# Engineering Physics Questions and Answers – Dielectric Breakdown

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This set of Engineering Physics Multiple Choice Questions & Answers (MCQs) focuses on “Dielectric Breakdown”.

1. Which of the following is the correct expression for the dielectric strength?
a) d/Vb
b) Vb/d
c) Vbd
d) Vb/d2

Explanation: The dielectric strength is also defined as the maximum potential gradient to which a dielectric can be subjected with insulation breakdown. The expression is: Vb/d, where Vb is the breakdown voltage and d is the thickness of the specimen.

2. Mica has a dielectric strength of _____________
a) 50 X 106 V/m
b) 100 X 106 V/m
c) 150 X 106 V/m
d) 200 X 106 V/m

Explanation: Mica has one of the highest dielectric strengths of 100 X 106 V/m. It is widely used for insulation purposes.

3. Beyond breakdown voltage, dielectrics become insulating.
a) True
b) False

Explanation: Dielectrics are defined as materials devoid of free charges. After the breakdown voltage, it loses its insulating property and becomes conducting.

4. Dielectric breakdown occurs in gases due to ____________
a) Corona Discharge
b) Dielectric heating
c) Intrinsic breakdown
d) Defect breakdown

Explanation: Dielectric breakdown occurs in gases due to the corona discharge in non-uniform fields or due to photoionization and collision of electrons with gas atoms.

5. In which type of breakdown, an avalanche of electrons is formed?
a) Defect Breakdown
b) Thermal Breakdown
c) Intrinsic Breakdown
d) Electrochemical Breakdown

Explanation: At high Electric fields, electrons are ejected which are accelerated through the material, producing an avalanche of electron. Such a breakdown is called intrinsic breakdown.

6. The presence of porosity in dielectrics is detrimental for higher dielectric strength.
a) True
b) False

Explanation: The more holes of cracks there are in the dielectric solid, the more ionization of gases would take place in that pores at higher electric field. This leads to thermal stress which generates cracks. Thus, the presence of porosity is detrimental for higher dielectric strength.

7. The process of breakdown is accelerated by the presence of ____________
a) Impurities
b) Conduction
c) Humidity
d) Magnetic Field

Explanation: The process of breakdown of liquids is accelerated by the presence of impurities or high concentration of mobile ions (e.g. Na+, Li+) in liquids.

8. The most common mode of breakdown in ceramic insulators is _____________
a) Defect Breakdown
b) Thermal Breakdown
c) Intrinsic Breakdown
d) Electrochemical Breakdown

Explanation: Thermal breakdown occurs due to local heating or conduction losses which generate heat at the rate faster than it can be dissipated. This is most common in ceramics.

9. Which region is for instrinsic breakdown?

a) A
b) B
c) C
d) D

Explanation: The given figure shows the variation of breakdown strength with log time. The A region is of intrinsic breakdown, b is for Streamer, C is for thermal and D is electrochemical breakdown.

10. Which of the following shows the voltage-current relation before the breakdown?
a)
b)
c)
d)

Explanation: Before gas breakdown, there is a non-linear relation between voltage and current as shown in the following diagram.

Sanfoundry Global Education & Learning Series – Engineering Physics.

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