A metal wire of length $L$ and radius $r$ has a resistance $R$. If a wire of the same metal of length $2 L$ and radius $3 r$ is taken, then what will be its resistance?
Balancing point of a potentiometer shifts from a length of 60 cm to 40 cm by shunting the cell with a $4 \Omega$ resistance. What is the internal resistance of the cell?
A current $I=5 \mathrm{~A}$ flows along a thin wire shaped as shown in figure. The radius of curved part of the wire is equal to $R=100 \mathrm{~mm}$, the angle $2 \phi=90^{\circ}$. The magnitude of magnetic field at the point $O$ is approximately
$$ \left(\text { use, } \frac{\mu_0}{4 \pi}=10^{-7} \mathrm{~T} \mathrm{~mA}^{-1}\right) $$

A toroid has a core (non-ferro magnetic) of inner radius 24 cm and outer radius 26 cm around which 2000 turns of a wire is wound. If the current in the wire is 12 A , the magnetic field inside the core of the toroid is
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