiences a force. What would happen if this conductor were moving or if the strength or direction of the magnetic field were to change? Michael Faraday, an English scientist, studied these problems initially and in 1831, postulated the principles of electromagnetic induction. Electromagnetic induction refers to the production of an electromotive force across a conductor exposed to varying amounts of magnetic fields.

Let us now do a small experiment to answer a few questions.

Take a coil of a vast number of turns. We will label one end of the coil as A and another end as B. Connect the two ends of the coil to a galvanometer. Now, take an active magnet and move the north pole of the magnet into the coil, towards the B end of the loop. What do you note?

electromagnetic induction

Image from NCERT Science Textbook

You will see that there is a momentary deflection of the galvanometer. Now, reverse the magnet and move the south pole of the magnet towards the B end of the coil. What do you note? You will note a deflection in the galvanometer. But, this time, the direction of the deflection would be opposite to the previous deflection. Therefore, we can conclude that movement of a magnet in respect of the coil induces the production of a current in the coil. Similar results can be got by moving the coil instead of the magnet.

Now let us set up another experiment.

Take two turns of coils. Let us name these coils as A and B. The coil?A and B will need to have a different number of turns. For the purpose of the experiment, we shall assume that coil A has 100 turns and coil B has 50 turns.We will also need a battery, one galvanometer and a core of non-conducting material (a paper or cardboard core).We will set up the equipment in such a way that both coil A and B are wound over the non-conducting core and are a few centimetres away from each other. We will now connect coil A to a battery and coil B to a galvanometer. What would happen when current flows through coil A?

We will note a momentary deflection of the galvanometer. Now, disconnect the battery from coil A. You will note another momentary deflection of the galvanometer. However, this time, the direction of deflection would be opposite to the previous instance. Can you explain the results of the above experiment?

electromagnetic induction

Image from NCERT Science Textbook

We have previously learnt that when current passes through a conductor, it produces a magnetic field around the wire. In the above experiment, as the current across coil A varies, the magnetic field around coil A also changes. The change in the magnetic field around coil A leads to a change in the magnetic field around coil B as well. Any change in the magnetic field around a conductor will induce production of a potential difference in the conductor and this leads to setting up of an electric current within the conductor.

The above experiments are simple demonstrations of the principle of electromagnetic induction.

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Solenoids- A versatile device with multiple uses

Solenoids are a coil of wire wound into a tight helix. The term Solenoid was coined by the French Physicist Andre-Marie Ampere. The term solenoid may have different meanings in different fields of science. In physics, a solenoid often refers to a tightly wound coil whose length is greater than that of the diameter of the coil. There is a core of soft iron inside the coil, and when current passes through the coil, it produces a uniform magnetic field around the solenoid. Here, the solenoid is working as an electromagnet, creating a controlled magnetic field.

solenoids

Image from Wikipedia

In engineering, the term solenoid usually refers to a transducer device that converts energy into motion (linear). The term solenoid may also refer to an integrated appliance- the solenoid valve of solenoid switch. In the above devices, a solenoid actuates a pneumatic or hydraulic valve or a switch, thus helping us control the device. Examples include the automobile starter solenoid and the solenoid bolts.

Types of Solenoids

There are two types of solenoids:-

  1. Infinite continuous solenoid

  2. Finite continuous solenoid

Infinite continuous solenoid

An infinite continuous solenoid, as the name implies has an infinite length, but a finite diameter. The word continuous here refers to a sheet of conducting material rather that separate coils.

Finite continuous solenoid

A finite continuous solenoid, in contrast with an infinite continuous solenoid, has a finite length and diameter. The term continuous here also refers to a sheet of conducting material rather than different coils.

Uses of solenoids

Solenoids have many uses in physics, engineering, and in everyday life. Following are some uses of solenoids:-

  1. Electromechanical solenoids– Electromechanical solenoids consists of a helical coil of wire with a movable core. The movable core or the armature is free to move in and out of the coil. The armature usually provides mechanical force to another object, for example, a pneumatic valve. The force applied to the armature depends on the inductance of the coil and the current flowing through the coil. Thus, by altering the current flowing through the coil, we can modulate the force applied to the armature. Some examples of electromechanical solenoids include electronic paintball markers, pinball machines, dot-matrix printers and fuel injection systems.

