Physics · Atoms · NEET
Kinetic energy is ALWAYS positive. KE = (1/2)mv^2 can never be negative because both m and v^2 are positive. In a Bohr orbit KE = +13.6 Z^2/n^2 eV. Only the potential energy and the total energy carry a negative sign. Students lose marks by writing KE negative - never do that.
Total Energy = KE + PE. In a Bohr orbit KE = -(1/2)PE (this comes from the Coulomb force providing the centripetal force). Substituting: Total = -(1/2)PE + PE = (1/2)PE. So PE = 2 x Total. Because Total is negative, PE is also negative and larger in size. In numbers: PE = 2E, and PE = -2 x KE.
Use two quick rules. KE = -(Total Energy) = -(-3.4) = +3.4 eV. PE = 2 x (Total Energy) = 2 x (-3.4) = -6.8 eV. Check: KE + PE = 3.4 + (-6.8) = -3.4 eV, which matches the total. This is exactly NEET 2019, answer 3.4 eV, -6.8 eV.
KE : Total Energy = 1 : -1. Since KE = -(Total Energy), the two have equal size but opposite signs. This is NEET 2018, correct option 1 : -1. Do not simplify it to 1 : 1 - the sign is the whole point of the question.
No. The RATIO 1 : -2 : -1 is fixed for every orbit and every hydrogen-like atom, no matter what n or Z is. Only the VALUES change (they scale as Z^2/n^2). So a question can give you any energy value and you apply the same three rules.
The ratio of kinetic energy to the total energy of an electron in a Bohr orbit of the hydrogen atom is:
The total energy of an electron in an atom in an orbit is -3.4 eV. Its kinetic and potential energies are, respectively:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is 1 : -2 : -1 for every orbit and every hydrogen-like atom. KE is positive, PE is negative and double the size, and Total Energy is negative and equal in size to KE.
KE = (1/2)mv^2 is always positive. The electron is bound to the nucleus, so its potential energy is negative and larger in size than KE. Adding them gives a negative total, which shows the electron is trapped (bound). A negative total energy means energy must be supplied to free the electron.
PE = -2 x KE. The potential energy is twice the kinetic energy in size but negative. This is a direct result of the Coulomb attraction acting as the centripetal force in the Bohr model.
No. The ratio 1 : -2 : -1 stays the same for all orbits (any n) and all hydrogen-like ions (any Z). Only the actual numbers change, scaling as Z^2/n^2.
Use KE = -(Total Energy) and PE = 2 x (Total Energy). These two one-line rules solve almost every NEET numerical on this concept in seconds.
