Students should refer to the below Magnetic Effect of Electric Current Class 10 notes prepared as per the latest curriculum issued by CBSE and NCERT. These notes and questions are really useful as they have been developed based on the most scoring topics and expected questions in upcoming examinations for Class 10. Magnetic Effect of Electric Current is an important topic in Science Class 10 which if understood properly can help students to get very good marks in class tests and exams.
Magnetic Effect of Electric Current Class 10 Notes and Questions PDF Download
Read the notes below which will help you to understand all important and difficult topics in this chapter. There are some topics in Magnetic Effect of Electric Current chapter which you should understand carefully as many questions can come from those parts. Our team of teachers have designed the revision notes so that its helpful for students to revise entire course prior to the class tests.
Magnet: Magnetic field and magnetic field lines, Magnetic field due to a current carrying conductor, Right hand thumb rule, Magnetic field due to current through a circular loop. Magnetic field due to current in a solenoid.
Magnet is an object that attracts objects made of iron, cobalt and nickle. Magnet comes to rest in North – South direction, when suspended freely.
Use of Magnets: Magnets are used
• in refrigerators.
• in radio and stereo speakers.
• in audio and video cassette players.
• in children’s toys and.
• on hard discs and floppies of computers.
Properties of Magnet
• A free suspended magnet always points towards the 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.
• Like poles of magnets repel each other while unlike poles of magnets attract each other.
Magnetic field: The area around a magnet where a magnetic force is experienced is called the magnetic field. It is a quantity that has both direction and magnitude, (i.e., Vector quantity).
Magnetic field and field lines: 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.
The magnetic field is represented by 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 mimicks 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.
Strength of magnetic field: The closeness of field lines shows the relative strength of magnetic field, i.e. closer lines show stronger magnetic field and vice – versa. Crowded field lines near the poles of magnet show more strength.
Properties of magnetic field lines
(i) They do not intersect each other.
(ii) It is taken by convention that magnetic field lines emerge from North pole and merge at the South pole. Inside the magnet, their direction is from South pole to North pole. Therefore magnetic field lines are closed curves.
Magnetic field lines due to current a current carrying straight conductor
A current carrying straight conductor has magnetic field in the form of concentric circles, around it. Magnetic field of current carrying straight conductor can be shown by magnetic field lines.
The direction of magnetic field through a current carrying conductor depends upon the direction of flow electric current.
Let a current carrying conductor be suspended vertically and the electric current is flowing from south to north. In this case, the direction of magnetic field will be anticlockwise. If the current is flowing from north to south, the direction of magnetic field will be clockwise.
The direction of magnetic field, in relation to direction of electric
current through a straight conductor can be depicted by using the Right Hand Thumb Rule. It is also known as Maxwell’s Corkscrew Rule.
Right-Hand Thumb Rule: If a current carrying conductor is held by right hand, keeping the thumb straight and if the direction of electric current is in the direction of thumb, then the direction of wrapping of other fingers will show the direction of magnetic field.
Maxwell’s Corkscrew rule: As per Maxwell’s Corkscrew Rule, if the direction of forward movement of screw shows the direction of the current, then the direction of rotation of screw shows the direction of magnetic field.
Properties of magnetic field
• The magnitude of magnetic field increases with increase in electric current and decreases with decrease in electric current.
• The magnitude of magnetic field produced by electric current decreases with increase in distance and vice – versa. The size of concentric circles of magnetic field lines increases with distance from the conductor, which shows that magnetic field decreases with distance.
• Magnetic field lines are always parallel to each other.
• No two field lines cross each other.
Magnetic field lines due to a current through a circular loop
In case of a circular current carrying conductor, the magnetic field is produced in the same manner as it is in case of a straight current carrying conductor.
In case of a circular current carrying conductor, the magnetic field lines would be in the form of iron concentric circles around every part of the FllmSs periphery of the conductor. Since, magnetic field lines tend to remain closer when near to the conductor, so the magnetic field would be stronger near the periphery of the loop. On the other hand, the magnetic field lines would be distant from each other when we move towards the centre of the current carrying loop. Finally, at the centre, the arcs of big circles would appear as a straight line.
The direction of the magnetic field can be identified using Right Hand Thumb’s Rule. Let us assume that the current is moving in anti-clockwise direction in the loop. In that case, the magnetic field would be in clockwise direction, at the top of the loop. Moreover, it would be in an anti-clockwise direction at the bottom of the loop.
