Two cells are connected in opposition as shown. Cell $$\mathrm{E}_1$$ is of $$8 \mathrm{~V}$$ emf and $$2 \Omega$$ internal resistance; the cell $$\mathrm{E}_2$$ is of $$2 \mathrm{~V}$$ emf and $$4 \Omega$$ internal resistance. The terminal potential difference of cell $$\mathrm{E}_2$$ is __________ V.

A electron of hydrogen atom on an excited state is having energy $$\mathrm{E}_{\mathrm{n}}=-0.85 \mathrm{~eV}$$. The maximum number of allowed transitions to lower energy level is _________.

A capacitor of capacitance $$\mathrm{C}$$ and potential $$\mathrm{V}$$ has energy $$\mathrm{E}$$. It is connected to another capacitor of capacitance $$2 \mathrm{C}$$ and potential $$2 \mathrm{~V}$$. Then the loss of energy is $$\frac{x}{3} \mathrm{E}$$, where $$x$$ is _______.

The distance between object and its two times magnified real image as produced by a convex lens is $$45 \mathrm{~cm}$$. The focal length of the lens used is _______ cm.