MPC 1 exam june 2022 marking key.pdf
- 1. KMTC/QP-08/EBS KENYA MEDICAL TRAINING COLLEGE Department of Radiography and Imaging COLLEGE EXAMINATIONS FOR ACADEMIC YEAR 2021/2022 END OF SEMESTER EXAMINATIONS FOR DIPLOMA IN MEDICAL IMAGING SCIENCE MPC 1112: Medical Physics and Chemistry 1 Marking Key Instructions Answer all questions in sections A, B and C SECTION A: 20 MARKS 1. Which one of the following is not considered as a force a. Gravity b. Electrostatic c. Electricity d. Electromagnetic 2. The following are units of energy except? a. Joule b. Erg c. Calorie d. Watt 3. Which one of the following is not a Vector quantity a. Displacement b. Velocity c. Speed d. Acceleration 4. If the distance from a radiation source is halved, the radiation intensity increases by a factor of a. 22 b. 21 c. 2-1
- 2. KMTC/QP-08/EBS d. 2-2 5. In physics, the degree of distortion produced by stress is known as a. Stress b. Pressure c. Shear d. Strain 6. The purpose of X-ray transformer is most likely to change the a. Magnetic field b. Electrical voltage c. Power level d. Current intensity 7. Convert 1090 F to degrees centigrade a. 47 b. 45 c. 40 d. 43 8. Convert 700 F to degrees Kelvin a. 294 b. 273 c. 282 d. 268 9. Which one of the following is not a factor that that affects geometrical unsharpness during radiological procedures a. Target angle b. Tube focus c. Source to image distance d. Selected Kilovoltage 10. Which one of the following is not a factor that causes alternating voltage and current to be out of phase a. Inductance in AC circuit b. Capacitance in AC circuit
- 3. KMTC/QP-08/EBS c. Impedance d. Induced EMF in AC circuit 11. The study of the effects which occur when electricity is not in motion is known as? a. Electricity b. Electrostatics c. Electromagnetic d. Electrification 12. Which of the following statements is FALSE about magnets a. The number of lines of flux can be determined by the strength of the magnetic pole producing them b. Good permanent magnet will always retain their magnetic properties in spite of harsh treatment c. Diamagnetic substances show a strong magnetic effect when placed in a strong magnetic field d. Magnetisation of a magnetic substance increases as the magnetising force is increased 13. All magnetic lines of force crossing a given area are referred to? a. Magnetic field b. Magnetic field strength c. Magnetic flux d. Magnetic force 14. Which one of the following statements is true about properties of a magnet a. If a magnet is freely suspended it sets in any direction b. A magnet attracts small magnetic substances c. Magnetic effects are equally concentrated through the magnet d. Like poles attract unlike poles repel 15. In electrical conduction, the energy band arrangement in semi āconductors is quite similar to that of insulators, except that a. The conduction band in semi ā conductors is much closer to the valence band than is the case with insulators.
- 4. KMTC/QP-08/EBS b. The conduction band in semi ā conductors is similar to the valence band of the insulators c. The conduction band in semi ā conductors is much further to the valence band than is the case with insulators d. The conduction band in semi ā conductors is much thicker than the valence band than is the case with insulators 16. Which of the following statements is FALSE about Transformers a. The power dissipated in primary winding should be equal to the power dissipated in the secondary winding from an ideal point of view. b. The power dissipated in the secondary winding will always be more than power dissipated in the primary windings because of transformer losses. c. The losses originate from inductive and resistive effects within the transformer. d. Two main type of transformer losses are Core losses and Copper losses 17. The principle transformer losses in the windings that are due to ohmic resistance that are dissipated in form of heat and are proportional to the product of current squared and resistance (12 R) are known as a. Core losses b. Eddy current losses c. Hysteresis losses d. Copper losses 18. Which one of the following statements is FALSE about the peak factor a. It is the ratio of peak value to the root mean square (RMS) value of current, EMF or potential difference. b. It is the factor in which RMS values must be multiplied to give peak value c. Peak factor is equal to 0.707 d. If the wave is not sinusoidal the peak factor is determined by the particular shape of the curve 19. The thickness of a sheet of a particular absorbing material which if placed in a beam of X radiation will reduce the intensity of the beam to one half its value is known as a. Half value thickness b. 2mm aluminium equivalent
- 5. KMTC/QP-08/EBS c. Absorption coefficient d. Linear absorption coefficient 20. The statements below are all true about semiconductors except one. Identify the incorrect statement a. When certain elements are added to intrinsic semiconductors, the conductivity of the semiconductor increases. b. Elements added to semiconductor for the purpose of increasing their conductivity are called impurities c. Impurity elements improve conductivity by providing free electrons or holes which makes conduction possible. d. When the added impurity causes the availability of free electrons the semiconductor becomes known as a P type of semiconductor. SECTION B: 40MKS 21. State the factors that govern the rate at which x-ray tube target cools (5mks) i. The temperature of the target ii. Temperature difference between the target and the surrounding iii. The area of the target iv. The condition of the target surface v. The thermal conductivity of the metal with which it is in contact. 22. An exposure of 20MAS is required to x-ray the skull of a young lady at a source to Image Receptor Distance (SID) of 90cm. If the SID was reduced to 75cm, what would the exposure be if the same density on the film is to be attained (5mks) mAs1/mAs2 = d1 2 /d2 2 902 cm = 20MaS 752 cm = ?