Clock Face Rule: A current carrying loop works like a disc magnet. The polarity of this magnet can be easily understood with the help of Clock Face Rule. If the current is flowing in anti – clockwise direction, then the face of the loop shows north pole. On the other hand, if the current is flowing in clockwise direction, then the face of the loop shows south pole.
Magnetic field and number of turns of coil: 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.
The strength of the magnetic field at the centre of the loop(coil) depends on :
(i) The radius of the coil: The strength of the magnetic field is inversely proportional to the radius of the coil. If the radius increases, the magnetic strength at the centre decreases
(ii) The number of turns in the coil : As the number of turns in the coil increase, the magnetic strength at the centre increases, because the current in each circular turn is having the same direction, thus, the field due to each turn adds up.
(iii) The strength of the current flowing in the coil: As the strength of the current increases, the strength of three magnetic fields also increases.
Magnetic field due to a current in a Solenoid: Solenoid is the coil with many circular turns of insulated copper wire wrapped closely in the shape of a cylinder. 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, similar to a bar magnet, which shows that magnetic field is same at all points inside the solenoid.
Magnetic field produced by a solenoid is similar to a bar magnet.
The strength of magnetic field is proportional to the number of turns and magnitude of current.
By producing a strong magnetic field inside the solenoid, magnetic materials can be magnetized. Magnet formed by producing magnetic field inside a solenoid is called electromagnet.
Electromagnet, Fleming’s Left-Hand Rule, Electric motor, Electromagnetic induction, Fleming’s right hand rule, Electric generator and domestic electic circuits.
Electromagnet: An electromagnet consists of a long coil of insulated copper wire wrapped on a soft iron.
Magnet formed by producing magnetic field inside a solenoid is called electromagnet.
Force on a current carrying conductor in a magnetic field: A current carrying conductor exerts a force when a magnet is placed in its vicinity. Similarly, a magnet also exerts equal and opposite force on the current carrying conductor. This was suggested by Marie Ampere, a French Physicist and considered as founder of science of electromagnetism.
The direction of force over the conductor gets reversed with the change in direction of flow of electric current. It is observed that the magnitude of force is highest when the direction of current is at right angles to the magnetic field.
Fleming’s Left-Hand Rule: If the direction of electric current is perpendicular to the magnetic field, the direction of force is also perpendicular to both of them. The Fleming’s Left Hand Rule states that if the left hand is stretched in a way that the index finger, the middle finger and the thumb are in mutually perpendicular directions, then the index finger and middle finger of a stretched left hand show the direction of magnetic field and direction of electric current respectively and the thumb shows the direction of motion or force acting on the conductor. The directions of electric current, magnetic field and force are similar to three mutually perpendicular axes, i.e. x, y, and z-axes.
Many devices, such as electric motor, electric generator, loudspeaker, etc. work on Fleming’s Left Hand Rule.
Electric motor: A device that converts electrical energy to mechanical energy. It is of two types : AC and DC Motor.
Electrical energy is converted into mechanical energy by using and electric motor. Electric motor works on the basis of rule suggested by Marie Ampere and Fleming’s Left Hand Rule.
Principle of Electric Motor: When a rectangular coil is placed in a magnetic field and a current is passed through it, force acts on the coil, which rotates it continuously. With the rotation of the coil, the shaft attached to it also rotates.
Construction: It consists of the following parts :
• Armature: It is a rectangular coil (ABCD) which is suspended between the two poles of a magnetic field.
The electric supply to the coil is connected with a commutator.
• Commutator or Split – ring: Commutator is a device which reverses the direction of flow of electric current through a circuit. It is two halves of the same metallic ring.
• Magnet: Magnetic field is supplied bv a permanent magnet NS.
• Sliding contacts or Brushes Q which are fixed.
• Battery: These are consists of few cells.
Working: When an electric current is supplied to the coil of the electric motor, it gets deflected because of magnetic field. As it reaches the halfway, the split ring which acts as commutator reverses the direction of flow of electric current. Reversal of direction of the current, reverses the direction of forces acting on the coil. The change in direction of force pushes the coil, and it moves another half turn. Thus, the coil completes one rotation around the axle. Continuation of this process keeps the motor in rotation.
