CBSE Class 12 Physics Chapter 12: Electromagnetic Induction NCERT Solutions

The direction of the induced current in a closed loop is governed by Lenz's Law, which states that the induced current opposes the change in magnetic flux that causes it. We will analyze each situation:

1. (a) Magnet entering the coil: As the North pole of the magnet enters the coil, the magnetic flux through the coil increases. To oppose this increase, the induced current will create a magnetic field that repels the approaching North pole. This means the face of the coil near the magnet will behave like a North pole, and the induced current will flow counter-clockwise when viewed from the magnet's side.
2. (b) Magnet leaving the coil: As the North pole of the magnet leaves the coil, the magnetic flux through the coil decreases. To oppose this decrease, the induced current will create a magnetic field that attracts the receding North pole. This means the face of the coil near the magnet will behave like a South pole, and the induced current will flow clockwise when viewed from the magnet's side.
3. (c) Tapping key just closed: When the tapping key is closed, the current in the primary circuit starts flowing and increases from zero to its maximum value. This changing current produces a changing magnetic field, which induces a current in the nearby secondary coil. The direction of this induced current will oppose the change in flux. If the primary coil is on the left and the secondary on the right, the increasing current in the primary creates an increasing magnetic field to the right. The induced current in the secondary will create a magnetic field to the left, opposing the change. This typically results in a current flowing in a specific direction (e.g., clockwise or counter-clockwise depending on coil orientation and current direction in the primary). The solution indicates the direction is along yzxy.
4. (d) Rheostat setting changed: Changing the rheostat setting alters the current in the primary circuit, thus changing the magnetic field it produces. This change in magnetic flux induces a current in the secondary coil. The direction of the induced current opposes the change in flux caused by the rheostat adjustment. The solution indicates the direction is along zyxz.
5. (e) Tapping key just released: When the tapping key is released, the current in the primary circuit decreases from its maximum value to zero. This decreasing current produces a decreasing magnetic field, which induces a current in the secondary coil. The induced current will flow in a direction that opposes this decrease in flux. The solution indicates the direction is along xryx.
6. (f) Current decreasing at a steady rate: Similar to case (e), a decreasing current in one coil leads to a decreasing magnetic flux. The induced current in the nearby coil will oppose this decrease. The solution states that No current is induced. This specific outcome (no induced current) might occur if the magnetic field lines from the primary coil are lying entirely in the plane of the secondary loop, meaning the magnetic flux through the loop is zero and remains zero even as the current changes, or if the coils are positioned such that the flux change is negligible.

Note: The specific directions (e.g., yzxy, zyxz, xryx) depend on the exact configuration of the coils and the direction of current in the primary circuit, which are detailed in the figures not fully rendered here. The principles applied are based on Lenz's Law.