A uniform magnetic field of 500 G acts normal to a flat surface of area \(40\ \mathrm{cm^{2}}\). The magnetic flux through the surface is
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → \(2\times10^{-4}\ \mathrm{Wb}\)
When a uniform field \( B \) is normal to a flat area \( A \), the magnetic flux through it is \( \phi_B=BA \). Both quantities are first changed to SI units. One gauss is \( 10^{-4} \) tesla.
\( \begin{aligned} B &= 500\ \mathrm{G} \\ &= 500\times10^{-4}\ \mathrm{T} \\ &= 0.05\ \mathrm{T} \end{aligned} \)
\( \begin{aligned} A &= 40\ \mathrm{cm^{2}} \\ &= 40\times10^{-4}\ \mathrm{m^{2}} \\ &= 4\times10^{-3}\ \mathrm{m^{2}} \end{aligned} \)
\( \begin{aligned} \phi_B &= BA \\ &= 0.05\times4\times10^{-3} \\ &= 2\times10^{-4}\ \mathrm{Wb} \end{aligned} \)
NCERT (2026-27):
Magnetism and Gauss’s Law – “… The magnetic flux through ΔS is defined as ΔφB = B.ΔS, where B is the field at ΔS.”
Magnetic Field, Lorentz Force – “… Tesla is a rather large unit. A smaller unit (non-SI) called gauss (=10–4 tesla) is also often used.”