Magnetism and Electromagnetism Online Test 9th Science Lesson 5 Questions in English

Natural magnets exist in the nature. These kinds of magnets can be found in rocks and sandy deposits in various parts of the world. The strongest natural magnet is lodestone magnetite. The magnetic property in the natural magnets is permanent. It never gets destroyed.

The magnetic field is the region around the magnet where its magnetic influence can be felt. It is denoted by B and its unit is Tesla.

Captains of the ships effectively used the magnets to identify the direction of the ship in the sea. There are two kinds of magnets that we can see around us: Natural magnet and Artificial magnet. Natural magnet lodestones were used to make compasses in the olden days. Artificial magnets are made by us. The magnets available in the shops are basically artificial magnets.

Magnetic field can penetrate through all kinds of materials, not just air. The Earth produces its own magnetic field, which shields the earth’s ozone layer from the solar wind and it is important for navigation also.

A magnetic field line is defined as a curve drawn in the magnetic field in such a way that the tangent to the curve at any point gives the direction of the magnetic field. They start at north pole and ends at south pole. The magnetic field at a point is tangential to the magnetic field lines.

Magnetic flux is the number of magnetic field lines passing through a given area.

The number of magnetic field lines crossing unit area kept normal to the direction of field lines is called magnetic flux density.

Magnetic flux is denoted by ϕ and its unit is weber (Wb).

Some sea turtles (loggerhead sea turtle) return to their birth beach many decades after they were born, to nest and lay eggs. In a research, it is suggested that the turtles can perceive variations in magnetic parameters of Earth such as magnetic field intensity and remember them. This memory is what helps them in returning to their homeland.

Magnetic flux density unit is Wb/m2.

Magnetic lines of force start from the North Pole and end at the South Pole. They will be maximum at the poles than at the equator.

It was on 21st April 1820, Hans Christian Oersted, a Danish Physicist was giving a lecture. He was demonstrating electrical circuits in that class. He had to often switch on and off the circuit during the lecture. Accidentally, he noticed the needle of the magnetic compass that was on the table. It deflected whenever he switched on and the current was flowing through the wire. The compass needle moved only slightly, so that the audience didn’t even notice. But it was clear to Oersted that something significant was happening. He conducted many experiments to find out a startling effect, the magnetic effect of current.

The direction of the magnetic lines around a current carrying conductor can be easily understood using the right-hand thumb rule.

Hold the wire with four fingers of your right hand with thumbs-up position. If the direction of the current is towards the thumb then the magnetic lines curl in the same direction as your other four fingers. This shows that the magnetic field is always perpendicular to the direction of current.

The strength of the magnetic field at a point due to current carrying wire depends on: (i) the current in the wire, (ii) distance of the point from the wire, (iii) the orientation of the point from the wire and (iv) the magnetic nature of the medium. The magnetic field lines are stronger near the current carrying wire and it diminishes as you go away from it. This is represented by drawing magnetic field lines closer together near the wire and farther away from the wire.

H.A. Lorentz found that a charge moving in a magnetic field, in a direction other than the direction of magnetic field, experiences a force. It is called the magnetic Lorentz force.

The current carrying wire has a magnetic field perpendicular to the wire (by looking at the deflection of the compass needle in the vicinity of a current carrying conductor). The deflection of the needle implies that the current carrying conductor exerts a force on the compass needle.

In 1821, Michael Faraday discovered that a current carrying conductor also gets deflected when it is placed in a magnetic field. The magnetic field of the permanent magnet and the magnetic field produced by the current carrying conductor interact and produce a force on the conductor.

If a current, I is flowing through a conductor of length, L kept perpendicular to the magnetic field B, then the force F experienced by it is given by the equation, F = I L B. The above equation indicates that the force is proportional to current through the conductor, length of the conductor and the magnetic field in which the current carrying conductor is kept.

The angle of inclination between the current and magnetic field also affects the magnetic force. When the conductor is perpendicular to the magnetic field, the force will be maximum (=BIL). When it is parallel to the magnetic field, the force will be zero.

The force is always a vector quantity. A vector quantity has both magnitude and direction. It means we should know the direction in which the force would act. The direction is often found using what is known as Fleming’s Left-hand Rule.

Fleming’s Left-hand Rule states that while stretching the three fingers of left hand in perpendicular manner with each other, if the direction of the current is denoted by the middle finger of the left hand and the second finger is for direction of the magnetic field, then the thumb of the left hand denotes the direction of the force or movement of the conductor.

Force on the conductor = ILB
= 5 × 50 × 10-2 × 2 ×10-3
= 5 × 10-3 N

The first conductor will experience a force due to the magnetic field of the second conductor. These two forces will be equal in magnitude and opposite in direction. Using Fleming’s left-hand rule, we can find that the direction of the force on each wire would be towards each other when the current in both of them are flowing in the same direction, i.e., the wires would experience an attractive force.

