Choose the incorrect statement from the following regarding magnetic lines of field
The direction of magnetic field at a point is taken to be the direction in which the north pole of a magnetic compass needle points
Magnetic field lines are closed curves
If magnetic field lines are parallel and equidistant, they represent zero field strength
Relative strength of magnetic field is shown by the degree of closeness of the field lines
If the key in the arrangement is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane ABCD, the lines are
concentric circles
elliptical in shape
straight lines parallel to each other
concentric circles near the point O but of elliptical shapes as we go away from it
A circular loop placed in a plane perpendicular to the plane of paper carries a current when the key is ON. The magnetic field lines point from B to A. The N-pole of the resultant magnet is on the face close to
A
B
A if the current is small, and B if the current is large
B if the current is small and A if the current is large
For a current in a long straight solenoid N- and S-poles are created at the two ends. Among the following statements, the incorrect statement is
The field lines inside the solenoid are in the form of straight lines which indicates that the magnetic field is the same at all points inside the solenoid
The strong magnetic field produced inside the solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the coil
The pattern of the magnetic field associated with the solenoid is different from the pattern of the magnetic field around a bar magnet
The N- and S-poles exchange position when the direction of current through the solenoid is reversed
A uniform magnetic field exists in the plane of paper pointing from left to right. In the field an electron and a proton move as shown. The electron and the proton experience
forces both pointing into the plane of paper
forces both pointing out of the plane of paper
forces pointing into the plane of paper and out of the plane of paper, respectively
force pointing opposite and along the direction of the uniform magnetic field respectively
Commercial electric motors do not use
an electromagnet to rotate the armature
effectively large number of turns of conducting wire in the current carrying coil
a permanent magnet to rotate the armature
a soft iron core on which the coil is wound
In the arrangement shown there are two coils wound on a non-conducting cylindrical rod. Initially the key is not inserted. Then the key is inserted and later removed. Then
the deflection in the galvanometer remains zero throughout
there is a momentary deflection in the galvanometer but it dies out shortly and there is no effect when the key is removed
there are momentary galvanometer deflections that die out shortly; the deflections are in the same direction
there are momentary galvanometer deflections that die out shortly; the deflections are in opposite directions
Choose the incorrect statement
Fleming's right-hand rule is a simple rule to know the direction of induced current
The right-hand thumb rule is used to find the direction of magnetic fields due to current carrying conductors
The difference between the direct and alternating currents is that the direct current always flows in one direction, whereas the alternating current reverses its direction periodically
In India, the AC changes direction after every 1/50 second
A constant current flows in a horizontal wire in the plane of the paper from east to west. The direction of magnetic field at a point will be North to South
directly above the wire
directly below the wire
at a point located in the plane of the paper, on the north side of the wire
at a point located in the plane of the paper, on the south side of the wire
The strength of magnetic field inside a long current carrying straight solenoid is
more at the ends than at the centre
minimum in the middle
same at all points
found to increase from one end to the other
To convert an AC generator into DC generator
split-ring type commutator must be used
slip rings and brushes must be used
a stronger magnetic field has to be used
a rectangular wire loop has to be used
The most important safety method used for protecting home appliances from short circuiting or overloading is
earthing
use of fuse
use of stabilizers
use of electric meter
A magnetic compass needle is placed in the plane of paper near point A. In which plane should a straight current-carrying conductor be placed so that it passes through A and there is no change in the deflection of the compass? Under what condition is the deflection maximum and why?
No deflection occurs when the conductor is placed in the plane of the paper itself since the magnetic field due to the conductor is also vertical. Maximum deflection occurs when the conductor is perpendicular to the plane of the paper and current in it is maximum.
Under what conditions is a permanent electromagnet obtained if a current-carrying solenoid is used? Support your answer.
The solenoid should be connected to a direct current source.
The rod inside the solenoid should be made of magnetic material such as steel.
AB is a current-carrying conductor in the plane of the paper. What are the directions of magnetic fields produced at points P and Q? Given \( r_1 > r_2 \), where will the magnetic field be stronger?
The magnetic field at P is into the plane of the paper and at Q is out of the plane of the paper. The strength is larger at Q because \( r_2 < r_1 \).
A magnetic compass shows a deflection when placed near a current-carrying wire. How will the deflection of the compass get affected if the current in the wire is increased? Give a reason.
The deflection increases because the magnetic field strength is directly proportional to the current in the conductor.
An electric current through a conductor produces a magnetic field around it. Is there a similar magnetic field around a thin beam of moving (i) alpha particles, (ii) neutrons? Justify your answer.
(i) Yes, alpha particles are positively charged and constitute a current when in motion.
(ii) Neutrons are electrically neutral; hence they produce no magnetic field.
What does the direction of thumb indicate in the right-hand thumb rule? How is this rule different from Fleming’s left-hand rule?
The thumb indicates the direction of current in the conductor. Fleming’s left-hand rule gives the direction of force on a current-carrying conductor in an external magnetic field.
Meena draws magnetic field lines of a current-carrying circular loop. As she moves away from the centre she observes that the lines keep on diverging. How will you explain her observation?
The magnetic field strength decreases with increasing distance from the loop, indicated by the decrease in closeness of field lines.
What does the divergence of magnetic field lines near the ends of a current-carrying straight solenoid indicate?
The divergence shows that the magnetic field strength decreases beyond the ends of the solenoid.
Name four appliances where an electric motor is used as an important component. In what respect are motors different from generators?
Electric fans, mixers, washing machines, and computer drives use electric motors.
Motors convert electrical energy into mechanical energy, whereas generators convert mechanical energy into electrical energy.
What is the role of the two conducting stationary brushes in a simple electric motor?
