Question 11:
easyAssertion (A): Soft iron is used as transformer core.
Reason (R): Soft iron has narrow hysteresis loop.
Soft iron is preferred for transformer cores because its narrow hysteresis loop indicates low energy loss (heat) during repeated cycles of magnetisation and demagnetisation. This property is crucial for efficient transformer operation. Both Assertion and Reason are true, and Reason is the correct explanation of the Assertion.
Question 12:
easyAssertion (A): All magnetic materials lose their magnetic properties when strongly heated.
Reason (R): Most of the substances show diamagnetism.
Ferromagnetic and paramagnetic materials indeed lose their specific magnetic properties upon strong heating (e.g., ferromagnets become paramagnetic above Curie temperature). So, Assertion (A) is true. However, Reason (R) is false; while diamagnetism is a universal property, most substances exhibit paramagnetism (due to unpaired electrons) or ferromagnetism, which mask diamagnetism.