When two identical batteries of internal resistance $1 \Omega$ each are connected in series across a resistor $R$, the rate of heat produced in $R$ is $P_1$. When the same batteries are connected in parallel across $R$, the rate of heat produced is $P_2$. If $P_1=2.25 P_2$, then the value of $R$ is
The magnetic field at the centre of a long solenoid having 400 turns per unit length and carrying a current ' $i$ ' is $6.24 \times 10^{-2} \mathrm{~T}$. The magnetic field at the centre of another long solenoid having 200 turns per unit length and carrying a current $\frac{i}{2}$ is
If a proton of kinetic energy 8.35 MeV enters a uniform magnetic field of 10 T at right angles to the direction of the field, then the force acting on the proton is
(Mass of proton $=1.67 \times 10^{-27} \mathrm{~kg}$ and Charge of proton $=1.6 \times 10^{-19} \mathrm{C}$ )
A sample of a ferromagnetic iron in the shape of a cube of side $1.0 \mu \mathrm{~m}$ contains $8.7 \times 10^{28}$ atoms per cubic metre and the magnetic dipole moment of each iron atom is $93 \times 10^{-24} \mathrm{Am}^2$. Then, the maximum possible magnetic dipole moment (in $\mathrm{Am}^2$ ) of the sample is nearly
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