  2. Rotatory solenoids– A rotary solenoid is an electromechanical device that can rotate a ratchet mechanism when power is applied to the solenoid. Rotatory solenoids were first invented during WW II to control the release of bombs from aircraft accurately. However, they are still used in many modern devices.

  3. Pneumatic solenoid valve– A pneumatic solenoid valve is a switch that can route air into any pneumatic device.

  4. Hydraulic solenoid valve– The hydraulic solenoid valve is similar in construction to a pneumatic solenoid valve. The only difference is that these devices control the flow of hydraulic fluids instead of air.

  5. Automobile starter solenoids– As the name implies, these devices are used to start the car. Here, the solenoid receives a large current from the car battery and a small current from the ignition switch. When we turn the ignition switch, a low current pass to the solenoid and the starter solenoid closes a pair of heavy contacts, thus, allowing the larger current from the car battery to start the starter motor.

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What is a magnetic field?

Both electrical currents and magnetic materials produce a magnetic effect on them- a magnetic field. The magnetic field is a vector- it has both magnitude and direction.

In our previous articles on magnetism, we have learnt about the magnetic effects of electric current flowing through a straight wire. We also learned how to find the direction of the magnetic field. We also know that the strength of the magnetic field varies with the force of the electric current. In this article, we shall learn more about magnetic fields, with a particular focus on magnetic fields around a circular loops, coils and solenoids.

Let us do a small activity to find out about the magnetic field produced by a loop of wire. Take a straight wire and bend it to produce a circular loop. We have previously learned that the strength of the magnetic field is inversely proportional to the distance from the centre of the wire. Therefore, in the loop model, we will note that the magnetic field lines from each point on the loop converge at the centre of the loop to form a straight line. Using the right-hand thumb rule, we can infer that each point on the loop contributes to the magnetic field at the centre of the loop, and the direction of the field is the same.

The above method is a little-complicated way to find the direction of the magnetic field in a circular loop. There is a simpler method- the Maxwell?s corkscrew rule.

Imagine driving a corkscrew. If we drive in the direction of the current, then the direction of the corkscrew is the direction of the magnetic field.

What would happen to the magnetic field if we use many coils of the wire to make a circular loop? The direction of the magnetic field produced by each point on each coil would be the same. Therefore, according to vector physics, they would just add up. Therefore, more the number of coils, greater is the strength of the magnetic field.

magnetic field

Magnetic field through a solenoid

A solenoid is many turns of insulated copper wire wound in the form of a cylinder. The following diagram shows the direction of the magnetic fields through a current carrying solenoid. You can note that one end of the solenoid behaves like the magnetic north pole, and the other behaves like the south pole. In fact, the magnetic field produced by a solenoid is similar in all aspects with that of a bar magnet. This property is of great relevance in physics and everyday life. Using solenoids, we can now produce unyielding magnets that can be activated by switching on the current. . Such magnets are called as electromagnets.

Can you think of some uses for electromagnets?

Electromagnets are used in electric motors and generators. They are used in transportation- like subway cars. They are used to lift heavy loads- like an electromagnetic crane. They are also used in space crafts. The list of uses of electromagnets is endless.

 

Magnetic effects of electric current

The magnetic effects of electric current were first discovered by Hans Christian Oersted, a Danish physicist. Oersted, in 1820, discovered that a compass is deflected when an electric current is passed through a wire kept nearby. He initially postulated that magnetism radiates from all sides of the wire. However, after more experiments he concluded that electric current produces circular magnetics fields.

In the previous article on magnetism, we had performed an activity where we sprinkled iron filings over a sheet. What happens when the electric current passes through the wire? The iron filing organises themselves along the circular magnetic fields produced by the magnetic effects of electric current.