In commercial motor, electromagnet instead of permanent magnet and armature is used. Armature is a soft iron core with large number of conducting wire turns over it. Large number of turns of conducting wire enhances the magnetic field produced by armature.
Uses of motors :
• Used in electric fans.
• Used for pumping water.
• Used in various toys.
Electromagnetic Induction: Michael Faraday, an English Physicist is supposed to have studied the generation of electric current using a magnetic field and a conductor.
Electricity production as a result of magnetism (induced current) is called
When a conductor is set to move inside a magnetic field or a magnetic field is set to be changing around a conductor, electric current is induced in the conductor. This is just opposite to the exertion of force by a current carrying conductor inside a magnetic field. In other words, when a conductor is brought in relative motion vis – a – vis a magnetic field, a potential difference is induced in it. This is known as electromagnetic induction.
Fleming’s Right-Hand Rule: Electromagnetic induction can be explained with the help of Fleming’s Right Hand Rule. If the right hand is structured in a way that the index (fore ginger) finger, middle finger and thumb are in mutually perpendicular directions, then the thumb shows direction of induced current in the conductor, in conductor The directions of movement of conductor, magnetic field and induced current can be compared to three mutually perpendicular axes, i.e. x, y and z axes.
The mutually perpendicular directions also point to an important fact that when the magnetic field and movement of conductor are perpendicular, the magnitude of induced current would be maximum.
Electromagnetic induction is used in the conversion of kinetic energy into electrical energy.
Electric Generator: A device that converts mechanical energy into electrical energy is called an electric generator.
Electric generators are of two types: AC generator and a DC generator. Principle of electric generator: Electric motor works on the basis of electromagnetic induction.
Construction and Working: The structure of an electric generator is similar to that of an electric motor. In case of an electric generator, a rectangular armature is placed within the magnetic field of a permanent magnet. The armature is attached to wire and is positioned in a way that it can move around an axle. When the armature moves within the magnetic field, an electric current is induced. The direction of induced current changes, when the armature crosses the halfway mark of its rotation.
Thus, the direction of current changes once in every rotation. Due to this, the electric generator usually produces alternate current, i.e. A.C. To convert an A.C generator into a D.C generator, a split ring commutator is used. This helps in producing direct current.
Electrical generator is used to convert mechanical energy into electrical energy.
A.C and D.C Current
A.C – Alternate Current: Current in which direction is changed periodically is called Alternate Current. In India, most of the power stations generate alternate current. The direction of current changes after every 1/100 second in India, i.e. the frequency of A.C in India is 50 Hz. A.C is transmitted upto a long distance without much loss of energy is advantage of A.C over D.C.
D.C – Direct Current: Current that flows in one direction only is called Direct current. Electrochemical cells produce direct current.
Advantages of A.C over D.C
• Cost of generatior of A.C is much less than that of D.C.
• C can be easily converted to D.C.
• C can be controlled by the use of choke which involves less loss of power whereas, D.C can be controlled using resistances which involves high energy loss.
• AC can be transmitted over long distances without much loss of energy.
• AC machines are stout and durable and do not need much maintenance.
Disadvantages of AC
• AC cannot be used for the electrolysis process or showing electromagnetism as it reverses its polarity.
• AC is more dangerous than DC.
Domestic Electric Circuits: We receive electric supply through mains supported through the poles or cables. In our houses, we receive AC electric power of 220 V with a frequency of 50 Hz.
The 3 wires are as follows
• Live wire – (Red insulated, Positive)
• Neutral wire – (Black insulated, Negative)
• Earth wire – (Green insulated) for safety measure to ensure that any leakage of current to a metallic body does not give any serious shock to a user.
Short Circuit: Short-circuiting is caused by the touching of live wires and neutral wire and sudden a large current flows.
It happens due to
• damage pf insulation in power lines.
• a fault in an electrical appliance.
Overloading of an Electric Circuit: The overheating of electrical wire in any circuit due to the flow of a large current through it is called overloading of the electrical circuit.
A sudden large amount of current flows through the wire, which causes overheating of wire and may cause fire also.
Electric Fuse: It is a protective device used for protecting the circuit from short-circuiting and overloading. It is a piece of thin wire of material having a low melting point and high resistance.
• Fuse is always connected to live wire.
• Fuse is always connected in series to the electric circuit.
