Electromagnetism

Introduction

Electromagnetism is a branch of physics that studies the interactions between electric and magnetic fields. A field is a change in the space around a body. An electric field is produced as a charged body influences (by attracting or repelling) any other charged body near it. A magnetic field is produced by magnetic and moving, charged objects. They influence other magnetic and charged objects around them. 

When charges flow (current passes) through a wire, they produce electric as well as magnetic fields. These form an electromagnetic field which transmits electromagnetic waves into space. 

The earth also has an electromagnetic field produced in the outer core. It is composed of liquid iron and nickel. Within the outer core the temperature ranges from 4400°C in the outer regions to 6000 °C near the inner regions. 

The differences in temperature, pressure and composition within the outer core cause convection currents (cool, dense fluid sinks while warm, less dense fluid rises). This flow of liquid metal generates electric currents (metals have almost free electrons), which in turn produce magnetic fields. Thus the earth is like a giant electromagnet: a temporary magnet whose magnetic field is produced as a result of current.

The beautiful phenomena of northern lights (Aurora Borealis) and southern lights (Aurora Australis) are credited to the earth’s magnetic field. The magnetic field of the earth protects us from harmful solar storms (eruptions of charged gas particles) by deflecting them. 

When their intensity and number increases during solar maximum (period of greatest solar activity), the magnetic fields of the charged gas particles interact with the magnetic field of earth. Thus, the gas is first funneled down the earth’s magnetic field and then stretched on the other side, displacing the gas particles with it. While the earth’s magnetic field is coming back to its original place, some gas particles also come back with it to the poles (as shown in the images). 

From here the charged gas particles enter the atmosphere and collide with atoms of different elements, transferring energy or charge to them which is released as light. 

What about the colours? Green light, produced by oxygen atoms, is the main colour seen as oxygen takes longer to release energy than other elements, causing green light to be seen for more time. Pink, purple and blue are produced by nitrogen, red by oxygen at higher altitudes and yellow when blue and green lights mix. 

 

Magnetic Effect of Electric Current

Oersted’s experiment  

Current (moving charge) produces a magnetic field around it. This is called the magnetic effect of electric current. Hans Oersted discovered this phenomena in 1820 by accident when he happened to keep a compass next to a closed circuit. He noticed a deflection in the compass from the north-south direction (along the earth’s magnetic field). This could only happen in the presence of another magnetic field, implying that the current flowing in the wire was producing a magnetic field. This discovery was a breakthrough into a new branch of physics, electromagnetism, that runs our world today. 

The direction of deflection of the compass needle depends on the direction of flow of current in the wire and on the position of the compass (above or below the wire). As these factors change, the direction of magnetic field changes and the compass aligns itself in the changed direction. The direction of the magnetic field can be figured from the right hand thumb rule…

Let’s explore some cases! 

1. A wire AB in a circuit lies in the east-west direction and a compass is placed below it. No current is flowing in the wire, so no magnetic field is produced by it and no deflection occurs in the compass from its north-south direction.

2. If current is passed through the wire AB from west to east, a magnetic field is produced in the wire in the anticlockwise direction. The compass (its south pole in this case) tries to align itself in the direction of the magnetic field, causing the north pole of the compass to deflect towards the west.  

             

3. If current is passed through the wire AB from east to west, a magnetic field is produced in the wire in the clockwise direction. The compass (its south pole) tries to align itself in the direction of the magnetic field, causing the north pole of the compass to deflect towards the east. Thus, on reversing direction of current, direction of deflection of compass reverses. 

4. If we take the second case - current is passed through the wire AB from west to east- but now the compass is placed above the wire, then the direction of deflection of the north pole of the compass will reverse - towards east. This is because the south pole of the needle tries to align itself in the direction of the magnetic field. 

5. If we take the third case - current is passed through the wire AB from east to west- but now the compass is placed above the wire, then the direction of deflection of the north pole of the compass will reverse - towards west. This is because the south pole of the needle tries to align itself in the direction of the magnetic field. Thus, on changing the position of the compass above and below the wire, the direction of deflection of the compass reverses. 

Magnetic field due to current in a circular coil and cylindrical coil 

The direction of the magnetic field due to current in a loop or circular coil can be found using the right hand thumb rule. 

Looking at the image of the coil after flipping it by 90°, the magnetic field lines seem to be entering into the back face and coming out from the front face. Magnetic field lines always enter into the south pole and come out from the north pole, making the back face the south pole and the front face the north pole. The coil having 2 poles acts as a magnet with its own magnetic field. 

