# The physical world: How things work

Want to learn about electricity and current? Or fancy magnetism? It’s all here.

## 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?

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?

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.

## 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.

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.

## 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 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.

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

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.

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.

## Atomic Structure: Understanding the building blocks of matter

Atomic structure is the structure of the atom. Atoms are the smallest constituent of matter. All substances- solid, liquid and gas are made of atoms. Atoms are incredibly small- about 100 picometres or 10 billionths of a metre.

The atomic structure is comprised of the protons, neutrons and electrons arranged in a particular pattern. However, the atomic structure is not a clearly defined map or diagram. The boundaries of the atom are not well defined and depending on the energy level of the individual constituents; the atomic structure can vary significantly.

Every atom has the following three components- protons, neutrons and electrons.

The protons and neutrons are concentrated at the centre of the atom- the nucleus. The core constitutes over 99% of the mass of the atom. The proton is positively charged while the neutron does not carry any charge. The proton and the neutron are held together by nuclear forces- an unyielding force.

The proton is 1836 times as massive as the electron and is positively charged. The number of protons in an atom corresponds to the atomic number of the element.

The neutron is the heaviest component of the atomic structure. It is 1839 times as massive as the electron. The neutron does not carry any charge. The sum of the protons and the neutrons is the mass number of the element. A mass number is just a number, i.e. It is dimensionless. The actual mass of an atom at rest is measured differently. The unified atomic mass unit is the actual mass of an atom at rest. The Dalton (Da) is the unit used to measure the unified atomic mass unit. One Dalton is equal to one-twelfth of the mass of a free neutral atom of carbon-12.

The electrons are the smallest particles in the atomic structure. Electrons always rotate in an orbit around the nucleus. However, they (electrons) do not follow distinct paths. Electrons exist more like a cloud around the nucleus- the electron cloud. As the atomic structure is not well defined, the dimensions of an atom are described by the atomic radii. The atomic radius is the distance from the centre of the atom to the more out end of the electron cloud. The atomic structure is assumed to be spherical. However, the spherical fabric of the atom is correct only in a vacuum. Everywhere else, the atomic structure is subject to many variations due to external forces acting on the atom.

The electrons are negatively charged. Therefore, they are attracted to the protons by a strong electromagnetic force. The electrons continuously rotate in an orbit around the nucleus to counteract the electromagnetic force exerted by the protons in the nucleus.

Electrons behave as both a particle and a wave. In certain positions within the atomic structure, an electron can form a three-dimensional standing wave- a wave that does not move about the nucleus. The probability of an electron occupying such a position can be calculated using mathematical models- the atomic orbital. Only a few, discrete sets of orbitals are present around the nucleus (the other waves rapidly decay into more stable forms). Within the atomic structure, each orbital corresponds to a particular energy level. Electrons can move to a higher orbital by acquiring energy from a photon. Similarly, electrons can move to a lower orbital by spontaneously emitting energy in the form of photons. The phenomenon of movement of electrons can be readily observed by heating a metal rod. When the metal cylinder is exposed to fire, the electrons acquire energy and move to a higher orbital. When the rod is removed from the fire, the electrons lose energy by emitting photons and rapidly move back into lower orbitals. The hot red colour of the bar is due to the emission of photons by the electrons that are moving to a lower orbital.

## Diode: Directing electric current

A diode is an electrical device that allows the electric current to flow in one direction only. The most common diodes are semiconductor diodes. However, other types of diodes also exist. A diode by definition is a small signal device (I< 1A). Rectifiers are devices with I>1A.

Diodes are made up of p-type and n-type semiconductors. The lead connected to the p-type semiconductor is the anode and the one attached to the n-type semiconductor is the cathode. Therefore, the n-type end is the negative terminal, and the p-type end is the positive terminal.

If we connect a source of power (battery) in such a way that the positive terminal of the battery is connected to the anode and the negative to the cathode, current will flow through the diode. If we reverse the connections, then current would not flow. Thus, diodes act as a valve through which current can flow in a single direction only.

A diode is schematically represented like this:-

Image from Wikipedia

In this line diagram, the cathode is on the right, and the anode is on the left.

There are a few terms that describe the properties of a diode.

1. Forward bias allows current to flow through the diodes. Here, the positive side of the power source is connected to the anode and negative is connected to the cathode.

Image from Wikipedia

2. Reverse bias does not allow current to pass through a diode. Thus, diodes behave as an insulator (up to an extent).
3. Peak inverse voltage is the maximum reverse bias a diode can sustain. If the voltage increases beyond the peak inverse voltage, then the diode will break down and allow current to flow in reverse bias as well.
4. Forward voltage drop is the amount of energy that is lost within a diode. Therefore, a power source must produce a voltage in excess of the forward voltage drop to allow electric current to flow through ?diodes.
5. Different diodes can allow a different amount of current to flow through them. Thus, each diode has a maximum current rating. If we exceed this value, then the diode will be damaged.

