Aptitude Test Problems in Physics Science for Everyone by S Krotov ( PDFDrive.com )

3. Electricity and Magnetism 89

3.39. A plate A of a parallel-plate capacitor

is fixed, while a plate B is attached to the

wall by a spring and can move, remaining

parallel to the plate A (Fig. 97). After the

key K is closed, the plate B starts moving

and comes to rest in a new equilibrium po-

sition. The initial equilibrium separation d

between the plates decreases in this case

by 10%.

What will be the decrease in the equilib-

rium separation between the plates if the

key K is closed for such a short time that

the plate B cannot be shifted noticeably?

Fig. 97 Fig. 98

3.40. The circuit shown in Fig. 98 is made

of a homogeneous wire of constant cross

section.

Find the ratio 0 /0 ,12 34 of the amounts of

heat liberated per unit time in conductors

1-2 and 3-4.

3.41. The voltage between the anode and

the cathode of a vacuum-tube diode is U,

and the anode current is I.