• Fuse is always connected to the beginning of an electric circuit.
• Fuse works on the heating effect.
Magnetic field: The area around a magnet in which other magnet feels force of attraction or repulsion is called Magnetic field.
Magnetic field lines: The closed curved imaginary lines in the magnetic field which indicate the direction of motion of north pole in the magnetic field if a magnet is free to do so.
Properties of magnetic field lines.
• Magnetic Field lines originate from the north pole of a magnet and end at its south pole.
• Magnetic Field lines are denser near the poles but rarer at other places.
• The Magnetic Field lines do not intersect one another.
Oersted’s experiment: According to this experiment “A current carrying wire creates a magnetic field around it. The direction of magnetic field depends on the direction of current in conductor.”
• Magnetic field pattern due to straight current carrying conductor are concentric circles whose center lie on the wire.
• The direction of magnetic field due to straight current carrying conductor can be determined by Right hand thumb rule.
Right hand thumb rule: According to this rule “if current carrying conductor is held in the right hand in such a way that thumb indicate the direction of current, then the curled finger indicates the direction of magnetic field lines around conductor.”
Magnetic field pattern due to current carrying loop: The Magnetic field lines are circular near the current-carrying loop. As we move away from the loop, field lines form bigger and bigger circles. At the center of the circular loop, the magnetic field lines are straight.
The solenoid is an insulated and tightly wound long circular wire having large number of turns whose radius is small in comparison to its length. Magnetic field produced by a solenoid is similar to the magnetic field produced by a bar magnet.
Current carrying solenoid is called an electromagnet.
Properties of magnetic lines of force or magnetic field lines.
• These lines originate from the north pole and end at the south pole.
• The magnetic field lines of a magnet form a continuous closed loop.
• Two magnetic lines of force do not intersect each other.
• The tangent at any point on the magnetic line gives the direction of the magnetic field at the point.
Fleming’s left hand rule: According to this rule, “if the thumb, forefinger and middle finger of the left hand are stretched perpendicular to each other and if the fore-finger gives the direction of magnetic field, middle finger gives the direction of current, then the thumb will give the direction of motion or the force acting on the current-carrying conductor.”
Principle of an electric motor: A motor works on the principle that when a rectangular coil is placed in a magnetic field and current passes through it, a force acts on the coil which rotates it continuously.
When the coil rotates, the shaft attached to it also rotates. In this way the electrical energy supplied to the motor is converted into the mechanical energy of rotation.
Principle of an electric generator: It is based on the principle of electromagnetic induction. It states that “an induced current is produced in a coil placed in a region where the magnetic field changes with time.” The direction of induced current is given by Fleming’s right-hand rule. An electric generator converts mechanical energy into electrical energy.
Electromagnetic induction: The phenomenon of setting up of an electric current or an induced e.mi. by changing the magnetic lines of force by a moving conductor is called electromagnetic induction.
Maxwell’s right hand thumb rule: The direction of the current is given by Maxwell’s right-hand thumb rule, “If the current carrying conductor is gripped with the right hand in such a way that the thumb gives the direction of the current, then the direction of the fingers gives the direction of the magnetic field produced around the conductor.
Fleming’s left-hand rule: The direction of motion of a conductor in a magnetic field is given by Fleming’s left-hand rule. According to this rule, if the thumb, forefinger and middle finger of the left hand are stretched perpendicular to each other and if fore-finger gives the direction of the magnetic field and the middle finger gives the direction of current then, the thumb will give the direction of the motion of the conductor carrying the current.
Fleming’s right-hand rule: The direction of the induced current is given by Fleming’s right-hand rule. According to this rule if the thumb, forefinger and middle finger of the right hand are stretched perpendicular to each other and if the fore-finger gives the direction of the magnetic field and the thumb gives the direction of motion, then the middle finger will give the direction of the induced current in the conductor.
Very Short Anwer Type Questions
Question. A straight wire carrying electric current is moving out of a plane of paper and is perpendicular to it. What is the direction of the magnetic field?
Answer : Magnetic field lines will be concentric circles in the plane of the paper and in anti-clockwise direction which can be found out by applying Right hand thumb rule.
Question. What happens when an iron core is inserted into a current carrying solenoid?
Answer : When an iron core is inserted into a current carrying solenoid, strength of the magnetic field produced inside the solenoid increases and it forms an electromagnet.
