Concept test Flashcards
A beam of white light passes through a cloud of gas. The spectrum of the transmitted light contains a number of dark lines. Explain why these dark lines occur. (Quantum Physics)
The white light passing through the cloud of gas is a continuous spectrum of wavelengths, thus consisting of photons of a range of energies. The electrons of the gas have only certain allowed energy states. Electrons of the gas atoms will absorb the photons that will cause excitation from ground state to a higher allowed state, thus only certain characteristic wavelengths are absorbed. Because these wavelengths are absorbed, they are missing from the transmitted beam, and so dark lines occur at the positions of the characteristic wavelengths.
Explain why the energy of each energy level is negative.
The electron is attracted to the nucleus of the atom and hence its total energy is negative
Why does a magnet slow down when falling through a metal tube?
By Lenz’s Law: As the magnet falls, the tube experiences a change in magnetic flux linkage. By Faraday’s Law, an EMF and hence current is induced in the copper tube. To oppose the change in magnetic flux linkage, by Lenz’s Law, the induced current will be in a direction to produce a magnetic field. Therefore there will be a magnetic force to resist the motion of the falling magnet. As some kinetic energy is converted to the electric potential energy by the induced current which in turn dissipated as heat, the magnet falls slower.
What is a use of a laminated soft iron core in a transformer?
Soft iron core: It have excellent magnetic permeability, which focuses magnetic lines of force and reduces energy loss(reduces flux leakage). Because soft iron has a high permeability, it allows full coupling of magnetic flux from the main coil to the secondary coil in the core of a transformer(increase flux linkage). It is more easily magnetise and hence there is lesser time delay.
Laminate: It prevents the formation of eddy currents across the core and thus reduces the loss of energy.
Why is x-radiation emitted from the container walls when B-particles are absorbed by the wall?
Beta particles are high speed electrons with high kinetic energy. The Beta particles are stopped suddenly by the container, hence they undergo large deceleration, producing X-ray photons. Beta particles also knocks out electrons in inner shells. Hence, when electrons from outer shells de-excites to fill the vacancy, X-ray photons are produced.
Explain why the total change in the internal energy of the gas in the pump during a complete cycle must be zero.
A cyclic process brings the state of a system through a sequence of changes and finally returns back to its original state, that is, to its original internal energy.
