Notes: JKBOSE Chapter 4 Magnetic Effects Of Current

Table of Contents

Chapter 4 Magnetic Effects of Current

Q1. What are magnets and magnetism?

Ans. Magnets have been source of curiosity for ages. You can find magnets in places like laboratories, in toys, magnetic stickers that stick in refrigerator etc.

So, what is a magnet? The substances which have the property of attracting small pieces of iron, nickel and cobalt etc. are called magnets and this property of attraction is called magnetism.
Most of the metals like copper, gold, silver aluminium etc. are not attracted to magnets.
Magnets are found naturally in certain natural rocks and can also be made artificially by certain methods.
Artificially made permanent magnets are made in various shapes like bar, rod, disk, ring etc.
In a bar, rod and horse shoe magnets north and south poles are dictated by letters N and S or North Pole is indicated by a dot.
In disk and ring magnets one face is North Pole and another face is South Pole.
Permanent magnets are commonly used as a direction finding compass.

Q2. List some properties of magnet:

Ans. Attracts objects of iron, cobalt and nickel.

Force of attraction of a magnet is greater at its poles then in the middle.
Like poles of magnets repel each other while unlike poles of magnets attract each other.
A free suspended magnet always point towards north and south direction.
The pole of a magnet which points toward north direction is called North Pole or north seeking.
The pole of a magnet which points toward south direction is called South Pole or south seeking.

Q3. What is magnetic Field?

Ans. The space around a magnet in which the force of attraction and repulsion due to the magnet can be detected is called the magnetic field.

When we bring two magnets near each either they attract each other or they repel each other.
We can explain this force of attraction and repulsion between two magnets using the concept of Magnetic Field.
Magnets produces magnetic field in the space around it, which exerts force on any other magnet placed in this region. So it is the region around magnet within which its influence can be experienced.

NOTE:- Each point in the field of any magnet has a particular strength and magnetic field at each point has definite direction.

Q4. How do you find the direction of the magnetic field due to magnet at a point near it?

Ans. The direction of the magnetic field due to a magnet at a point near it can be found by placing a magnetic compass at that point. The compass needle gets deflected when it is placed near the magnet.

Q5. Explain Earth’s magnetism.

Ans. The earth has a magnetic field which we call as the earth’s magnetic field.

The magnetic field is tilted slightly from the Earth’s axis.
The core of earth is filled with molten iron (Fe) which give Earth its very own magnetic field.
This large magnetic field protects the Earth from space radiation and particles such as the solar wind.
The region surrounding Earth where its magnetic field is located is termed as the Magnetosphere.
Earth has a magnetic field that has a shape similar to that of a large bar magnet.
To the north is the magnetic north pole, which is really the south pole of Earth’s bar magnet. (It is because this pole attracts the north pole of the compass magnet)

Q6. What is magnetic field? What are Field Lines?

Ans. The influence of force surrounding a magnet is called magnetic field. In the magnetic field, the force exerted by a magnet can be detected using a compass or any other magnet.

Fig: Magnetic Field Lines

The imaginary lines of magnetic field around a magnet are called field line or field line of magnet. When iron fillings are allowed to settle around a bar magnet, they get arranged in a pattern which mimics the magnetic field lines. Field line of a magnet can also be detected using a compass. Magnetic field is a vector quantity, i.e. it has both direction and magnitude.

Direction of Field Line: Outside the magnet, the direction of magnetic field line is taken from north pole to South Pole. Inside the magnet, the direction of magnetic field line is taken from south pole to north pole.

Q7. List some properties of magnetic field lines:

Ans.

All field lines are closed curves.
Outside the magnet field lines emerge from North Pole and merge at South Pole.
Inside a magnet, the direction of field lines is from South Pole to its north pole.
Field lines never intersect each other.
Field lines are closed together near the poles and spread out away from them.
The field is stronger where the field lines are more closely spaced. So, the field is stronger near the poles then at other points.

Q8. What is a magnetic effect of Current?

Ans. Electricity and Magnetism are related phenomenon. When an electric current is passed through metallic conductor, it generates a magnetic field around it.

Q9. What is magnetic field due to current through straight Conductor?

Ans. Electric current through a straight Conductor generates magnetic field around it.

