capacitance and capacitors

Question 1

What is a capacitor?

Answer 1

A capacitor is a component that stores electrical charge in a circuit, usually for a short time.

Question 2

What are the main components of a capacitor?

Answer 2

Two metal plates or foils insulated from each other by a material called the dielectric.

Question 3

What is the role of the dielectric in a capacitor?

Answer 3

The dielectric increases the strength of the electric field between the plates, allowing more charge to be stored at a given voltage.

Question 4

What is the dielectric constant?

Answer 4

A measure of the effectiveness of the dielectric material in increasing the capacitor’s charge storage capacity.

Question 5

What are the schematic symbols for capacitors in different standards?

Answer 5

The International Electrotechnical Commission (IEC) symbol (Europe) and the American National Standards Institute (ANSI) symbol (USA).

Question 6

How is the polarity of an electrolytic capacitor indicated?

Answer 6

By a ‘+’ symbol next to the positive terminal and a longer positive lead; the negative terminal is marked with a solid black line or a ‘–‘ symbol and a shorter lead.

Question 7

What are the two main hazards when working with capacitors?

Answer 7

Overheating due to excess or reverse-polarity voltage and electric shock.

Question 8

What causes overheating in capacitors and what are the consequences?

Answer 8

Overheating is caused by excessive or reverse voltage, which can cause the capacitor to expand, explode, and potentially cause injury or fire.

Question 9

What precaution should be taken regarding the voltage rating of capacitors?

Answer 9

Choose a capacitor with a voltage rating at least twice that of the expected working voltage in the circuit.

Question 10

Why must electrolytic capacitors be used with direct current (DC)?

Answer 10

Because they are polarized and must be connected with the correct polarity to function properly.

Question 11

How can you identify the positive terminal on an axial capacitor?

Answer 11

The positive terminal is at the crimped end of the cylinder.

Question 12

Why is it important to discharge capacitors before working on a circuit?

Answer 12

To prevent receiving a dangerous electric shock from stored charge, especially if the voltage exceeds 65 V.

Question 13

What is a key safety measure when choosing or replacing a capacitor?

Answer 13

Select a capacitor with a voltage rating at least twice the expected working voltage.

Question 14

What happens when a voltage is applied across the two plates of a capacitor?

Answer 14

A current flows due to the electric field generated between the plates as the charge accumulates, eventually stopping as the charge builds up.

Question 15

What factors affect the amount of charge held by the plates of a capacitor?

Answer 15

Plate area, the distance between the plates, and the applied voltage.

Question 16

What is the relationship between charge (Q), capacitance (C), and voltage (V)?

Answer 16

Q=CV

Question 17

How is the amount of charge stored (Q) related to the applied voltage (V)?

Answer 17

The amount of charge stored is proportional to the applied voltage.

Question 18

What is the unit of capacitance?

Answer 18

The farad (F).

Question 19

How can the farad be expressed in terms of other units?

Answer 19

C=As/v =F

Question 20

What are the smaller units of capacitance commonly used in circuits?

Answer 20

Microfarads (μF), nanofarads (nF), and picofarads (pF).

Question 21

What three quantities does capacitance depend on?

Answer 21

Area of overlap of the plates, distance between the plates, and the dielectric material used.

Question 22

What is the formula for capacitance (C)?

Answer 22

C= εA/d where ε is the permittivity of the dielectric, A is the area of overlap, and d is the distance between the plates.

Question 23

What is the permittivity of free space (ε0)?

Answer 23

ε₀ =8.85×10⁻¹² F/m.

Question 23

What is permittivity (ε)?

Answer 23

Permittivity is a measure of how much electric field is reduced in the dielectric material. It is measured in farads per metre (F/m).

Question 24

What is the dielectric constant (εr)?

Answer 24

The dielectric constant is the ratio of the permittivity of a dielectric material to the permittivity of free space (ε0).

Question 25

How does the dielectric constant affect capacitance?

Answer 25

The higher the dielectric constant, the greater the capacitance.

Question 26

How is capacitance in practical capacitors increased without changing the overall area?

Answer 26

By constructing capacitor plates from several separate parallel layers, electrically connected.

Question 27

What are the two common methods of capacitor construction?

Answer 27

Using several parallel layers or rolling the plates.

Question 28

What is the benefit of using a dielectric material in capacitors?

Answer 28

It increases the capacitance and allows the capacitor to withstand higher voltages.

Question 29

What is an example of a practical application of dielectric constants in capacitors?

Answer 29

Measuring fuel levels in aircraft tanks using tubular capacitors with fuel as the dielectric.

Question 30

Why must water be drained from aircraft fuel tanks regularly in the context of capacitors?

Answer 30

Water has a high dielectric constant (80) which can falsify the fuel level measurement.

Question 31

What are paper or film capacitors?

Answer 31

Capacitors with two conducting layers separated by paper foil or plastic film, which are non-polar and can be used with both direct and AC voltage.

Question 32

What is unique about electrolytic capacitors?

Answer 32

They have one layer of aluminum foil and a second conducting layer of thickened electrolyte, and they must maintain the correct polarity to prevent damage.

Question 33

What happens if an electrolytic capacitor is connected with incorrect polarity?

Answer 33

The dielectric is destroyed, permanently damaging the capacitor.

Question 34

The dielectric is destroyed, permanently damaging the capacitor.

Answer 34

To accept or reject certain frequencies of AC input and tune circuits to select specific frequencies.

Question 35

What are trimmer capacitors?

Answer 35

Small variable capacitors used for fine-tuning capacitance in radio receivers and other applications.

Question 36

How is capacitance adjusted in a trimmer capacitor?

