Electrical Machines Questions and Answers_pag17

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C(x) = ε​0​A/x = 8.85∗10​-12​∗100∗(10​-3​)​2​/0.1∗10​-3​ = 8.85∗10​-12​ F 
W​fld​ = 1/2∗Cv = 1/2∗100​2​∗8.85∗10​-12​ = 44.3nJ 
6. A parallel plate capacitor is connected to a DC source. Now the plates are allowed to move a 
small displacement under the influence of force of attraction between the two plates. As a result 
____________ 
(i) charge on the plates increases 
(ii) charge on plates remains constant 
(iii) energy stored increases 
(iv) energy stored remains constant 
(v) electric field intensity is unchanged 
(vi) flux density increases 
Which of the above statements are correct? 
a) (ii),(iv),(v) 
b) (ii),(iii),(vi) 
c) (i),(iii),(v) 
d) (i),(iii),(vi) 
View Answer 
Answer: d 
Explanation:C = ε​0​A/x ⇒ C ∝ 1/x ⇒ If the force if of attraction type, then x decreases and C 
increases. 
q=Cv, v constant⇒ as C increases, q also increases. We also know that q=DA and if q 
increaes, D increases. 
Finally, W​fld​= 1/2 D​2​/ε​0​ and as D is increasing, W​fld​ increases. 
7. The force produced by electric field in a singly excited energy conversion device, using 
electric field as coupling medium can be obtained by ___________ 
a) use of field energy function only 
b) use of coenergy function only 
c) any of the mentioned 
d) none of the mentioned 
View Answer 
Answer: c 
Explanation: f​e​ = -∂W​fld​(q,x)/∂x 
f​e​ = -∂W​fld​
1​(q,x)/∂x 
8. Charge and voltage associated with electric field are analogous, respectively to __________ 
and ___________ in magnetic field. 
a) flux linkages and current 
b) flux density and current 
c) flux linkages and voltage 
d) MMF and current 
View Answer 
Answer: a