CHAPTER 13 QUESTIONS

13.1     QUESTIONS

1.            What is it in an atomic structure which differentiates between inert gases and some metals?  Name some examples.

2.            What charge does an electron carry?

3.           How do you explain an electric current in electron terms?  What is its direction?

4.           What is the difference between a primary and a secondary cell?

5.            Describe the elements of a ‘dry’ battery.

6.           What are the main differences between lead-acid and alkaline secondary cells?  What are the advantages of alkaline cells?

7.           Why must a lead-acid cell never be left in a discharged state?

8.           Why must battery rooms be ventilated?

9.           About what specific gravity reading would you expect a fully charged lead-acid cell to show?

10.         Approximately how much of the charge energy of a battery would you expect to recover in use?

11.          A battery is rated 250Ah at the 5-hour rate.  For how long could you draw 7A from it continuously?  If you drew 70A continuously, what effect would you expect on the battery’s performance?

12.         What effect would the discharging of a nickel-cadmium cell have on the electrolyte’s specific gravity?

13.         What is the difference between natural magnetism and electromagnetism?  Sketch an electromagnetic coil, illustrating the field direction.

14.         Name some uses for an electromagnet.

15.         What is the equivalent resistance of several resistors in series?

16.         What is the equivalent resistance of several resistors in parallel?

17.          Write down Ohm’s Law in its three variations, naming the units used.

18.          What is the rate of heat production when current flows in a conductor of known resistance?  How can the heat rate be reduced?

19.         What is understood by Faraday’s ‘Law of Electromagnetic Induction’?  Illustrate your answer by a sketch.

20.          How do you use Fleming’s Right-hand Rule for generated emf?

21.          What force acts on a conductor carrying current in a magnetic field?  Illustrate your answer with a sketch.

22.          Why does the current rise slowly when a voltage is applied to an inductive circuit?  What do you understand by ‘back-emf’, and what is it proportional to?

23.          For an inductor of inductance L henrys, write down the formula connecting L, the back-emf E and the rate of change of current.

24.          In the inductor of Q.23, if the current flowing at any instant is I, what is the energy stored in the inductor?  In what unit?

25.          What do you understand by the ‘time constant’ of an inductive (R-L) circuit?

26.          What happens if you suddenly switch off an inductive circuit?  How can any ill-effects be mitigated?

27.          In what unit is capacitance now measured?  Is it a practical unit?

28.          What do you understand by the ‘dielectric’ of a capacitor?  How does it function?

29.          A capacitor of capacitance C is charged to a voltage V.  What is the energy stored in it?  In what unit?

30.          Describe an electrolytic capacitor.  How does it compare with a normal type?

31.          Why must an electrolytic capacitor never be reverse-charged?  How is this to be avoided?

32.          What is the equivalent capacitance of a number of capacitors in series?

33.          What is the equivalent capacitance of a number of capacitors in parallel?

34.          What is the time constant of a capacitive (R-C) circuit?

35.          How is a capacitor/resistor combination used to provide a time delay to an operating circuit?  Make a sketch.

36.         What is the power transmitted in a d.c. circuit by a current of / amperes at a pressure of V volts?  If the resistance of the circuit is R ohms, express this power in two alternative forms.  What is the unit of the power?

37.          What is the equivalent in kW_m of 20 horsepower?

38.          What is the efficiency of a motor with full-load output of 50 hp and an electrical input of 45kW?

39.          Name any three classes of d.c. measuring instrument, indicating their principal features and differences.

40.          How can you tell a moving-iron from a moving-coil instrument simply by looking at it?

41.          What difference is there between the movements of a moving-iron ammeter and a moving-iron voltmeter?

42.          How does a dynamometer instrument operate as a wattmeter? Can it indicate reverse power flow?

43.          What is a ‘set-up zero’ instrument, and when is this feature used?