Let us do some more activities to understand the relationship between electricity and magnetism.

magnetic effects of current

Direction of magnetic fields

We will need a straight copper wire, two or three cells and a plug key. We will first connect these in a series so that the copper wire is directed along the north-south axis. Next, we will place a compass over the copper wire, keeping the needle parallel to the copper wire. Now, what happens when current is passed through the copper wire? The compass needle deflects. Now, what happens when the direction of the current is changed? The direction of the deflection of the needle is in the opposite direction. Why does this happen?

The change in the direction of deflection can be explained by the right-hand thumb rule. Imagine that you are holding the current carrying wire in your hand in such a manner that the thumb points towards the direction of the current and the fingers are wrapped around the wire. The direction of the fingers would depict the direction of the magnetic fields.

The right hand thumb rule

The right hand thumb rule

Now, let us get back to our activity. We will modify our experiment to include a rheostat to control the flow of the current. What happens when we increase or decrease the flow of current through the wire? We will note that the deflection of the compass increases progressively as the strength of the current increases. Thus, the magnetic effects of electric current are directly proportional to the force of the current.

What happens when the compass is moved away from the copper wire? We will note that the deflection of the compass progressively decreases when the compass is moved away from the copper wire. Therefore, we can conclude that the magnetic effects of electric current reduce in intensity as we move farther away from the current-carrying wire.

So what have we learnt about the magnetic effects of electric current in this article?

  1. An electric current passing through a conductor produces a circular magnetic field around the wire.
  2. The direction of the magnetic field can be deduced by the right-hand thumb rule.
  3. The strength of the magnetic field increases when the force of the current passing through the conductor.
  4. The strength of the magnetic field decreases as we move away from the centre of the conducting core.

What is magnetism?

What is magnetism? Can you think of a few uses of magnetism in everyday life?

Magnetism is a physical a phenomena mediated by magnetic fields and forces. Magnets and magnetism play a significant role in our lives. Magnets form the core component of many machines, starting from the simplest motor to the most complicated power generation equipment. Magnets have also been a navigational aid for many millennia. In fact, many of the modern inventions like the radio, TV, and mobiles would not be possible without magnetism.

Magnetism is mediated by magnetic fields. So, what is a magnetic field? Let us do a simple experiment to study magnetic fields.

Take a sheet of paper and place a bar magnet over the centre of the sheet. Now, sprinkle some iron filings uniformly across the sheet of paper. Now, gently tap the paper. What do you see?

You will note that the iron filings have neatly rearranged themselves. If one connects these iron filings in a line, you will note that the lines goes from one end of the magnet to another. These lines are called as magnetic fields.

Magnetic fields have both direction and magnitude. By convention, magnetic fields emerge from the North Pole and enter the South Pole. Within the magnet, these lines move from the South to the North Pole. Thus, a magnetic field is a closed loop. The magnitude of the magnetic field denotes the strength of the field. The magnetic field is strongest in those regions where the magnetic field lines converge. Therefore, the magnetic field is strongest at the poles.

Sources of Magnetism

There are two sources of magnetism:-

  • Electrical Current
  • Magnetic moments of elementary particles

An electric current passing through a conductor produces a magnetic field around that wire. We will learn about the magnetic effects of electricity in another article.

We have previously determined that all elements are made up of a central nucleus around which electrons are in an orbit. These electrons are always spinning around the nucleus. Also, they are constantly rotating around their own axis, similar to the earth?s rotation around its axis. The spinning produces a magnetic field. Most of the times, the electrons are arranged in such a way that the magnetic field produced by different electrons is cancelled out, leaving the element non-magnetic. However, when a magnetic substance enters a magnetic field, the electrons line up along the field lines, producing a net magnetic field.

Do you know that the earth?behaves like a magnet! Due to earth?s rotation around its axis, a magnetic field is set up. The tip of a compass would line up along the Earth magnetic field and point towards the magnetic north. The magnetic and geographic north are different points on earth. The geographic north corresponds with the North Pole. The magnetic north keeps changing due to changes in the earth?s magnetism. In 2001, it was located at?81.3?N 110.8?W. In 2015, it is projected to be located at 86.3?N 160.0?W. The magnetic north is moving towards Russia at a speed of 55-60 miles/year.

Magnetism has a myriad use in our life. We will learn more about the uses of magnetism in further articles.

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