funky qns Flashcards
Explain briefly, in words, why the e.m.f. is sinusoidal. (N08/III/2)
(c) As the coil rotates at a constant angular speed, the angle between the normal of the area of the coil and the magnetic field varies sinusoidally. This causes the magnetic flux linkage of the coil to varies sinusoidally with time. From Faraday’s law of induction, the induced e.m.f. is E= −NΦ / dt and it will be sinusoidal as well.
Comments: Most candidates merely identified the positions (frequently incorrectly) for maximum and minimum e.m.f. They then assumed a sinusoidal e.m.f. would exist. It was expected that a reference would be made to the dependence of the e.m.f. on the angle between the coil and the field and that this angle varies sinusoidally with time.
By considering the components of the velocity parallel to the magnetic field and at right-angles to the magnetic field, describe and explain qualitatively the motion of the electron in the field. (N14/III/7(e)
Electron moves at constant v∥ in the direction parallel to magnetic field. This is because v∥ does not contribute any force and hence there is no acceleration in the direction parallel to magnetic field.
Electron moves in a uniform circular motion in the plane perpendicular to magnetic field. This is because of v⊥which results in the electron experiencing a magnetic force F perpendicular to v⊥ and magnetic field. This provides a centripetal force for the electron to move in a circular motion. (F = B q v⊥, B is the magnetic flux density and q is the charge of the electron)
Hence the electron moves in a helical path with constant pitch* in the direction parallel to magnetic field.
Use Faraday’s law of electromagnetic induction to explain how an e.m.f. is obtained across the secondary coil of a transformer
The alternating voltage V1 generates changing magnetic field in the primary coil. The soft iron core in the transformer links the magnetic field lines to the secondary coil. So the secondary coil experiences changing magnetic flux linkage.
Since Faraday’s law of induction states that induced emf is directly proportional to the rate of change of magnetic flux linkage, Hence emf is induced in the the secondary coil.
Explain, in terms of the energy changes of the emitted electrons, why there is a minimum potential difference, Vs, to reduce the current to zero. (N15/II/3)
When the electrons are emitted from electrode M, they have a range of kinetic energies. By applying a lower potential at C and higher potential at M, the photoelectrons will lose their kinetic energies and gain electric potential energies as they move towards C. In order to reduce the current to zero, the most energetic electrons will need to come to rest at electrode C and their gain in electric potential energy will be eVs, the other electrons with lesser energy will have lost their kinetic energies before they can reach plate C.
By reference to binding energy per nucleon, explain why energy is released in this fission reaction. (N11/III/5)
The binding energy per nucleon of uranium is lower than either barium or krypton. Therefore the total binding energy before reaction (binding energy of U) is less than the total binding energy of the products after the reaction (sum of binding energy of Ba and Kr). The greater binding energy per nucleon of the products after the reaction means that the products are more stable. The difference between the total BE of the uranium and its products give the energy released in the reaction.
Comments: Few answers link total binding energy and binding energy per nucleon. Most were content to comment on relative values of BE and then to state that energy would be released (as stated in the question).