1. Magnetic Field intensity increases on the increasing the current in the conductor
2. Magnetic field decrease as the distance increase from the conductor
3. Magnetic Field direction can be find using Right Hand Thumb Rule

Q10. What is Right Hand Thumb rule?

Ans. When you are holding a current-carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then your fingers will wrap around the conductor in the direction of the field lines of the magnetic field

Q11. What is magnetic field due to a Current through a Circular Loop?

Ans. a) As with straight conductor, the magnetic field lines would be in the form of concentric circles around every part of the periphery of the conductor.

The magnetic field would be stronger near the periphery of the loop as magnetic field lines tend to remain closer when near the conductor.
The magnetic field lines would be distant from each other when we move towards the centre of the current carrying loop. At the centre, the arcs of big circles would appear as straight lines

Q12. What is magnetic field due to current through a coil having number of turns?

Ans. We know that the magnetic field produced by a current-carrying wire at a given point depends directly on the current passing through it and the current in each circular turn has the same direction Therefore, Magnitude of magnetic field gets summed up with increase in the number of turns of coil. If there are ‘n’ turns of coil, magnitude of magnetic field will be ‘n’ times of magnetic field in case of a single turn of coil.

Q13. What is Solenoid?

Ans. A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid Magnetic Effect of Current carrying Solenoid

A current carrying solenoid produces similar pattern of magnetic field as a bar magnet. One end of solenoid behaves as the north pole and another end behaves as the south pole. Magnetic field lines are parallel inside the solenoid; like a bar magnet; which shows that magnetic field is same at all points inside the solenoid

Q14. What is an Electromagnet?

Ans. When a piece of magnetic material, like soft iron is placed inside a solenoid,the strong magnetic field produced inside a solenoid magnetise the soft iron and it behaves like strong magnet. The magnetism in the soft iron is temporary and it becomes null when the current is switched off. This type of magnet is called Electromagnets. So electromagnets are temporary magnets

Q15. What is force on a current carrying conductor in a Magnetic Field?

Ans. When a current carrying, conductor is placed in a magnetic field, it experienced a force. The direction of force depends on the direction of the current and direction of the Magnetic Field. The direction of the force can be found using Fleming Left hand rule

Q16. What is Fleming Left hand rule?

Ans. Stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor

Q17. Explain principle, construction and working of Electric Motor

Ans. An electric motor is a device which works on the above principle. Here the electrical energy is converted to mechanical energy. Here a current carrying conductor is placed in the magnetic field and force acts on the conductor and it rotates and do the mechanical work. This is used in electric fans .

Working of electric Motor

An electric motor, consists of a rectangular coil ABCD of insulated copper wire. The coil is placed between the two poles of a magnetic field such that the arm AB and CD are perpendicular to the direction of the magnetic field. The ends of the coil are connected to the two halves P and Q of a split ring.
When the electric current is switched on, Current in the coil ABCD enters through conducting brush X and flows back to the battery through brush Y. Current in arm AB of the coil flows from A to B. In arm CD it flows from C to D, that is, opposite to the direction of current through arm AB. On applying Fleming’s left hand rule for the direction of force on a current-carrying conductor in a magnetic field We find that the force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards. Thus the coil and the axle O, mounted free to turn about an axis, rotate anti-clockwise.
At half rotation, Q makes contact with the brush X and P with brush Y. Therefore the current in the coil gets reversed and flows along the path DCBA. A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator. The reversal of current also reverses the direction of force acting on the two arms AB and CD. Thus the arm AB of the coil that was earlier pushed down is now pushed up and the arm CD previously pushed up is now pushed down. Therefore the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.

Q18. What is Electromagnetic Induction?

Ans. Electromagnetic induction is the production of induced current in a coil placed in a region where the magnetic field changes with time.

The magnetic field may change due

relative motion between the coil and a magnet placed near to the coil.
If the coil is placed near to a current-carrying conductor, the magnetic field may change either due to a change in the current through the conductor or due to the relative motion between the coil and conductor.

The direction of the induced current is given by the Fleming’s right-hand rule.

Q19. What is Fleming’s Right Hand rule?

Ans. Stretch the thumb, forefinger and middle finger of right hand so that they are perpendicular to each other. If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion of conductor, then the middle finger will show the direction of induced current

Q20. Explain the working of Electric Generator

Ans. It is a device which converts mechanical energy into electrical energy. It is based on electromagnetic induction principle as explained above.