Uses of Diodes

Diodes have many scientific and industrial applications. Diodes are used in radio transmission, power conversion, protection against voltage surges, logic gates, to measure temperature, and to steer the direction of electric current.

Diodes are also used in musical equipment like a keyboard. Electronic musical keyboards use keyboard matrix circuits to reduce the amount of wiring. However, if several keys are pressed simultaneously, current can flow backwards through the channel. This (backflow) can trigger phantom keys and ghost sounds are heard. By soldering diodes to the keyboard matrix, it is possible to eliminate this problem.

Particle detectors that detect ionising radiation also contain diodes.

Finally, are diodes and batteries similar?

A diode is a valve that directs the flow of current. On the other hand, a battery is a power source. Diodes will not conduct electricity unless connected to an energy supply like a battery. Therefore, a diode and a battery are not the same.

## Newton’s Law (s) of motion

Newton’s Law (s) are the bedrock of classical mechanics. On 05 July 1687 Sir Issac Newton, an English physicist and mathematician published a set of three laws of motion. These (laws) laid the foundations of classical mechanics and described the relationship between an object, force and resulting motion. These laws of motions were subsequently called Newton’s law (s). We shall now discuss each of the three Newton’s law (s) in detail here.

Newton’s law No. 1

When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.

In his first law, Newton expanded on the concept of Inertia as expounded by Galileo. According to the first Newton’s law, all objects that are moving will continue to move unless an external force acts on it. The first law is similar to the concept of inertia put forth by Galileo.

Imagine a moving car. According to this (Newton’s law) law, the car should keep moving unless an external force acts on it. But in practice, the car will come to a stop once it runs out of fuel. How do we explain this discrepancy?

This discrepancy can be explained if we account for the frictional forces that are acting on the car. We will discuss friction in more detail in another article.

Newton’s Law No. 2

The vector sum of the external forces F on an object is equal to the mass (m) of that object multiplied by the acceleration vector (a) of the object: F = ma.

The second Newton’s law is a powerful tool that allows for quantitative calculation of force and acceleration. In the above equation force on an object is a product of its mass and acceleration. As Newton’s law (s) are valid only for constant mass systems, we can conclude that acceleration of an object is proportion to the force acting on it. Here both force and acceleration are vectors, and the direction of the force is the direction of the acceleration. The first and second Newton’s law differ from the ideas of Aristotle significantly. Aristotle propounded that force maintains the velocity of an object. Aristotle’s views seem to be more plausible based on common sense. However, they (Aristotle’s views) are not true. Aristotle erred to account for the frictional forces. Therefore, his observations are at variance with that of Newton.

Newton’s law No. 3

When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

The third Newton’s law can be summarised in a simple sentence- “every action has an equal and opposite reaction”. This is the law that has made most of our air, space and water travel possible.

Do you know how a jet engine works?

A jet engine is similar to a balloon filled with air. As long as the mouth of the balloon is shut, there is no net force acting on the balloon. However, as soon as the mouth of the balloon is opened, air from inside the balloon escapes through the hole. The balloon now experiences a force or thrust that propels it forward. The force pushing the balloon forward is equal to the force exerted by the escaping air.

Image from howthingsfly.si.edu

In a jet engine, fuel is burnt at a rapid pace, and a large quantity of hot gas (mainly carbon dioxide) is produced. This (hot gas) is extruded through a small nozzle at the back of the engine. The force exerted by this gas is equal to the force acting on the engine to push it forward.

So how does a rocket fly?

Rockets operate on the same principle. However, as rockets fly in space, they have to carry their source of oxygen. Most sophisticated rockets carry liquid oxygen as the oxidising agent.

## Spherical lens

What is a spherical lens? We have previously learned about spherical mirrors. We learned that the reflecting surface of a spherical mirror was part of a sphere. Similarly, the refracting surface of a spherical lens is part of a sphere. You will note that here we have used the word refracting rather than reflecting. Thus, a lens causes refraction of light.

A spherical lens is of two types- convex and concave.

A convex spherical lens is thicker at the centre and thinner at the edges. Light rays converge on entering a convex lens thus earning the name-converging lens.

A concave spherical lens is thicker at the edges and thinner at the centre. Light rays diverge from a concave lens, hence the name diverging lens.

There are a few terms that one should know about when studying spherical lens.

Optical centre: The central point of the lens

Centre of curvature: Previously, we have learned about the centre of curvature of spherical mirrors. A similar concept works in a spherical lens as well. Each surface of the lens is a part of a sphere. The centre of each sphere is the centre of curvature of the surface of the lens. As a spherical lens has two refracting surfaces, each lens has two centre of curvature- C1 and C2.

Principal Axis of the lens: A line drawn through the centre of curvatures of the spherical lens.

Principal focus (convex lens): A point on the principal axis where light rays parallel to the principal axis converge after passing through the lens.

