NCERT Class X Science Class: Chapter –13. Magnetic Effects of Electric Current – Part-6

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Question 27:

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?

Answer:

The magnetic field pattern due to a circular coil is shown in the given figure. At every point of current carrying circular loop the concentric circles representing the magnetic field around it becomes larger and larger as we move away from the wire. At the centre of the loop the field appears as straight line.

Image Magnetic field pattern

Image Magnetic Field Pattern

Image Magnetic field pattern

The magnetic field produced by current carrying circular wire at a given point, depend on (i) Amount of current flowing through the wire The strength of magnetic field (B) due to current carrying circular wire is directly proportional to the amount of current (J) flowing through it. i.e.,

(ii) Number of turns of the circular wire The strength of the magnetic field due to current carrying wire is directly proportional to the number of turns (N)oi the wire. i.e.,

If there is a circular coil having n turns, the field produced is n times as large as that produced by a single turn. It happens because the current in each circular turn has same direction and the field due to each turn just adds up.

Question 28:

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?

Answer:

Explain According to Fleming’s left hand rule, stretch the thumb, forefinger and central finger of your left hand such that they are mutually perpendicular. If the fore finger points in the direction of magnetic field and the central in the direction of current, then the thumb will point in the direction of motion or force acting on the conductor.

Image Magnetic field and finger points

Image Magnetic Field and Finger Points

Image Magnetic field and finger points

Question 29:

Draw a labelled circuit diagram of a simple electric motor and explain its working. In what way these simple electric motors are different from commercial motors?

Answer:

The circuit diagram of a simple electric motor is shown below:

Image Split rings and Brushes

Image Split Rings and Brushes

Image Split rings and Brushes

Working:

  1. Let coil ABCD be in horizontal position, when the key is switched on, the current flows indirection DCBA and then leaves through the brushes ,via ring

  2. Applying Fleming’s left-hand rule, no force acts on arm 6C and AD as they are parallel to magnetic field, arm A8 experiences a force in downward direction and arm CD experiences an equal force in upward direction. A torque acts on the coil and it rotates in anti-clockwise direction,

  3. While rotating, the coil reaches the vertical position, brushes loose contact with the rings and current stops flowing, But the coil does not stop due to inertia of motion,

  4. When the coil passes the vertical position, the rings automatically change their positions and come in contact with opposite brushes.

  5. This reverses the direction of current through the coil but the direction of current on right-hand side remains same.

  6. So, the force on right hand side is always upward and a force on left-hand side is always in downward direction. Thus, the coil continues to anti-clockwise direction.

A commercial electric motor is one which uses the following (i) An electromagnet in place of permanent magnet. (ii) Large number of turns conducting wire in current carrying coil. (iii) A soft iron core on which the coil is wound. The combination of soft iron core and coil is an armature. It enhances the power of motor.

Thus, commercial electric motors do not use permanent magnet to rotate the armature because permanent magnets are weak and do not produce strong magnetic field in the region.

Question 30:

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.

Answer:

This phenomenon was discovered by Michael Faraday and Joseph Henry in 1831. The phenomenon of generation of an electric current in a closed circuit from magnetic effects i.e., by changing the magnetic field is called electromagnetic induction. Electric current produced due to this phenomenon is called induced current. The potential difference set in the circuit due to induced current is called induced emf. Experiment Let two different coils of copper wire, namely coil 1 and coil 2, having large number of turns are arranged as per the circuit diagram as shown in figure.

Image Coil - 1 and Coil - 2

Image Coil - 1 and Coil - 2

Image Coil - 1 and Coil - 2

The coil 1, having larger number of turns, is connected in series with a battery and a plug key whereas the other coil 2 is connected with a galvanometer as shown in circuit diagram. When key K is closed, the galvanometer shows a deflection in its needle instantly jumps to one side and just as quickly returns to zero, indicating a momentary current in coil 2.

This happens due to an external magnetic field passing through the loop increases. When key K is open, the coil 1 is disconnected from the battery. The needle momentarily moves, but to the opposite side. It means that now the current flows in the opposite direction in coil2. This happens due to an external magnetic field passing through the loop decreases.