A cell of emf $$E$$ is connected across an external resistance $$R$$. When current '$$I$$' is drawn from the cell, the potential difference across the electrodes of the cell drops to $$\mathrm{V}$$. The internal resistance '$$r$$' of the cell is

Beams of electrons and protons move parallel to each other in the same direction. They

A long straight wire of radius '$$a$$' carries a steady current $$I$$. The current is uniformly distributed across its area of cross-section. The ratio of magnitude of magnetic field $$\vec{B}_1$$ at $$\frac{a}{2}$$ and $$\vec{B}_2$$ at distance $$2 a$$ is

$$\vec{E}$$ and $$\vec{B}$$ represent the electric and the magnetic field of an electro- magnetic wave respectively. The direction of propagation of the wave is along