When there is current, the magnetic field is produced and the current carrying conductor behaves like a magnet. You may now wonder how was it possible for a lodestone to behave like a magnet when there was no current passing through it. Only in the twentieth century, we understood that the magnetic property arises due to the motion of electrons in the lodestone.
In the circuit the electrons flow from negative of the battery to positive of the battery and constitutes current. As a result, it produces magnetic field. In natural magnets and artificial magnets that we buy in shops, the electrons move around the nucleus constitutes current which leads to magnetic property.

An electric motor is a device which converts electrical energy into mechanical energy. Electric motors are crucial in modern life. They are used in water pump, fan, washing machine, juicer, mixer, grinder etc. when electric current is passed through a conductor placed normally in a magnetic field, a force is acting on the conductor and this force makes the conductor to move.

The speed of rotation of coil can be increased by: i. increasing the strength of current in the coil. ii. increasing the number of turns in the coil. iii. increasing the area of the coil and iv. increasing the strength of the magnetic field.

When it was shown by Oersted that magnetic field is produced around a conductor carrying current, the reverse effect was also attempted. In 1831, Michael Faraday explained the possibility of producing an e.m.f across the conductor when the magnetic flux linked with the conductor is changed.

All these observations made Faraday to conclude that whenever there is a change in the magnetic flux linked with a closed circuit an emf is produced and the amount of emf induced varies directly as the rate at which the flux changes.

In the Faraday’s experiment, current (or voltage) is generated by the movement of the magnet in and out of the coil. The greater the number of turns, the higher is the voltage generated.

The change in the magnetic flux linked with a closed circuit an emf is produced and the amount of emf induced varies directly as the rate at which the flux changes. This emf is known as induced emf.

The phenomenon of producing an induced emf due to change in the magnetic flux linked with a closed circuit is known as electromagnetic induction.

F = I L B = (4I) × (L/2) × (3 B) = 6 F
Therefore, the force increases six times.

The direction of the induced current was given by Lenz’s law, which states that the induced current in the coil flows in such a direction as to oppose the change that causes it. The direction of induced current can also be given by another rule called Fleming’s Right Hand Rule.

Michael Faraday (22nd Sep,1791–25th Aug, 1867) was a British Scientist who contributed to the study of electromagnetism and electrochemistry. His main discoveries include the principles underlying electromagnetic induction, diamagnetism and electrolysis.

Fleming’s Right hand rule is also called ‘generator rule’.

An alternating current (AC) generator, consists of a rotating rectangular coil ABCD called armature placed between the two poles of a permanent magnet.

The two rings S1 and S2 are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field. Outer ends of the two brushes are connected to the external circuit. ed with the coil changes. This change in magnetic flux will lead to generation of induced current.

To get a direct current (DC), a split ring type commutator must be used. With this arrangement, one brush is at all times in contact with the arm moving up in the field while the other is in contact with the arm moving down. Thus, a unidirectional current is produced. The generator is thus called a DC generator.

Transformer is a device used for converting low voltage into high voltage and high voltage into low voltage. It works on the principle of electromagnetic induction.

Transformer consists of primary and secondary coil insulated from each other. The alternating current flowing through the primary coil induces magnetic field in the iron ring. The magnetic field of the iron ring induces a varying emf in the secondary coil.

The transformer used to change a low alternating voltage to a high alternating voltage is called a step-up transformer. i.e., Vs > Vp. In a step-up transformer, the number of turns in the secondary coil is more than the number of turns in the primary coil (Ns > Np).
The transformer used to change a high alternating voltage to a low alternating voltage is called a step-down transformer (Vs < Vp). In a step-down transformer, the number of turns in the secondary coils are less than the number of turns in the primary coil (Ns < Np).

The formulae pertaining to the transformers are given in the following equations
Es / E p = Ns / N p or
Number of primary turns Np/ Number of secondary turns Ns = Primary voltage Vp/ Secondary voltage Vs
Number of secondary turns Ns/ Number of primary turns Np = Primary current Ip/ Secondary current Is

A transformer cannot be used with the direct current (DC) source because, current in the primary coil is constant (ie. DC). Then there will be no change in the number of magnetic field lines linked with the secondary coil and hence no emf will be induced in the secondary coil.

In a transformer, Es / E p = Ns / Np
Es = (Ns / Np) × E p
= 8/800 × 220 = 220/100 = 2.2 volt

Magnetic levitation (Maglev) is a method by which an object is suspended with no support other than magnetic fields. In maglev trains two sets of magnets are used, one set to repel and push the train up off the track, then another set to move the floating train ahead at great speed without friction. In this technology, there is no moving part.

Nowadays electromagnetic fields play a key role in advanced medical equipment’s such as hyperthermia treatments for cancer, implants and magnetic resonance imaging (MRI). Many of the medical equipment’s such as scanners, x-ray equipment’s and other equipment’s also use the principle of electromagnetism for their functioning.

The space surrounding a bar magnet in which its influence in the form of magnetic force can be detected, is called magnetic field.