The brushes are connected to the battery and touch the outer sides of the split ring whose inner sides are insulated and attached to the axle.
What is the difference between a direct current and an alternating current? How many times does AC used in India change direction in one second?
DC flows in one direction only, while AC reverses direction periodically.
In India, AC frequency is 50 Hz, so it changes direction 100 times per second.
What is the role of a fuse used in series with any electrical appliance? Why should a fuse with a defined rating not be replaced by one with a larger rating?
A fuse protects appliances from short-circuiting or overloading by blowing off when excess current flows. A fuse with a larger rating would not blow in time and may damage appliances.
Why does a magnetic compass needle pointing North and South in the absence of a nearby magnet get deflected when a bar magnet or a current carrying loop is brought near it? Describe some salient features of magnetic lines of field concept.
A magnetic compass needle aligns itself along the earth’s magnetic field in the absence of any other magnetic influence. When a bar magnet or a current-carrying loop is brought near it, the magnetic field created by them interacts with the earth’s magnetic field. This modifies the net magnetic field at the position of the compass and causes the needle to deflect.
Magnetic field lines emerge from the North pole and enter the South pole. They represent both the direction and the magnitude of the magnetic field. The magnetic field strength is indicated by the closeness of field lines. Field lines never cross each other because two different magnetic field directions cannot exist at the same point. If field lines are parallel and equispaced in a region, the magnetic field is considered uniform.
With the help of a labelled 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 to find direction of magnetic field associated with a current carrying conductor?
The magnetic field around a current-carrying straight conductor consists of concentric circles centred on the wire. The direction of the magnetic field depends on the direction of current in the conductor.
According to the right-hand thumb rule, if the current-carrying conductor is held in the right hand such that the thumb points in the direction of current, then the fingers curled around the conductor give the direction of magnetic field lines. This rule helps determine the orientation of the circular field around the straight conductor.
Explain with the help of a labelled diagram the distribution of magnetic field due to a current through a circular loop. Why is it that if a current carrying coil has \( n \) turns the field produced at any point is \( n \) times as large as that produced by a single turn?
The magnetic field produced by a current-carrying circular loop appears as concentric circles near the wire and becomes nearly straight near the centre of the loop. The field is strongest at the centre of the loop.
If there are \( n \) turns in the circular coil, each turn carries the same current and produces a magnetic field of the same magnitude at the centre. These individual fields add up in the same direction. Therefore, the total magnetic field at the centre becomes \( n \) times the magnetic field produced by a single turn.
Describe the activity that shows that a current-carrying conductor experiences a force perpendicular to its length and the external magnetic field. How does Fleming's left-hand rule help us to find the direction of the force acting on the current carrying conductor?
When a current-carrying conductor is placed in a region of an external magnetic field, it experiences a force. This can be demonstrated by suspending a current-carrying wire between the poles of a magnet. When current flows through the wire, the conductor is seen to move, showing that a force acts on it. When the direction of current is reversed, the direction of motion is also reversed, confirming the dependence on current direction.
Fleming’s left-hand rule states that if the thumb, forefinger, and middle finger of the left hand are stretched mutually perpendicular, the forefinger represents the direction of magnetic field, the middle finger represents the direction of current, and the thumb gives the direction of force or motion. This helps determine the direction of force on the conductor.
Draw a labelled circuit diagram of a simple electric motor and explain its working. In what way are simple electric motors different from commercial motors?
A simple electric motor works on the principle that a current-carrying coil in a magnetic field experiences a torque that rotates the coil. When current flows through the rectangular coil placed between two magnetic poles, opposite forces act on the two sides of the coil, producing rotation. A split-ring commutator reverses the direction of current in the coil every half-turn, ensuring continuous rotation in the same direction.
Commercial motors differ in that they use an electromagnet instead of a permanent magnet, have many turns of conducting wire in the coil to increase force, and use a soft iron core on which the coil is wound to enhance the magnetic effect.
Explain the phenomenon of electromagnetic induction. Describe an experiment to show that a current is set up in a closed loop when an external magnetic field passing through the loop increases or decreases.
Electromagnetic induction is the process by which a change in magnetic field linked with a closed conductor induces an electric current in it. The induced current arises due to the changing magnetic flux through the loop.
To demonstrate this, take two coils wound on a soft iron core. Connect one coil to a battery and a switch, and the other coil to a galvanometer. When the switch of the first coil is closed or opened, the galvanometer in the second coil shows a momentary deflection, indicating that a current is induced whenever the magnetic field through the second coil changes. Increasing or decreasing current in the primary coil similarly induces current in the secondary coil.
Describe the working of an AC generator with the help of a labelled circuit diagram. What changes must be made in the arrangement to convert it to a DC generator?
An AC generator works on the principle of electromagnetic induction. A coil is rotated in a magnetic field, causing the magnetic flux through the coil to change continuously. This produces an alternating induced emf in the coil. Slip rings attached to the coil allow the alternating current to be taken out through brushes.
To convert an AC generator into a DC generator, the slip rings must be replaced with a split-ring commutator. This ensures that the direction of current in the external circuit remains the same, producing direct current instead of alternating current.
Draw an appropriate schematic diagram showing common domestic circuits and discuss the importance of fuse. Why is it that a burnt out fuse should be replaced by another fuse of identical rating?
Domestic wiring consists of live, neutral, and earth wires connected to various household devices through switches and distribution circuits. Fuses are included in the live wire to protect appliances from overloading or short-circuiting.
A fuse melts when excessive current flows, thereby breaking the circuit and preventing damage. A burnt-out fuse must be replaced with one of the same rating because a fuse with a higher rating may not blow when needed, potentially damaging appliances.