Question. Define the term induced electric current.
Answer : Induced electric current: It is the current which is created due to the relative motion of coil or magnet. The induced current is found to be the highest when the direction of motion of the coil is at right angles to the magnetic field.
Question. What is indicated by crowding of magnetic field lines in a given region?
Answer : The crowding of magnetic field lines in a given region indicates that the magnetic field is stronger in that region.
Question. The change in magnetic field lines in a coil is the cause of induced electric current in it. Name the underlying phenomenon.
Answer : The change in magnetic field lines in a coil is the cause of induced current in it and the phenomenon is electromagnetic induction.
Short Answer
Question. Under what conditions permanent electromagnet is obtained if a current carrying solenoid is used? Support your answer with the help of a labeled circuit diagram.
Answer : The following conditions are required to obtain permanent electromagnet when a current carrying solenoid is used:
(1) Rod inside the solenoid should be made of magnetic material like steel which should retain magnetic properties for a long time after magnetization.
(b) The current through the solenoid should be direct current.
(c) The number of turns in the solenoid should be large and closely packed, so that a strong uniform magnetic field inside it is produced.
Question. State the rule to determine the direction of a (A) magnetic field produced around a straight conductor-carrying current, (B) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (C) current induced in a coil due to its rotation in a magnetic field.
Answer : Rule to determine the direction:
(A) Right hand thumb rule which states that that if one holds a straight current carrying conductor with right hand such that the thumb points towards the direction of current, then fingers will wrap around the conductor in the direction of field lines of the magnetic field.
This rule determines the magnetic field produced around a straight conductor carrying current.
(B) Fleming’s Left Hand Rule, which states that if the first finger points in the direction of magnetic field and second finger in the direction of current, then the thumb will
point in the direction of motion or the force acting on the conductor.
This determines force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it.
(C) Fleming’s Right Hand Rule, which states that if the forefinger indicates the direction of the magnetic field and thumb shows the direction of motion of conductor, then
the middle finger will show the direction of induced current.
This rule determines the current induced in a coil due to its rotation in a magnetic field.
Question. What are the differences between permanent magnet and electromagnet?
Answer : Difference between permanent magnet and electromagnet:
Question. (A) Draw the pattern of magnetic field lines due to a magnetic field through and around a current carrying circular loop.
(B) Name and state the rule to find out the direction of magnetic field inside and around the loop.
Answer : (A)
(B) The magnetic field lines are concentric circles at every point of a current carrying circular loop. The direction of magnetic field of every section of the circular loop can be found be using the right hand thumb rule.
(1) The magnetic field lines are near circular at the points where the current enters or leaves the coil.
(2) Within the coil, the field lines are in the same direction.
(3) Near the centre of the coil, the magnetic field lines are almost parallel to each other.
(4) At the centre of the coil, the plane of magnetic field lines is at right angle to the plane of the circular coil.
Question. With the help of a labeled circuit diagram,illustrate the pattern of field lines of the magnetic field around a current-carrying straight long conducting wire. How is the right hand thumb rule useful in finding the direction of a magnetic field associated with a current-carrying conductor?
Answer : To be able to easily comprehend the direction of field lines around a current carrying conductor,we fix the straight conducting wire (PQ) at the centre (O) of a cardboard and sprinkle some iron fillings on the cardboard. Now we connect the conducting wire to terminals of a battery through a variable resistor, an ammeter (A) and a key (K).
As you can see in the image above, when current is passed through the conductor, iron fillings rearrange themselves in concentric circles (due to magnetic fields produced).
The arrows in the circles show the direction of magnetic field lines. The arrow just above P (or after Q) shows the direction of current flow.
We used Fleming right-hand thumb rule (as shown in the image below) to identify the flow of current and field lines.
In this rule, if a current carrying straight wire conductor is held by the right hand such that the thumb points towards the direction of current, then the fingers will wrap around the
conductor in the direction of the field lines of the magnetic field.
Question. A current through a horizontal power line flows in west to east direction.
(A) What is the direction of the magnetic field at a point directly above it and at a point directly below it?
(B) Name the rule used to determine:
(1) The direction of force when a current carrying wire is placed in a strong magnetic field.
(2) Magnetic field in a current carrying conductor.
Answer : (A) The current is in the east-west direction.
