470 ALTERNATING CURRENTS 



3. What is the average power of a circuit in which the current leads the 

 voltage by 90? 



4. When the current and the voltage differ in phase by an angle which 

 is less than 90 but greater than zero, what is the general character of the 

 power curve? Make a sketch. What is meant by power-factor? What is 

 power-factor equal to, numerically? 



6. When an alternating voltage is impressed across a resistance, what 

 phase relation exists between this voltage and the resulting current? How 

 may the value of the current be calculated, knowing the voltage and the 

 resistance? 



6. What is the effect of inductance on the building up of a current in a 

 circuit across which a steady direct-current voltage is impressed? What 

 occurs when the current attemps to die out in an inductive circuit? 



7. State how the current may be prevented from reaching its Ohm's 

 law value in an inductive circuit. 



8. What effect does inductance in an alternating-current circuit have 

 upon; (a) the phase angle between the current and the voltage; (6) the 

 magnitude of the current? What is the effect of frequency upon the 

 magnitude of the current? What is "inductive reactance?" 



9. What in general is the effect of capacitance upon the flow of. current 

 in any electric circuit? How does capacitance affect (a) the- phase angle 

 between the current and the voltage in an alternating-current circuit; 

 (6) the magnitude of the current? 



10. What is the effect of frequency upon the magnitude of the current 

 in a condensive circuit? What is condensive reactance? What is the value 

 of the average power taken by a perfect condenser? 



11. What is the phase relation existing between the current and the 

 voltage across the resistance in a circuit containing resistance and inductance 

 in series? Between the current and the voltage across the inductance? 

 How is the line voltage obtained? 



12. What is meant by impedance? How may the angle between the 

 line voltage and the current be determined? What relation does this 

 angle bear to the power in the circuit? 



13. What is the phase relation existing between the current and the 

 voltage across the resistance in a circuit containing resistance and capaci- 

 tance in series? Between the current and the voltage across the capaci- 

 tance? What phase relation exists between the line voltage and the 

 current? 



14. Sketch the vector diagram of a circuit containing resistance, inductance 

 and capacitance all in series. How may the circuit phase-angle be found? 



15. What is meant by "resonance" in an alternating-current circuit? 

 What phase relation exists between the line voltage and the line current? 

 What is the numerical relation existing between the inductive voltage and 

 the condensive voltage? Show that with both inductance and capacitance 

 in a series circuit, the voltage across each can be several times the line voltage. 



16. In practice, why is parallel grouping of resistances, inductances, 



