Magnetic Effect of Electric current
One Marks Questions
Answer: When an electric current flows through a conductor, a magnetic field is produced around it. This phenomenon is called as magnetic effect of electric current.
Answer: The region around a magnet in which the force of the magnet can be experienced is called as magnetic field.
Answer: The current carrying conductor should be kept in a magnetic field such that the direction of current is perpendicular to magnetic field to get maximum displacement.
Answer: An electric motor is a rotating device that coverts electrical energy to mechanical energy.
Answer: Split rings act as commutator. They help to change the direction of current in a coil for every half rotation.
Answer: Motor works on a principle The current carrying conductor kept in a magnetic field experiences a mechanical force.
Answer: The phenomenon of electromagnetic induction is the production of induced current in a coil placed in a region where the magnetic field changes with time.
Answer: Generator works on the principle of electromagnetic induction. The phenomenon of electromagnetic induction is the production of induced current in a coil placed in a region where the magnetic field changes with time.
Answer:
Live wire – red
neutral wire – black and
ground wire - green
Answer: 50 Hz
Answer: DC power can be transmitted over long distances without much loss of energy.
Answer: Unity among the people of India.
Answer: 220 V
Two Marks Questions
Answer: When current-carrying straightconductor is held in right hand such that the thumb points towards the direction of current, Then other fingers will wrap around the conductor in the direction of the field lines of the magnetic field. This is known as the right-hand thumb rule.
Answer: stretch the thumb, forefinger and middle finger of 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.
Answer: Stretch the thumb, forefinger and middle finger of right hand so that they are perpendicular to each other, as shown in. 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. This rule is called Fleming’s right-hand rule.
Answer: 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.
| AC | DC |
|---|---|
| Changes direction periodically | Flows along single direction |
| Can be transmitted over long distances without muh loss of energy | There is a loss of energy during transmission of DC |
Answer: The use of an electric fuse prevents the electric circuit and the appliance from a possible damage by stopping the flow of unduly high electric current. The Joule heating that takes place in the fuse melts it to break the electric circuit.Thus it prevents electric circuit during short circuit.
Three and Four Marks Questions
Answer:
• The field lines emerge from north pole and merge at the south pole. Inside the magnet, the direction of field lines is from its south pole to its north pole. Thus the magnetic field lines are closed curves.
• The relative strength of the magnetic field is shown by the degree of closeness of the field lines.
• The field is stronger, that is, the force acting on the pole of another magnet placed is greater where the field lines are crowded.
• No two field-lines are found to cross each other.
Answer:
• Magnetic field lines around current carrying straight conductor are concentric circles.
• The magnetic field produced by a given current in the conductor decreases as the distance from it increases.
• The concentric circles representing the magnetic field around a current-carrying straight wire become larger and larger as we move away from it.
Answer:
• The magnetic field produced by a current-carrying straight wire depends inversely on the distance from it.
• At every point of a current-carrying circular loop, the concentric circles representing the magnetic field around it would become larger and larger as we move away from the wire.
• At the centre of the circular loop, the arcs of these big circles would appear as straight lines.
• The magnetic field produced by a current-carrying wire at a given point depends directly on the current passing through it. Therefore, if there is a circular coil having n turns, the field produced is n times as large as that produced by a single turn.
Answer:
* The magnetic field around solenoid is similar to that in bar magnet.
* one end of the solenoid behaves as a magnetic north pole, while the other behaves as the south pole.
* The field lines inside the solenoid are in the form of parallel straight lines.
* This indicates that the magnetic field is the same at all points inside the solenoid. That is, the field is uniform inside the solenoid.
Answer:
a. When bar magnet is pushed into the coil of insulated copper wire connected to a galvanometer, an induced current is set-up in the coil due to change of magnetic field through it. As a result galvanometer gives a deflection.
b. When the bar magnet is withdrawn from inside the coil, again an induced current is set up in the coil. As a result galvanometer gives deflection in the reverse direction.
c. If magnet is held stationary inside the coil , then there is no induced current in the coil. Because there is no change in the magnetic field. As a result , galvanometer does not show any deflection.
Answer:
Construction :
• 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. The inner sides of these halves are insulated and attached to an axle.
• The external conducting edges of P and Q touch two conducting stationary brushes X and Y, respectively.
Working :
• Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through brush Y.
• The 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.
• 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.
• 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.
Answer:
• An electric generator consists of a rotating rectangular coil ABCD placed between the two poles of a permanent magnet.
• The two ends of this coil are connected to the two rings R1 and R2. The inner side of these rings are made insulated.
• The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively.
• The two rings R1 and R2 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 galvanometer to show the flow of current in the given external circuit.
Working :
• When the axle attached to the two rings is rotated such that the arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet.
• When the coil ABCD is rotated clockwise in the arrangement 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. If there are larger numbers of turns in the coil, the current generated in each turn adds up to give a large current through the coil. 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.