Working of electric Generator

When the coil ABCD is rotated clockwise . By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions AB and CD. Thus an induced current flows in the direction ABCD. This means that the current in the external circuit flows from B2 to B1.
After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both the arms change, giving rise to the net induced current in the direction DCBA. The current in the external circuit now flows from B1 to B2. Thus after every half rotation the polarity of the current in the respective arms changes. Such a current, which changes direction after equal intervals of time, is called an alternating current (abbreviated as AC). This device is called an AC generator.
To get a direct current (DC, which does not change its direction with time), 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.

AC Current

which current direction changes after equal intervals of time, then the current is called an alternating current (abbreviated as AC).

Direct Current

When the current flows in the same direction and does not change direction, it is called Direct current (DC)

AC generator

Electric Generator can be used to generate both the AC and DC current. When it is designed to produce AC current, it is called AC generator DC generator

Electric Generator can be used to generate both the AC and DC current. When it is designed to produce DC current, it is called DC generator Facts about AC and DC currents

Most power stations constructed these days produce AC.
In India, the AC changes direction after every 1/100 second, that is, the frequency of AC is 50 Hz.
An important advantage of AC over DC is that electric power can be transmitted over long distances without much loss of energy

Q21. What is Galvanometer?

Ans. A galvanometer is an instrument that can detect the presence of a current in a circuit. The pointer remains at zero (the centre of the scale) for zero current flowing through it. It can deflect either to the left or to the right of the zero mark depending on the direction of current

TEXTUAL QUESTIONS

Q1. Why does a compass needle get deflected when brought near a bar magnet?

Ans. The compass needle is a small magnet. When the compass needle is brought close to a bar magnet, the magnetic field lines of the compass needle interact with the magnetic field lines of the bar magnet, which causes the compass needle to deflect.

Q2. Draw magnetic field lines around a bar magnet.

Ans. Magnetic field lines of a bar magnet emerge from the North Pole and terminate at the South Pole, as shown in the figure below.

Q3. List the properties of magnetic field lines.

Ans. The properties of magnetic field lines are as follows:

Magnetic field lines do not intersect with each other.
They emerge from the North Pole and terminate at the South Pole.
Inside the magnet, the direction of the field lines is from the South Pole to the North Pole.

Q4. Why don’t two magnetic field lines intersect each other?

Ans. If two magnetic field lines intersect, then at the point of intersection, the compass needle shows two different directions, which is not possible. Hence they do not intersect with each other.

Q5. Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.

Ans. For the downward direction of the current, the direction of the magnetic field will be as if emerging from the table outside the loop and merging with the table inside the loop. Similarly, for current flowing in an upward direction, the direction of the magnetic field will be as if they are emerging from the table outside the loop and merging with the table inside the loop, as shown in the figure.

Q6. The magnetic field in a given region is uniform. Draw a diagram to represent it.

Ans.

Q7. In Activity 13.7, how do we think the displacement of rod AB will be affected if (i) current in rod AB is increased; (ii) a stronger horse-shoe magnet is used; and (iii) length of the rod AB is increased?

Ans. A current-carrying conductor, when placed in a magnetic field, experiences force. The magnitude of this force will increase with the increase in the amount of current,

the length of the conductor and the strength of the magnetic field. Hence, the strength of the magnetic force exerted on the rod AB and its displacement will increase if

The current in rod AB is increased
A stronger horseshoe magnet is used
When the length of the rod AB increases

Q8. A positively-charged particle (alpha-particle) projected towards the west is deflected towards north by a magnetic field. The direction of magnetic field is

towards south
towards east
downward
upward

Ans. The direction of the magnetic field can be determined using Fleming’s Left-hand rule. According to the rule, if we arrange our thumb, forefinger and the middle finger of the left hand right perpendicular to each other, then the thumb points towards the direction of the magnetic force, the middle finger the direction of current and the forefinger the direction of magnetic field. Since the direction of the positively charged particle is towards the west, the direction of the current will also be towards the west. The direction of the magnetic force is towards the north. Hence the direction of the magnetic field will be upward according to Fleming’s Left-hand rule.

Q9. State Fleming’s left-hand rule.

Ans. Fleming’s Left-hand rule states that if we arrange our thumb, forefinger and middle finger of the left hand at right angles to each other, then the thumb points towards the direction of the magnetic force, the forefinger points towards the direction of the magnetic field and the middle finger points towards the direction of the current.

Q10. What is the principle of an electric motor?

Ans. The working principle of an electric motor is based on the magnetic effect of current. A current-carrying conductor, when placed in a magnetic field, experiences force and rotates. The direction of the rotation of the conductor can be determined by Fleming’s Left-hand rule.

Q11. What is the role of split ring in an electric motor?

Ans. The split ring plays the role of a commutator in an electric motor. The commutator reverses the direction of the current flowing through the coil after each half-rotation of the coil. Due to this reversal of current, the coil continues to rotate in the same direction.

Q12. Explain different ways to induce current in a coil.

Ans. Following are the different ways to induce a current in a coil:

If the coil is moved rapidly between the two poles of a horseshoe magnet, an electric current is induced in the coil.
When a magnet is moved relative to the coil, an electric current is induced in the coil.

Q13. State the principle of an electric generator.

Ans. An electric generator works on the principle of electromagnetic induction. In a generator, electricity is generated by rotating a coil in the magnetic field.

Q14. Name some sources of direct current.

Ans. DC generators and cells are some sources of direct current.

Q15. Which sources produce alternating current?

Ans. Power plants and AC generators are some of the sources that produce alternating current.

Q16. Name two safety measures commonly used in electric circuits and appliances.

Ans. The safety measures commonly used in electric circuits are as follows:

Fuse : Each circuit should be connected to a fuse because a fuse prevents the flow of excessive current through the circuit. When the current in the circuit exceeds the maximum limit of the fuse element, the fuse melts to stop the flow of current protecting the appliance connected to the circuit.
Earthing : Earthing protects the user from electric shocks. Any leakage of current in an appliance is transferred to the ground by earthing, and the people using the appliance are prevented from getting electrocuted.

Q17. An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.

Ans. The current drawn by the electric oven can be calculated using the formula

P = V × I I = P/V

Substituting the values, we get I = 2000 W/220 V = 9.09 A

The current drawn by the electric oven is 9.09 A which exceeds the safe limit of the circuit. This causes the fuse to melt and break the circuit.

Q18. What precaution should be taken to avoid the overloading of domestic electric circuits?

Ans. A few of the precautions to be taken to avoid the overloading of domestic electric circuits are as follows:

Connecting too many devices to a single socket should be avoided
Using too many appliances at the same time should be avoided
Faulty appliances should not be connected to the circuit

Q19. List two methods of producing magnetic fields.

Ans. Following are the methods of producing magnetic fields:

By using a permanent magnet, we can produce a magnetic field, and it can be visualized by spreading iron fillings on white paper and keeping a magnet beneath the paper.
A current-carrying straight conductor produces magnetic field.
Different types of conductors, such as solenoid and circular loops, can be used to see the presence of a magnetic field.

Q20. How does a solenoid behave like a magnet? Can you determine the north and south poles of a current–carrying solenoid with the help of a bar magnet? Explain.

Ans. A solenoid is a long coil of circular loops of insulated copper wire. The magnetic field produced around the solenoid when the current is passed through it is similar to the magnetic field produced around the bar magnet when a current is passed through it. The figure shown below shows the arrangement of magnetic fields produced around the solenoid when current is passed through it. When the north pole of the bar magnet is brought close to the end connected to the negative terminal of the battery, the solenoid repels the battery. As like poles repel each other, we can infer that the end connected to the negative terminal behaves as a north pole while the end connected to the positive terminal behaves as a south pole.

Q21. When is the force experienced by a current–carrying conductor placed in a magnetic field largest?

Ans. When the direction of the current is perpendicular to the direction of the magnetic field, the force experienced is the largest.

Q22. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?

Ans. The direction of the magnetic field can be determined using Fleming’s Left-hand rule. The direction of the magnetic field will be perpendicular to the direction of the current and the direction of deflection, i.e., either upward or downward. The direction of the current is from the front wall to the back wall because negatively charged electrons move from the back wall to the front wall. The direction of the magnetic force is rightward. Hence, using Fleming’s left-hand rule, it can be concluded that the direction of the magnetic field inside the chamber is downward.