Applying the right-hand thumb rule, we get that the direction of magnetic field at a point above the wire is from south to north or anticlockwise direction. The direction of magnetic field at a point directly below the wire is north to south or clockwise direction.
(B) (1) According to Fleming’s left-hand rule, hold the forefinger, the centre finger and the thumb of your left hand at right angles to one another.
Adjust your hand in such a way that the forefinger points in the direction of the magnetic field and the centre finger points in the direction of current, then the direction in which the thumb points, gives the direction of the force acting on the conductor.
(2) According to Maxwell’s right hand thumb rule: Imagine that you are holding the current-carrying wire in your right hand so that your thumb points in the direction of current, then the direction in which your fingers encircle the wire will give the direction of the magnetic field lines around the wire.
Question. What happens to the magnetic field lines due to a current-carrying conductor when the current is reversed?
Answer : When we reverse the direction of electric current flowing in the current-carrying conductor, then the direction of magnetic field lines produced by it are also reversed.
LONG ANSWER
Question. (A) What is an electromagnet? List any two uses.
(B) Draw a labelled diagram to show how an electromagnet is made.
(C) State the purpose of soft iron core used in making an electromagnet.
(D) List two ways of increasing the strength of an electromagnet if the material of the electromagnet is fixed.
Answer : (A) An electromagnet is a temporary strong magnet. Its magnetism is only for the duration of current passing through it. The polarity and strength of an electromagnet can be changed. Uses of electromagnet-
Electromagnets are used:
(1) in electrical appliances like electric bell, electric fan etc.
(2) in electric motors and generators.
(3) in radios, television, microphone etc.
(4) in separating iron from non-magnetic material.
(5) in magnetising steel bars. (Any two)
(B)
(C) Soft iron increases the strength of the electromagnet.
(D) Ways of increasing the strength of an electromagnet:
(1) If we increase the number of turns in the coil, the strength of electromagnet increases.
(2) If the current in the coil is increased, the strength of electromagnet increases.
Question. What is a solenoid? Draw the pattern of magnetic field lines of (1) a current carrying solenoid and (2) a bar magnet. List two distinguishing feature between the two fields.
Answer : Solenoid:
A solenoid is a coil of many circular turns of wire wrapped in the shape of a cylinder. The magnetic field produced by a solenoid is very similar to that of a bar magnet and is uniform
inside the solenoid.
(1) Diagram showing pattern of field lines of a current carrying solenoid:
(2) Diagram showing pattern of field lines of a bar magnet:
Question. Answer the following questions:
(A) What is solenoid? Draw field lines of the magnetic field through and around a current-carrying solenoid.
(B) If field lines of a magnetic field are crossed at a point, what does it indicate?
Answer : (A) Solenoid: A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.
Magnetic field around a current carrying solenoid is shown in the figure.
These appear to be similar to that of a bar magnet. One end of the solenoid behaves like North Pole and the other end behaves like the South Pole. Magnetic field
lines inside the solenoid are in the form of parallel straight lines. This means that the field is same at all the points inside the solenoid.
(B) No two field-lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.
Question. A coil of insulated copper wire is connected to a galvanometer. What would happen if a strong bar magnet is
(A) pushed into the coil?
(B) withdrawn from inside the coil?
(C) held stationary inside the coil?
Give Justification for each observation.
Answer : (A) When a strong bar magnet is pushed into a coil of insulated copper wire connected to a galvanometer, we will observe a deflection in the galvanometer in one direction. This is because current is induced in the coil in a particular direction (either clockwise or anti clockwise) due to an increasing magnetic field. This phenomenon is called electromagnetic induction.
(B) When the magnet is withdrawn from the coil, we will again observe a deflection in the galvanometer but in direction opposite to that observed in (A).
This is because current is again induced in the coil but in an opposite direction due to a decreasing magnetic field.
(C) No deflection will be observed when the coil is held stationary as no current will be induced since magnetic field is not changing.
Question. (A) What are magnetic field lines? How is the direction of magnetic field at a point in a magnetic field determined using field lines?
(B) Two circular coils ‘X’ and ‘Y’ are placed close to each other. If the current in the coil ‘X’ is changed, will some current be induced in the coil ‘Y’? Give reason.
(C) State ‘Fleming’s right hand rule”.