- 6. KMTC/QP-08/EBS 752 x20 = 13.88MaS 902 23. The voltage from a transformer is expected to be 120,000volts, but only 105,000volts are obtained. Express the difference and the obtained values as percentage of the expected value (5mks) Difference=150,000 in 120,000 =15: 120 = 15/120Ć 100 =12.5% Obtained =105,000 in 120,000 = 105/170Ć100 = 87.5% 24. Describe heat transfer in solids (5mks) Conduction Conduction is the transfer of heat in solids. Metals are relatively good conductors thus transfer heat more readily. When one meter of a bar metal is heated molecules in the area acquire more thermal energy. The vigorous vibration of these molecules brings them into near contact with neighbouring molecules whose vibration increases by the transfer of thermal energy, resulting from molecular activity. They in turn cause the next layer of molecules to become more active and in this way the temperature rises along the metal bar. If metal bar is held in hand while being heated the transfer of heat would be quickly appreciated. 25. State 5 information that are included in the rating of x-ray transformer. (5mks) i. The maximum peak secondary voltage under no-load. ii. The maximum permissible current which may flow for no longer than one second (momentary or intermittent loading condition). iii. The maximum permissible current which can be allowed to flow continuously (continuously loading).
- 7. KMTC/QP-08/EBS iv. The percentage permissible over load. v. Other information relating to insulation, permitted temperature rise under certain conditions during operation. 26. List 5 types of Transformers that are found in X-ray equipment (5mks) i. The high tension transformer ii. The tube filament heating iii. The valve filament heating transformer iv. The autotransformer v. The variac transformer 27. The size and the shape of the core of the transformer are determined by the following factors: (5mks) i. Circuit design ii. Functional requirement iii. Required percentage of efficiency iv. Space available v. Required E.M.F 28. Briefly describe the periodic table of elements (5mks) Periodic table of elements. The periodic table of elements list matter in order of increasing complexity, beginning with hydrogen (H). An atom of hydrogen contains one proton in its nucleus and one electron outside the nucleus. Helium (He), second atom in the table contain two protons two neutrons and two electrons. Third atom lithium (Li), contain three protons, four neutrons and three electrons. The number of electrons to the outermost shell its equal to its group in the periodic table. The number of electrons in the outermost shell determines the valence of an atom. The number of the outermost electron shell of an atom is equal to its period in the periodic table.
