ah de Graatt geherators like the one shown in Figure I are used to produce very

Question

ah de Graatt geherators like the one shown in Figure I are used to produce very high Voltages. In the igure, the + sighs represent positive charge and the -signs represent negative charge. in this common Van de Graaff generator, charge is separated by the frictional contact of the belt and the lower pulley shown. Positive charge collects on the lower pulley and an equal amount of negative charge spreads out along the inside of the belt. Electrons from the ground are attracted to the outside of the belt by the net positive charge on the lower portion of the belt- pulley system. These electrons travel up the belt and are transferred to the dome, which is a hollow metal sphere. A high negative charge density can be built up on the dome, because the electrons from the outside of the belt do not experience a repulsive force from the charge built up on the outside of the sphere. The electric potential of the dome is V Er where E is the electric field just outside the dome and r is the radius. The charges on the surface of the dome do not affect the electric field inside the cavity. The potential that can build up on the dome is limited by the dielectric strength of the air, which is about 30,000 v/cm for dry air at room temperature. When the electric field around the dome reaches the dielectric strength of the air, air molecules are ionized. This enables the air to conduct electricity Van de Graaff generators are routinely used in college physics laboratories. When a student gets within a few inches of a Van de Graaff generator, she may draw a spark with an instantaneous current of 10 upper pulley belt lower pulley -16) ground

Explanation / Answer

given

V = Er ( electric potential of the dome)

E = 30000 V/cm = 30,00,000 V/m

i = 10 A

t = 1 us

a. Most of the enrgy is dissipated inthe air ( while ionising it) as air offers less resistacne than the human body

hence human body passes very less energy

hence option b

b. r = 10 cm

Vm = Er = 30,00,000*0.1 = 3,00,000 V

Vm = 300 kV

option e

c. once the air molecules become ionised, charge can flow into the air and hecne the maximum charge onthe dome is limited by the dielectric strength of air, and hence the maximum potential

option c

d. the conducting dome shields the effect of the charges outside ( no force is felt by the charges inside)

hence

option d

e. r = 0.1 m

q = -1 C

i = 0.5 A, for 3 s

q' = -1 + 0.5*3 = 0.5 C

then discharged to q = 0 C

then recharged with instantaneous current of 10 A

the sphere has highert potential when it has highest charge, that should be q' = 0.5 C

hence option a. just after being charged by 0.5 A current

Related Questions

Navigate

• Browse (All)
• Browse A
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.