The force between two point charges kept with a separation of 9 cm in air is 98 N . If a dielectric slab of constant 4, thickness 6 cm and another dielectric slab of constant 9 , thickness 3 cm are introduced between the two charges, then the new force becomes

Three point charges shown in the figure lie along a straight line. The energy required to exchange the position of central charge with one of the negative charges is

A capacitor of capacitance $2 \mu \mathrm{~F}$ is charged to 50 V and then disconected from the source. Later the gap between the plates of the capacitor is filled with a dielectric material. If the energy stored in the capacitor is decreased by $25 \%$ of its initial value, then the dielectric constant of the dielectric material is

A wire of resistance $100 \Omega$ is stretched, so that its length increases by $20 \%$. The stretched wire is then bent in the form of a rectangle whose length and breadth are in the ratio $3: 2$. The effective resistance between the ends of any diagonal of the rectangle is