Principal focus (concave lens): A point on the principal axis from where light rays (initially parallel to the principal axis) appear to diverge after passing through the lens.

Focal length (f): The distance between the principal focus and the optical centre.

Laws of refraction through a spherical lens

1. A ray of light parallel to the principal axis after refraction passes through the principal focus on the other side of a convex lens or appears to diverge from the principal focus on the same side of a concave lens.
2. A ray appearing to meet at the principal focus of a concave lens or passing through the principal focus of a convex lens, after refraction emerges parallel to the principal axis.
3. A ray passing through the optical centre of a concave of or convex lens does not deviate.

Characteristics of image formed by convex spherical lens

?

Characteristics of image formed by concave spherical lens

## Refraction of light

What do you know about the refraction of light? Can you think of an experiment that can practically demonstrate the refraction of light?

Let us device a fun experiment to explain the refraction of light. Take a bucket and draw a circle on the bottom using a waterproof marker. Fill up the bucket with water. Now try to drop a coin in such a way that it will drop exactly on the circle we have drawn. Pretty difficult to drop the coin exactly in the circle. Isn?t it! Why is it so difficult to drop the coin exactly within the circle? The light rays from the circle change its path when it enters the air from water. We have previously learned that the speed of light is different in a different medium. The speed of light is dependent on the density of the medium. The difference in the speed of light in various medium leads to the refraction of light.

There are a few rules of refraction of light.

1. The refracted ray, incident ray and the normal to the interface between two media at the point of incidence, all lie on the same plane.
2. For a given pair of media, the sine of the angle of incidence and the angle of refraction of light has a constant ratio. This law is called Snell?s law. This law can be represented by the following formula:-

Sine i/ Sine r= constant= 1n2

This constant is also called as the relative refractive index.

The refractive index is of two types- absolute refractive index and relative refractive index. Absolute refractive index (n) is derived from the following formula:-

n= speed of light in vacuum/ speed of light in medium = c/v

Consider a ray of light moving from one medium to another. The relative refractive index is the refractive index of one medium with respect to another medium. Therefore, relative refractive index is given by the following formula:-

1n2 = n2/n1= v2/v1

Here v2 and v1 is the speed of light in medium 2 and medium one respectively.

Can we predict the direction of refraction of light? When the light goes from a rarer medium to a denser medium, the ray of light bends towards normal while it bends away from normal when entering a rarer medium from a denser medium.

In conclusion, refraction of light is caused by the different speed of light in different mediums. Refraction of light is responsible for some wonderful optical phenomenon like the deep blue colour of the sky and twinkling of the stars. We shall learn more about these natural phenomena in subsequent articles.

Here is an excellent collection of CBSE books for class 10 science.

## The human eye

The human eye is one of the five sense organs. It perceives the meaning of light. It is because of the human eye that we can see the varied and beautiful world around us. There are two eyes, located within a bony socket called the orbit. The bones of the skull and face form the boundaries of the orbital cavity. Within the orbital cavity, lies the eye, the extra-ocular muscles, loose connective tissue, blood vessels, nerves and the optic nerve.

The outermost covering of the human eyes is sclera, a thick, fibrous sheath. It functions to protect the eyeball from injury. The white colour of the eye is because of the sclera.

The cornea is the transparent portion of the eye that light to enter the eye. ?In previous chapters, we have learned about refraction. The cornea can be considered to be a refractive medium that brings the light rays to focus. Most of the refraction occurs at the out surface of the cornea.

Human eye: A cross-section.

The Iris, a muscular diaphragm, controls the entry of light into the human eye. How does the Iris monitor the entry of light into the human eye? Iris does so by controlling the size of the pupil, a small aperture. Have you noted what happens when you shine a torch into the eye? The pupil constricts, thus reducing the amount of light entering the human eye.

Immediately behind the Iris, lies the lens. The lens is flattened towards the front and it is suspended in the eye by muscles called the ciliary muscles. These muscles help with the process of adaptation. The ciliary muscles can increase or decrease the focal length of the lens to enable the eye to focus on objects across a broad range of focal lengths.

The light from a source is refracted by the cornea and the lens and is brought to focus on the light sensitive layer of the human eye- the retina.The retina contains two types light sensitive cells- the rods and cones. The rods, numbering 120 million are more numerous and are more sensitive to light. On the other hand, cones, numbering 6-7 million sense colour. The cones are concentrated around the centre of the retina, an area called the optic disc. Within the optic disc lies the fovea. Cones are present in the highest concentration in the fovea. Therefore, colour perception is best at the fovea.

The light that is refracted by the cornea and the lens forms an inverted real image over the retina. The image is transmitted to the occipital lobe of the brain by the visual pathways comprising the two optic nerves, optic tracts and their projections. The human brain processes the signals. Therefore, we only see the erect images.

Thus, the human eye behaves like a camera for all purposes. There is one difference though- the human eye can appreciate the depth of field, which a camera sorely lacks.