Electricity

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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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.

How does the electric motor work?

The electric motor is ubiquitous in everyday life. Most equipment used in our homes and offices require the presence of an electric motor. Some examples include washing machines, dishwashers, and air conditioners. Even computers and printers require an electric motor to function. So, what is an electric motor?

In the previous articles on energy, we have learned that energy can neither be created nor destroyed. It can merely be transformed from one form to another. An electric motor is a device that converts electrical energy into mechanical energy.

How many of you know how an electric motor works?

When electric current flows through a conductor suspended in a magnetic field, it experiences a force. The direction of the force can be illustrated by a simple rule called the Fleming?s left-hand rule. The Fleming?s law states that the direction of the current, magnetic field and force are perpendicular to each other. Thus, if we stretch the thumb, index and middle finger of the left hand so that they are roughly perpendicular to each other. Then, the index finger will point in the direction of the magnetic field. The middle finger will point in the direction of the electric current, and the thumb will point in the direction of the force being experienced by the conductor. An electric motor uses this principle to convert electrical energy into mechanical energy.

The central component of an electric motor is a rectangular coil of insulated copper wire. The coil is sandwiched between two strong magnets. Previously, electric motors used natural magnets, but nowadays, electro-magnets are commonly used. The accompanying picture schematically depicts an electric motor.

Electric Motor

Schematic diagram of an electric motor

In this diagram, current enters the coil ABCD through a conducting brush ?X?. In arms AB, the current flows from A to B. The current flows in the opposite direction in arms CD (C to D).

On applying the Fleming?s rule, we will note that the arm AB is pushed down while the arm CD is pushed up. ?After half a rotation, the direction of the current is reversed?in the coil, and the arm CD is now pushed down while the arm AB is pushed up. Thus, the coil ABCD completes a full rotation.

In the accompanying diagram, you note that there is a split ring around the axle. The split ring functions to reverse the direction of the current every half rotation. The split ring is also called a commutator.

Modern electric motors have an enormous number of turns in the coil. Also, the coil is would over a central core made of soft iron. The soft iron core is called an armature. These modifications enhance the power output of an electric motor.

Thus, we can convert electrical energy into mechanical energy by using the magnetic properties of electric current. The reverse can also be done, i.e. mechanical energy can be converted into electrical energy. The machine that does this job is called an electric generator. We will learn about the electric generator in another article.

You can read more articles on electricity and magnetism in our class 10 science notes.

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Electric Potential

Electric potential is the potential energy stored in an electric charge. Let us start with an analogy. Water flows in the taps only because the valve is connected to an overhead tank. By virtue of its location, the water in the reservoir has potential energy. On opening the valve, the water in the reservoir that is at high pressure flows to a low-pressure region, i.e., the tap. Water will not flow if the tank is at the same level as the tap. Electric potential also works on a similar principle.

A positive charge (source charge) anywhere in space will exert a repulsive force on another positive charge (test charge) in its vicinity. Work will have to be done to move a test charge against the repulsive strength of the source charge. The work required to move a test charge from point A to B would depend on the quantity of charge and the initial and final position of the particle. The path taken by the test charge will not influence the amount of work done. Therefore, electric potential is a conservative force.

The SI unit of electric potential is Volt (V). Alessandro Volta was an Italian physicist after whom the unit of electric potential is named. By definition, when 1 Joule (J) of work is required to move a charge of 1 Coulomb (C), the electric potential is taken as 1 Volt (V). The following formula will represent the relationship between electric potential, work and charge:-

1 Volt= 1 Joule/1 Coulomb

1 V = 1 JC-1????????????????????

Let us take another example. The batteries we use in remotes and other electronic equipment are graded in Volts. A standard AA battery is 1.5 Volts. What does this mean?

In an AA battery, 1 Coulomb of charge moving from the negative to the positive terminal will be capable of doing work equivalent to 1.5 Joules. Now there are different types of AA batteries. A Duracell and ordinary alkaline battery have the same voltage. A Duracell lasts much longer. How is this possible? 1 C of charge moving from the negative to the positive terminal does work equivalent to 1.5 J in both batteries. ?However, a Duracell carries more charges than a regular battery. Therefore, it lasts longer, but the rate of work is the same as that of a standard AA battery.

Let us now talk about something that we all use routinely- mobile phones. How many ways are there to charge a cell phone? We could connect it to an electric socket, or use a car charger or connect it to our computers by using a USB cable. Have you noted how much time does it take to charge a mobile entirely using the above methods? Try it home and you find an interesting fact. Plugging your mobile into an electric socket is the quickest way to charge the mobile. A car charger takes more time and charging through a computer is the slowest. Why does it take more time for a car charger to charge a mobile?

The car charger is marked 12 V and the electric sockets in our houses are 240 V. What does this mean? It means that 1 C of charge moving from the negative to the positive terminal will do work equivalent to 12 J and 240J respectively. Therefore, an electric socket can charge a mobile ten times faster than a car charger.

To conclude, electric potential is the potential energy stored in an electric charge. The electric potential difference is the difference in the electric potential between two points in a circuit. We discuss the potential difference in another article.

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What is Electricity?

Have you ever wondered why our lives are so dependent on electricity?? We use electricity in nearly every sphere of our life. You are reading this article on a machine powered by electricity. The fans and air conditioners that keep us fresh run on electricity. The heaters that keep us warm also run on electric power. We use electric power to cook, light bulbs, watch TV and even run our vehicles. So, imagine how our life would be without electric power!

So, what makes electric power so popular?

Electric power is a clean, convenient and easily controllable form of energy. Electricity does not emit carbon dioxide or smoke (though the power plants that produce electricity emit smoke and other pollutants). It can move through wires along great distances (even thousands of kilometres) with very little loss during transport. It is easy to control, even huge electric substations are controlled by flipping a simple switch. And most importantly, it is versatile. A range of appliances can run on electricity- TV, music systems, motors, vehicles, heaters, etc.

So, what exactly is an electric current?

The flow of electric charges is known as electricity. We now know that electricity works by the flow of electrons through a wire. However, the phenomenon of electricity was discovered long before electrons. Therefore, for many centuries, electricity was considered to represent the flow of positive charges, and thus began the practice of labelling the electrodes as positive and negative. Conventionally, the direction of the flow of positive charges is the direction of the electricity. Electrons carry a negative charge. Therefore, their direction of movement is opposite that of the direction of the current.

Take a torch for example. It contains a bulb, a switch and the batteries. If you have seen a battery, you would have noticed that one end is flat, and the other end has a knob. The flat end is the negative electrode and the one with the knob is the positive terminal. When the switch is flipped on, it completes the circuit. The electrons then flow through the filament in the bulb, producing light. This same principle applies to most other appliances.

Can the electric charge be measured?

Coulomb (C) is the measure of electric charge. One Coulomb is the amount of charge present in 6 x 1018 electrons.

Can Electric current be measured?

Ampere is the measure of electric current. This SI unit is named after a French scientist, Andre-Marie Ampere. One Ampere is one Coulomb of charge flowing per second. An ammeter is an instrument that measures the electric current. This device is connected in series, and when electric current flows through this device, it can measure the amount of electric current flowing through the circuit.

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