When the conductivity of a semiconductor is only due to the breaking of the covalent bonds, the semiconductor is called

A coil having effective area A, is held with its plane normal to magnetic field of induction B. The magnetic induction is quickly reduced by $$25 \%$$ of its initial value in 2 second. Then the e.m.f. induced across the coil will be

At constant temperature, increasing the pressure of a gas by $$5 \%$$ its volume will decrease by

Half life of radio-active element is 1600 years. The fraction of sample remains undecayed after 6400 years will be