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
View Answer

Answer: b
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.
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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
View Answer

Answer: b
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
View Answer

Answer: b
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
View Answer

Answer: a
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
View Answer

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

Answer: a
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
View Answer

Answer: a
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
View Answer

Answer: b
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?
engineering-physics-questions-answers-dielectric-breakdown-q9
a) A
b) B
c) C
d) D
View Answer

Answer: a
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.
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10. Which of the following shows the voltage-current relation before the breakdown?
a) engineering-physics-questions-answers-dielectric-breakdown-q10a
b) engineering-physics-questions-answers-dielectric-breakdown-q10b
c) engineering-physics-questions-answers-dielectric-breakdown-q10c
d) engineering-physics-questions-answers-dielectric-breakdown-q10d
View Answer

Answer: d
Explanation: Before gas breakdown, there is a non-linear relation between voltage and current as shown in the following diagram.
engineering-physics-questions-answers-dielectric-breakdown-q10d

Sanfoundry Global Education & Learning Series – Engineering Physics.

To practice all areas of Engineering Physics, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn