Moving Charges and Magnetism | Class 12 Physics Notes
By ConceptScroll Team · Published on 17 July 2026 · 3 min read
Moving Charges and Magnetism – this guide gives you a concise, exam-ready overview of Moving Charges and Magnetism from Class 12 Physics, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
4.2 MAGNETIC FORCE
This section begins by recalling the electric field E produced by a static charge Q, given by E = Q r̂ / (4πε₀ r²), where r̂ is the unit vector along the displacement vector r. The force on a test charge q in this field is F = qE. The electric field is a vector field defined at each point in space and can vary with time. It obeys the principle of superposition, meaning fields due to multiple charges add vectorially.
Analogously, moving charges or currents produce a magnetic field B(r), also a vector field obeying superposition. The force on a charge q moving with velocity v in the presence of electric field E and magnetic field B is given by the Lorentz force:
F = q [E(r) + v × B(r)] = F_electric + F_magnetic
The magnetic force depends on the charge q, velocity v, and magnetic field B. It is perpendicular to both v and B, given by the right-hand rule for the vector cross product. The force is zero if the velocity is parallel or antiparallel to B or if the charge is stationary. The SI unit of magnetic field B is the tesla (T), defined as the magnetic field that exerts a force of one newton on a charge of one coulomb moving at one meter per second perpendicular to the field. The smaller unit gauss (G) is also used, where 1 G = 10⁻⁴ T. The Earth's magnetic field is about 3.6 × 10⁻⁵ T.
Extending this to a current-carrying conductor, the force on a straight rod of length l carrying current I in a magnetic field B is F = l × B, where l is a vector along the direction of current with magnitude l. For wires of arbitrary shape, the total force is the vector sum (integral) of forces on infinitesimal elements.
📊 Diagram: Figure 4.3 shows a wire suspended in a uniform horizontal magnetic field balancing gravity; Figure 4.4 illustrates velocity along x-axis and magnetic field along y-axis with directions of Lorentz force on electron and proton.
🧪 Activity: Demonstration of force on current-carrying wire in magnetic field balancing weight.
🔗 Connection: Prepares for analysis of motion of charged particles in magnetic fields, including circular and helical trajectories.
Frequently asked questions
The magnetism of a magnet is due to
spin motion of electrons
Charge through a cross-section of a conductor is given by Q= 5 t 2 - 2t coulomb. Find the average current through the conductor in the interval t 1 = 2 s to t 2 = 4 s.
28 A
The potential at a point, due to a positive charge of 100μC at a distance of 9 m, is
10 5 V
A parallel plate capacitor is charged. If the plates are pulled apart,
the potential difference increases
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