Exams › JEE Main › Physics

An electron and a proton, initially at rest and infinitely far apart, are released and pulled toward one another by their mutual attraction. Which statement about their kinetic energies is correct?

1. The electron's kinetic energy exceeds the proton's
2. The electron's kinetic energy is smaller than the proton's
3. Both have equal kinetic energy
4. It cannot be decided; it depends on their separation

Correct answer: The electron's kinetic energy exceeds the proton's

Solution

Internal forces are equal and opposite, so momentum is conserved; both acquire equal magnitude of momentum p. Kinetic energy = p²/(2m). Since the electron has much smaller mass, its kinetic energy (inversely proportional to mass for equal p) is far greater than the proton's.

Related JEE Main Physics questions

• If an electric field vector E exists in a material whose dielectric constant is K, and ε0 denotes the permittivity of free space, which expression gives the electric displacement vector?
• If a unit positive charge is taken from point A to point B along the same equipotential surface, then what can be said about the potential difference V_A - V_B?
• Two thin metallic spherical shells are placed concentrically with radii R1 and R2, where R1 > R2. If the shells carry charges Q1 and Q2 respectively, what is the electric potential at a point located at distance r such that R2 < r < R1? (Take k = 1/(4πϵ0))
• An alpha particle is made to move through a potential difference of 10⁶ V. What kinetic energy does it gain?
• A parallel-plate capacitor has its entire gap filled with a dielectric whose dielectric constant changes with position x according to K(x) = K0 + λx, where λ is a constant. How is its capacitance C related to the vacuum capacitance C0?
• Two thin circular rings of radius R each are kept on the same axis with their centres separated by a distance d. One ring carries charge +q and the other carries charge −q. What is the potential difference between the centres of the two rings?

⚔️ Practice JEE Main Physics free + battle 1v1 →