Q23. Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a splitring in an electric motor?

Ans. An electric motor is a device that converts electrical energy to mechanical energy. It works on the principle of the magnetic effect of current. The figure listed below shows a simple electric motor.

When current is made to flow through the coil MNST by closing the switch, the coil starts to rotate in the anticlockwise direction. This is due to the downward force acting on the length MN and simultaneously an upward force acting along the length ST. As a result of which, the coil rotates in the anticlockwise direction. Current in the length MN flows from M to N, and the magnetic fields act from left to right normal to the length MN. According to Fleming’s Left-Hand rule, a downward force acts along the length MN. Similarly, the current along the length ST flows from S to T and the magnetic field acts from left to right. Therefore, an upward force acts along the length ST. These two forces together cause the coil to rotate anti-clockwise. After half a rotation, the position of MN and ST interchange. The half-ring C comes in contact with brush B and the half-ring D comes in contact with rush C. Hence the direction of current in the coil MNST gets reversed.

Q24. Name some devices in which electric motors are used.

Ans. A few devices in which electric motors are used are:

Electric fans
Water pumps
Mixers
Washing machines

Q25. A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary inside the coil?

Ans. (i) When a bar magnet is pushed into the coil, a current is induced in the coil momentarily. As a result, the

galvanometer deflects in a particular direction momentarily.

When the bar magnet is withdrawn from inside the coil, a current is induced momentarily but in the opposite direction, and the galvanometer deflects in the opposite direction momentarily.
When the bar magnet is held stationary inside the coil, no current will be induced. As a result, there will be no deflection in the galvanometer.

Q26. Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.

Ans. When the current in coil A changes, the magnetic field associated with it also changes. As a result, the magnetic field around coil B undergoes change. The change in the magnetic field of coil B induces a current in it.

Q27. State the rule to determine the direction of a (i) magnetic field produced around a straight conductor-carrying current, (ii) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.

Ans. (i) The rule used to determine the direction of the magnetic field produced around a straight conductor-carrying

current is Maxwell’s right-hand thumb rule.

The rule used to determine the force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it is the Fleming’s left hand rule.
The rule used to determine the current induced in a coil due to its rotation in a magnetic field is Fleming’s right-hand rule.

Q28. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

Ans. The electric generator converts mechanical energy into electrical energy. The working principle of the electric generator is electromagnetic induction. It generates electricity by rotating a coil in the magnetic field. The figure below shows the construction of a simple AC generator.

In the diagram,

A and B are brushes. C and D are slip rings. X is the axle.

G is the galvanometer.

When the axle X is rotated clockwise, MN moves upwards while ST moves downward. The movement of MN and ST in the magnetic field results in the production of electric current due to electromagnetic induction. MN moves upwards, and the magnetic fields act from left to right. Therefore, according to Fleming’s right hand rule, the direction of the induced current will be from M to N along the length MN. Similarly, the direction of the induced current will be from S to T along the length ST. The direction of the current in the coil is MNST. Hence, the galvanometer shows a deflection in a particular direction.

After half a rotation, length MN starts moving downwards while the length ST starts moving upwards. Now, the direction of the induced current reverses to TSNM. Since the direction of the induced current reverses every half rotation, the current induced is known as alternating current.

Function of Brushes

Brushes are kept pressed onto two slip rings separately. The outer ends of the brushes are connected to the galvanometer. Thus, brushes help in transferring current from the coil to the external circuit.

Q29. When does an electric short circuit occur?

Ans. Listed below are two instances of when a short-circuit can occur:

When too many appliances are connected to a single socket or when high power rating appliances are connected to a light circuit, the resistance of the circuit becomes low. As a result, the current flowing through the circuit becomes very high. This condition results in a short circuit.
When live wires whose insulation has worn off come in contact with each other, the current flowing in the circuit increases abruptly, which results in a short circuit.

Q30. What is the function of an earth wire? Why is it necessary to earth metallic appliances?

Ans. The metallic body of electric appliances is earthed by means of an earth wire. Any leakage of electric wire is transferred to the ground by means of the earth wire. This prevents the user of the electric appliance from getting electric shocks. This is the reason why it is important for metallic appliances to be earthed.