The polarity of the faces can also be figured using the clock rule. If the current around a face is in the clockwise direction, the face acts as the south pole. While if the current around a face is in the anticlockwise direction, the face acts as the north pole. This also makes the back face of the coil the south pole and the front face the north pole.

Similar is the situation of current in a solenoid - a cylindrical coil with a length greater than the diameter. A straight wire can be wound cylindrically by first making 2 rectangular cuts in a cardboard: parallel to each other, at a distance. Then the wire is to be passed through a corner in one of the cuts, and taken out from the back through the second cut. Again pass the wire through the first cut, and continue the process for the entire length of the cuts. 

In a solenoid a magnetic field is produced due to current. Its direction will be from the north to south pole. The 2 ends of the solenoid behave like the 2 poles of a magnet. We can make out the polarity of the ends by using the clock rule which uses direction of current to deduce polarity of end. 

  On looking from the south end On looking from the north end 

It might be hard to comprehend that current flows in opposite directions in opposite faces of a coil and in opposite ends of a solenoid. The current is actually flowing in one direction only, it’s just the perspective that differs. To understand better, try taking a small object, say a pen. With the cap facing you, rotate the pen clockwise. Then slowly, without stopping or changing the direction of rotation, move the pen such that the back of it faces you. It will appear to be rotating in the opposite direction - anticlockwise. 

Lorentz Force 

Lorentz Force is the force experienced by a moving charge (or current carrying wire) in a magnetic field, moving in a direction other than the direction of the magnetic field. This force is perpendicular to and depends on the direction of the magnetic field and current. Let’s demonstrate this! 

A wire is suspended in circuit, giving it freedom to move. A magnet is brought near the wire such that a part of the wire is between the north and south poles of the magnet. Also, the direction of flow of current is perpendicular to the direction of the magnetic field. 

The current flows in the x-direction while the magnetic field acts in the y-direction. These are acting in the plane of paper. A plane is any 2-d or flat surface that extends indefinitely in both directions. Our paper is just a part of the x-y plane (ignoring its thickness, it has 2 dimensions- x and y). The current and magnetic field also act in the x-y plane. Thus, they act in the same plane as that of paper.                                                                                                           

The force acts in a direction perpendicular to the magnetic field and current- that is perpendicular to the x and y directions- it acts in the z direction. We can also say that the force acts perpendicular to the plane of paper (magnetic field and current act in the plane of paper) - acts either out of the paper or into the paper.

Let’s look at cases of different directions of current and magnetic field. If no current is passed in the wire no force will act on it and it will not move as charges are not moving.       

When current is passed in the wire in the positive x direction (from left to right), the force acts in the positive z direction. This causes the wire to move out of the plane of paper (forward). 

When current is passed in the wire in the negative x direction (reversed), the force acts in the negative z direction (reversed). This causes the wire to move into the plane of paper (backward) . 

Now, current is passed in the wire in the positive x direction, but the direction of the magnetic field is reversed by reversing the polarities of the magnet or by holding it after flipping. Then the force acts in the negative z direction (reversed). This causes the wire to move into the plane of paper.

 

With lorentz force, we can convert electrical energy (passed through the wire) to kinetic energy (of motion of wire). This concept is used in motors which are used in fans, mixers, washing machines, automobiles, etc.

To conclude, electromagnetism - the interactions between electric and magnetic fields - is an amazing concept. It is credited with the creation of the beautiful northern and southern lights. It is also used in multiple day to day devices, thus running our world. There are still so many concepts and applications of electromagnetism to explore…

Answers to questions in the previous blog-:

Q  If an electrical circuit were analogous to a water circuit at a water park, then the battery voltage would be comparable to _____.

a. the rate at which water flows through the circuit

b. the speed at which water flows through the circuit

c. the distance that water flows through the circuit

d. the water pressure between the top and bottom of the circuit

e. the hindrance caused by obstacles in the path of the moving water

Ans  d. the water pressure between the top and bottom of the circuit

In a circuit, the battery establishes an electric potential difference across the two ends of the external circuit and thus causes the charge to flow. The battery voltage is the numerical value of this electric potential difference. In an analogous manner, the water pump creates a difference in water pressure between the top and the bottom of the water slide and thus causes water to flow down the slide. 

2.  If a battery provides a high voltage, it can ____.

a. do a lot of work over the course of its lifetime

b. do a lot of work on each charge it encounters

c. push a lot of charge through a circuit

d. last a long time

Ans  b. do a lot of work on each charge it encounters

The voltage of a battery is the potential energy difference across its terminals for every Coulomb of charge. A high voltage battery maximizes this ratio of energy/charge by doing a lot of work on each charge it encounters.