Answer 36

By changing the space between the plates using a small screw that pushes the plates together.

Question 37

how do you read a four colour band capacitor?
e.g; yellow, brown, red and green

Answer 37

1st digit, 2nd digit, 3rd band multiplier and 4th band tolerance,
e.g;
1st band yellow = 4
2nd band brown = 1
3rd band red = x100
4th band green = 5%

Question 38

What happens to capacitance when capacitors are connected in series?

Answer 38

When capacitors are connected in series, the total capacitance decreases compared to each individual capacitor.

Question 39

How is the total capacitance calculated for capacitors in series?

Answer 39

The total capacitance
𝐶T of capacitors connected in series is given by: 1/CT = 1/C¹ +1/C² + 1/C³……1/Cⁿ

Question 40

Why is the total capacitance smaller than the smallest individual capacitor in a series circuit?

Answer 40

In a series circuit, the effective capacitance is reduced because the capacitors share the applied voltage, resulting in an overall decrease in stored charge.

Question 41

How are voltages distributed in a series capacitor circuit?

Answer 41

The sum of the voltages across the series capacitors equals the total voltage across the equivalent capacitor
𝐶𝑇. Voltages are distributed in inverse proportion to the capacitances: larger capacitors have smaller voltages across them, and vice versa.

Question 42

What happens when capacitors are connected in parallel?

Answer 42

When capacitors are connected in parallel, the total capacitance
𝐶𝑇 is the sum of the individual capacitances
CT = C¹+C²+C³+….Cⁿ

Question 43

What is the voltage distribution in a parallel capacitor circuit?

Answer 43

In a parallel capacitor circuit, each capacitor has the same voltage across it as the total voltage applied to the circuit. This ensures equal voltage distribution across all capacitors in parallel.

Question 44

What determines the time it takes for a capacitor to charge or discharge?

Answer 44

The time taken for a capacitor to charge or discharge is determined by the product of the resistance R and the capacitance
C. This product RC is known as the time constant.

Question 45

Define the time constant in a capacitor-resistor circuit.

Answer 45

The time constant
RC describes the time it takes for the voltage across a capacitor to change approximately 63% of its total change during charging or discharging.

Question 46

How is the time constant calculated?

Answer 46

The time constant
RC is calculated as the product of the resistance R in ohms (Ω) and the capacitance C in farads (F).

Question 47

What happens to the current and voltage during capacitor charging?

Answer 47

During capacitor charging, the current decreases exponentially as the voltage across the capacitor increases. Initially, the current is high and decreases over time due to the resistor limiting it.

Question 48

Describe the charging curve of a capacitor.

Answer 48

At 𝑡 = 𝑅𝐶, the capacitor is charged to approximately 63% of its maximum voltage (supply voltage). After t=5RC, the capacitor is considered fully charged, with the voltage approaching the supply voltage.

Question 49

How does capacitor discharge occur?

Answer 49

When a capacitor discharges through a resistor, the current follows a similar exponential decay as during charging. The voltage across the capacitor decreases over time.

Question 50

Why is a resistor often used in series with a capacitor during charging?

Answer 50

A resistor limits the initial charging current when a capacitor is connected to a voltage source. This prevents damage to the capacitor and other components due to excessive current.

Question 51

How does the capacitor voltage change during charging and discharging?

Answer 51

During charging, the capacitor voltage increases exponentially toward the supply voltage. During discharging, the capacitor voltage decreases exponentially toward zero as it releases stored energy.

Question 52

How can you visually inspect a capacitor for potential faults?

Answer 52

Visual inspection can reveal bulging of the casing and electrolyte leakage, which are signs of potential capacitor failure.

Question 53

How do you test a capacitor using an ohmmeter?

Answer 53

Discharge: Start by touching the capacitor’s leads together to discharge it.
Testing:

~For small capacitors (<1uF), use [Rx1] or [Rx10] on the ohmmeter.

~For larger capacitors (>1uF), use [Rx100] or higher.

~Connect the positive ohmmeter probe to the positive capacitor lead and the negative probe to the negative lead.

~The meter should show zero resistance and then swing to infinity as the capacitor charges.

~Repeat with the probes reversed (negative probe to positive lead).

~The meter should again show zero resistance initially and then swing to infinity.

Question 54

What do different ohmmeter readings indicate about the capacitor’s condition?

Answer 54

~Normal Capacitor: Shows zero resistance initially and then swings to infinity as it charges and holds the charge.

~Open Circuit (Fault): Shows infinite resistance on both tests.

~Shorted Capacitor (Fault): Shows zero or near-zero resistance and does not swing to infinity.

Question 55

What are catastrophic failures of capacitors?

Answer 55

Catastrophic failures include short circuits due to dielectric breakdown or open circuits caused by connection failures.

Question 56

What are degradation failures of capacitors?

Answer 56

Degradation failures occur gradually and involve a decrease in leakage resistance, leading to increased leakage current, equivalent series resistance, or dielectric absorption.

Question 57

What is excessive equivalent series resistance in capacitors?

Answer 57

Excessive equivalent series resistance can be caused by defective lead to plate contact, resistive leads, or resistive plates, particularly in AC circuits.

Question 58

Why are electrolytic capacitors particularly susceptible to failures?

Answer 58

More than 40% of defective capacitors have excessive leakage current, a common issue with electrolytic capacitors. They are also prone to dielectric absorption, where they retain a residual charge even after appearing discharged.

Question 59

Why is dielectric absorption dangerous?

Answer 59

Dielectric absorption can make a capacitor appear discharged, but it may still hold a significant charge. Disconnecting leads after shorting can release this stored charge unexpectedly, posing a safety hazard.