- 8. KMTC/QP-08/EBS All atoms that have one electron in the outer shell lie in group 1 of the periodic table. Atoms with two electrons in the outer most shell fall in group II. When 8 electrons are in the outer shell the shell is filled. Atoms with filled outer shell lie in group VII, the noble gases and are very stable. The orderly scheme of atom progression from smallest to largest atom is interrupted in the fourth period. instead of adding an electron to the next outer shell, electrons are added to an inner shell. Atoms associated with this phenomenon are called transitional elements. Chemical properties of transitional elements depend on the number of electrons in the two outermost shells. SECTION C ā 40MKS 29. Discuss various types of electron emission processes (20mks) ELECTRON EMISSION Electron Emission is the process which electrons leave material. Electron Emitter or an emitting surface is the surface from which electrons escape. Electron emission can be produced more easily and in greater quantity in a low pressure gas or vacuum in a sealed container such as a glass tube. The following are various types of electron emission process Thermionic Electron Emission Thermionic emission is the emission of electrons from the surface of metal as a result of absorption of thermal energy. When a metal is heated, its atom absorbs thermal energy. At a particular temperature, some of the electrons in the metal may acquire enough energy to allow them to move a small distance away from the surface of the metal, (Thermionic emission). Electrons will be more easily emitted in greater number if the pressure and therefore density of the surrounding air is reduced. For example at a temperature of 22000 C pure tungsten will emit a useful number of electrons through thermionic electron emission.
- 9. KMTC/QP-08/EBS Field Electron Emission If two electrodes sealed in a glass tube which usually contains gas at low pressure, are connected to a source of unidirectional E.M.F an electric field will be produced between the electrodes. The intensity of the field can be made high to cause emission of electrons to take place from the negative electrode (the cathode). The electron acquires the necessary energy for emission from the electric field and this is called field emission or cold cathode emission. When electrons are emitted from the cathode they ionize gas molecules in either path and so release more electrons. Photoelectric Emission When high frequency electromagnetic radiation (light and ultra ā violet radiations) is incident on the surface of the suitable material, electrons in atoms at the surface of the material absorb energy from the radiation. If sufficient energy is absorbed the electrons will be emitted from the surface. This is called proto-electric emission by which photo-electrons are emitted. The frequency of the incident radiation at which photoelectric emission will occur with a given material is called the threshold frequency. Emission of electrons will not occur if the frequency of radiation is below threshold value. Secondary Electron Emission Electrons travelling at high velocity have high kinetic energy. If Beam of such electrons is used to bombard the surface of a metal, the temperature of the metal will be increased and this will make electrons in the metal more susceptible to emission. Some of these electrons will acquire energy from the bombarding (primary) electrons and may then be emitted. Electrons emitted in this way are called secondary electrons. They are usefully employed in variety of electronic devices e.g. television camera tubes.
- 10. KMTC/QP-08/EBS 30. a. An electric current cannot be directly observed but can be detected by effects it produces. Describe the principal effects of direct current. (10mks) Principal effects of direct current. An electric current cannot be directly observed but can be detected by effects it produces. These effects include; i. Production of heat, e.g., heating of filament in thermionic values and x ray tubes. Production of light in lamps ii. Establishment of a magnetic field in the vicinity of the current -carrying conductor. iii. Electrolysis ā breaking down a dissolved compound into its constituent elements. Chemical effects of passing an electric current through certain solution e.g., dilute solution of sulphuric acids, bubbles of gas appear on the immersed ends of the wire. I.e. oxygen is produced on one end and hydrogen is produced on the other end. iv. Mechanical effects. When an induction coil is energized a series of electrical discharges can be produced between its terminals. If a sheet of paper is placed in the path of the discharge and then held up to alight it will be seen that the sparks have punctured a number of fine pin-holes through the sheet. v. Luminous effect is produced by the discharge of electricity through a gas at low pressure. The production of luminosity by electric discharge in ions -pressure gas is widely applied in various forms of lighting e.g., fluorescent tubes
- 11. KMTC/QP-08/EBS b. Describe the principle of a simple alternator (10mks) Principle of the alternator A simple alternator is also called dynamo or synchronous generator. From the above diagram, the conductor in the form of a rectangular winding, coil or loop, is mounted on (but insulated from) a shaft, and arranged concentrically in the magnetic field between the poles of a magnet. The end of the conductor loop is connected to metal collars S1 and S2 which are called slip rings or collector rings. The slip rings are insulated from the shaft and from each other and rotate with the conductor. Stationary conductors B1 and B2 called brushes are in contact with the rotating slip rings. It is through these brushes and slip rings that conductors are connected to an external circuit which is to be supplied with alternating emf. External circuit is represented in the figure above as load resistance R. When an alternating emf is being induced in the rotating conductors it will drive an alternating current in the complete circuit; that is, from the conductor via a slip ring to a brush, then through the external circuit, and back to the other brush and slip ring and to the conductor again.