# High Voltage Engineering Questions and Answers – Van de Graaff Generator

This set of High Voltage Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Van de Graaff Generator”.

1. Van de Graff generators are ________ voltage and ________ power devices.
a) high, low
b) low, high
c) low, low
d) high, high

Explanation: Van de Graff generators are high voltage and low power devices. Their power rating rarely exceeds few tens of kilowatts. It is an electrostatic machine that generates very high voltages. They are useful in low-current applications.

2. The generator is usually enclosed in earthed metallic cylindrical vessel.
a) True
b) False

Explanation: The generator is usually enclosed in earthed metallic cylindrical vessel. They work under high-pressure gases. It is operated under pressure or in vacuum. It is an electrostatic machine that generates very high voltages. Their power rating rarely exceeds a few tens of kilowatts.

3. What is the speed of the electric motor which drives the belt?
a) 100-200m/min
b) 10-20m/min
c) 1000-2000m/min
d) 10000-20000m/min

Explanation: The belt is drive by an electric motor. The speed of the electric motor ranges between 1000-2000m/min. The belt moves through an insulated metal electrode. The charge is sprayed to this moving belt.

4. The charge is sprayed to the moving belt from the corona points at a potential of ________
a) 20-200kV
b) 10-100kV
c) 100-1000kV
d) 200-2000kV

Explanation: Van de Graff generator is an electrostatic machine which generates very high voltages. The corona points spray charge on the moving insulated belts at a potential between 10-100Kv above the earth. Later the charge is removed and collected from the belt.

5. The potential of high voltage electrode at any instant is __________
a) V=$$\frac{C}{Q}$$
b) V=$$\frac{Q}{C}$$
c) V=$$\frac{2Q}{C}$$
d) V=$$\frac{2C}{Q}$$

Explanation: The potential of the high voltage electrode at any instant is V=$$\frac{Q}{C}$$ above the earth. Q is the is the charge stored and C is the capacitance of the high voltage electrode. The potential of high voltage often rises.

6. The high voltage electrode rises at a rate ___________
a) $$\frac{I}{C}$$
b) $$\frac{2I}{C}$$
c) $$\frac{C}{I}$$
d) $$\frac{2C}{I}$$

Explanation: The high voltage electrode rises at a rate $$\frac{I}{C}$$ where C is the capacitance of the high voltage electrode to earth and I is the net charging current.
$$\frac{dV}{dt}=\frac{1}{C} \frac{dQ}{dt} = \frac{I}{C}$$

7. What is the shape of the high voltage electrode?
a) rectangular
b) triangular
c) oval
d) spherical

Explanation: The shape of the high voltage is spherical. It is made with re-entrant edges to avoid corona and local discharges. The spherical shape can also avoid high surface field gradients.

8. The charging of belt is done by ___________
a) lower spray points
b) upper spray points
d) middle level spray points

Explanation: The belt is charged from the corona spray points which is situated at a lower position. These spray points are sharp needles. They are connected to a dc source of 10-100kV. The corona is maintained between the moving belts and the needles.

9. What is the charging current for unit surface area of the belt?
a) I=b v σ/2
b) I=2b v σ
c) I=b v σ
d) I=4b v σ

Explanation: The charging current for unit surface area of the belt is given by I=b v σ. Here b is the breadth of the belt in meters, v is the velocity of the belt is m/s and is the surface charge density in coulombs/m2.

10. The pressure of the high-pressure gaseous medium ranges between __________
a) 5-15atm
b) 10-20atm
c) 2-3atm
d) 20-22atm

Explanation: The generator works under high-pressure gaseous medium whose pressure ranges from 10-15atm. The gas may be nitrogen, air, Sulphur hexafluoride or air -Freon mixture.

11. What is the charging current for unit surface area of the belt with b=3m, v=$$\frac{3m}{s}$$ and σ=1.4 x 10-5 C/m2.
a) 120 μA
b) 126 μA
c) 12 μA
d) 10 μA

Explanation: The charging current for unit surface area of the belt is given by I=b v σ. Here b is the breadth of the belt in meters, v is the velocity of the belt is m/s and is the surface charge density in coulombs/m2.
I=3 x 3 x 1.4 x 10-5A
I=126 μA.

12. What must be the value of surface charge density in a Van de Graff generator?
a) less than or equal to 1.4 x 10-5 C/m2
b) greater than or equal to 1.4 x 10-5 C/m2
c) less than or equal to 4 x 10-5 C/m2
d) greater than or equal to 4 x 10-5 C/m2

Explanation: The value of surface charge density in a Van de Graff generator must be less than or equal to 1.4 x 10-5 C/m2. This is to have a safe electric field intensity normal to the surface. The surface charge density greater than this value can be unsafe.

13. The Van de Graff generators have high current output.
a) True
b) False

Explanation: The Van de Graff generator is an electrostatic machine which has low current output. This serves as a major disadvantage of Van de Graff generators. They produce high voltage. The produced voltage can be stabilized up to 0.01%.

14. What is the potential of the spherical electrode in terms of electric field E?
a) V=$$\frac{E}{r}$$
b) V=$$\frac{2E}{r}$$
c) V=Er
d) V=2Er

Explanation: The potential of the spherical electrode is V=E r where E is the electric field and r is the radius of the sphere. The spherical electrode maintains a uniform electrical field E. It is the most favorable shape for an electrode.

15. What is the charging current for unit surface area of the belt with b=1m, v=10m/s and σ=19.283 x 10-6C/m2.
a) 1.9283 μA
b) 10 μA
c) 19.283 μA
d) 192.83 μA

Explanation: The charging current for unit surface area of the belt is given by I=b v σ. Here b is the breadth of the belt in meters, v is the velocity of the belt is m/s and is the surface charge density in coulombs/m2.
I=1 x 10 x 19.283 x 10-6A
I=192.83 μ.

Sanfoundry Global Education & Learning Series – High Voltage Engineering.

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