**CBSE class XII past Questions and Answers**

**Q.1)** What is the effect of introducing a dielectric medium between the plates of a parallel plate condenser charged to a certain potential V ?

**Answer:** The capacitance increases by k times when the dielectric slab of dielectric constant k is introduced. But the charge on the capacitor remains constant (in this question). So, the potential of the capacitor will decrease by a factor of k. [ since, V = Q/C ].

**Q.2)** A condenser is connected across a source of emf E. If a dielectric slab is introduced between the plates of the condenser, the condenser will draw more charge from the cell. Justify.

**Answer:** Initially the capacitor will be charged to a potential of E. Say, it now has a charge Q. When the dielectric slab of dielectric constant k is introduced, the capacitance increases by k times. So, the potential of the capacitor will tend to decrease (since V=Q/C). So, more charge will flow from the battery (source of emf) to keep the potential constant at E. Therefore finally, the capacitor will have more charge than Q.

**Q.3)** A hollow metal sphere of radius 5 cm is charged such that potential on its surface is 10 V. What is the potential at the centre of the sphere? [CBSE 2011]

**Answer:** Hollow metal sphere means a hollow conducting sphere. For such a sphere, the potential inside the sphere is same as the potential on its surface. So, the potential at the centre of the sphere will be 10 V.