Answer : (A) Magnetic field lines are the imaginary lines drawn around a magnet which describe the magnetic field around a magnet and indicate the direction in which a hypothetical North
pole would move if placed at that point.
The direction of magnetic field at a point in a magnetic field is determined by drawing a tangent at that point.
(B) Yes, if current is changed in coil X, a current will be induced in the coil Y. When current in coil X changes, the magnetic field associated with it also changes, thereby changing the
magnetic field lines associated with the coil Y. This induces a current in coil Y by the process of electro magnetic induction..
(C) Fleming’s right hand rule:
Stretch the forefinger, the central finger and the thumb of the right hand perpendicular to each other so that the forefinger indicates the direction of the field, and the thumb is in the direction of motion of the conductor.
Then, the central finger shows the direction of current induced in the conductor.
Question. (A) Why don’t two magnetic field lines ever intersect each other? Explain.
(B) ‘‘The magnetic field is said to be uniform inside a current carrying solenoid.’’ Why?
(C) State Fleming’s left hand rule.
(D) Enlist two factors that enhance the power of commercial motors.
Answer : (A) Two magnetic field lines can never cross each other because it would mean that at the point of intersection the compass needle would point towards two directions
simultaneously which is not possible.
(B) The magnetic field lines inside a current carrying solenoid are in the form of parallel straight lines. This indicates that the magnetic field is the same (uniform)
at all points inside the solenoid.
(C) Fleming’s left hand rule:
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 the magnetic field, the second finger in the direction of current, then the thumb will point in the direction of motion.
(d) Any two factors:
(i) Strength of electromagnet
(ii) Large number of coil/turns of the conducting wire
(iii) A soft iron core on which the coil is wound.
Question. A student fixes a sheet of white paper on a drawing board. He places a bar magnet in the centre of it. He sprinkles some iron filings uniformly around the bar magnet. Then he taps the board gently and observes that the iron filings arrange themselves in a particular pattern.
(A) Why do the iron filings arrange in a pattern?
(B) What do the lines along which the iron filings align represent?
(C) What does the crowding of iron filings at the end of the magnet indicate?
(D) List the properties of magnetic field lines.
Answer : (A) Iron filings arrange in a pattern because a magnetic field exists around a magnet and force is exerted by the magnet within its magnetic field.
(B) The lines represent magnetic field lines.
(C) The crowding of iron filings at the end of the magnet indicates that the strength of magnetic field is maximum near the poles of the magnet.
(D) Properties of magnetic field lines:
(1) Magnetic field lines are closed continuous curves.
(2) The tangent at any point on the magnetic field lines gives the direction of magnetic field at that point.
(3) No two magnetic field lines can intersect each other.
(4) They are crowded in a region of strong magnetic field and are far from each other in a region of weak magnetic field.
Question. (A) Name and state the rule to determine the direction of force experienced by a current carrying straight conductor placed in a uniform magnetic field which is perpendicular to it.
(B) Draw a labelled diagram of an electric motor.
Answer : (A) The rule is Fleming’s Left Hand Rule.
Statement: Stretch the forefinger, the central finger and the thumb of your left hand mutually perpendicular to each other.
If the forefinger points towards the direction of the field and the central finger that of the current, then the thumb will point towards the direction of motion of the conductor, i.e.,
force.
(B) Diagram of electric motor
Question. (A) A coil of insulated wire is connected to a galvanometer. What would be observed if a strong bar magnet with its south pole towards one face of the coil is:
(i) moved quickly toward it?
(ii) moved quickly away from it?
(iii) held stationary near it?
(B) Name the phenomena involved.
(C) State the conclusion based on the observations in (i), (ii) and (iii).
Answer : (A) A coil of insulated wire is connected to a galvanometer. When a strong bar magnet with its south pole facing towards one face of coil is:
(i) Moved quickly towards: These is a momentary deflection in the needle of the galvanometer, say to the left. This indicates the presence of current in the coil.
(ii) Moved quickly away from it: The galvanometer is deflected towards the left, showing that current is now set up in the direction opposite to the first, say to the right.
(iii) Held stationery near it: The galvanometer does not show any deflection indicating that no current is produced in the coil.
(B) The phenomenon involved is Electromagnetic induction.
(C) Conclusion: The motion of strong bar magnet with respect to coil produces an induced potential difference, which sets up an induced electric current in the coil.
