3.9 Capacitance & Capacitors Flashcards
What is the primary function of capacitors in a circuit?
To store charge for a short time
Capacitors are essential components in electrical circuits, particularly for filtering, timing, and energy storage applications.
What are the main components of a simple capacitor?
Two metal plates and a dielectric material
The dielectric material increases the strength of the electric field between the plates.
What does the ‘dielectric constant’ measure?
The effectiveness of the dielectric material
A higher dielectric constant indicates a better ability to store charge.
What are the two standard symbols for capacitors based on different standards?
IEC standard (Europe) and ANSI standard (USA)
The symbols may vary in appearance but represent the same component.
What additional marking is present on electrolytic capacitors?
’+’ symbol next to the positive terminal
This marking indicates the polarity of the capacitor.
What are the two distinct hazards associated with capacitor usage?
Overheating and electric shock
Understanding these hazards is crucial for safe handling and operation of capacitors.
What causes overheating in capacitors?
Excess voltage or reverse-polarity voltage
Overheating can lead to capacitor expansion and explosion.
What is the consequence of applying excessive voltage to a capacitor?
It can cause the capacitor to explode
Explosions can result in local damage, personal injury, and fire hazards.
What should be considered when choosing or replacing a capacitor?
Select a capacitor with a voltage rating at least twice the expected working value
This ensures safe operation under normal circuit conditions.
What is the required current type for electrolytic capacitors?
Direct current (DC)
Using AC can damage electrolytic capacitors due to their polarity requirement.
What marking indicates the positive terminal on electrolytic capacitors?
’+’ symbol and a longer positive lead
The negative terminal may have a solid black line or a ‘–’ symbol.
What is a crucial safety step before working on a circuit with capacitors?
Ensure capacitors are fully discharged
This is especially important if the voltage exceeds 65 V to prevent electric shock.
Fill in the blank: Capacitors can store electric charge for a _______.
long time
This characteristic can pose risks if not properly managed.
True or False: Axial capacitors always have a positive terminal at the crimped end.
True
This design helps in identifying the correct polarity for connections.
What are the factors affecting the capacitance of a capacitor?
- Plate area
- Distance between the plates
- Applied voltage
The capacitance is influenced by these factors as they determine the amount of charge stored.
What happens when a voltage is applied across the plates of a capacitor?
A current flows due to the electric field generated between the plates as the charge accumulates until the current stops.
This current stops due to the accumulation of charge on the plates.
What is the relationship between the amount of charge stored and the applied voltage?
The amount of charge stored is proportional to the applied voltage.
This proportionality forms the basis for understanding capacitance.
What symbol is used for the constant of proportionality in capacitance?
C
C represents capacitance in electrical formulas.
What is the unit of capacitance?
The farad
Capacitance is measured in farads, which can be expressed in terms of ampere, seconds, and volts.
What does a capacitor with a capacitance of 1 farad signify?
A current of 1 amp flowing for 1 second produces an increase in Potential Difference (PD) of 1 volt across the capacitor.
This definition illustrates the relationship between capacitance, current, and voltage.
What are the smaller units used for capacitance in electrical circuits?
- Microfarads
- Nanofarads
- Picofarads
1 farad is a large value, hence smaller units are commonly used.
What three quantities determine the capacitance of a capacitor?
Capacitance depends on:
* Area of overlap of the plates
* Distance between the plates
* Dielectric material used
What is the formula for capacitance?
C = (ε * A) / d
Where C is capacitance in farads (F), A is the area of overlap of the plates in square metres (m²), d is the separation of the plates in metres (m), and ε is the permittivity of the dielectric in farads per metre (F/m)
Why are capacitor plates usually constructed from several separate parallel layers?
To maintain the total area while changing the dimensions of the capacitor
How does compact capacitor construction increase capacitance?
By utilizing both sides of the plates
What is another common method of capacitor construction that increases capacitance?
Rolling the plates to utilize both sides
What is the role of the dielectric material in a capacitor?
Increases capacitance and helps withstand high voltages
What is the permittivity of free space symbolized as?
ε0
What is the value of the permittivity of free space (ε0)?
8.85 x 10^-12 F/m
What is the dielectric constant of air compared to a vacuum?
The same when measured to four significant figures
Fill in the blank: The dielectric constant of fuel is ______.
2.7
How does the dielectric constant of fuel affect capacitance in a sensor?
Increases capacitance when used as the dielectric in a tubular capacitor
What must be drained regularly from the fuel tank to prevent measurement falsification?
Water
What are the two conducting layers in paper or film capacitors?
Metal foil or an evaporated metal layer
True or False: Paper or film capacitors are polar.
False
Describe the construction of an electrolytic capacitor.
One layer of aluminium foil as the positive plate and a thickened or solid electrolyte as the negative plate
What happens if an electrolytic capacitor is operated with the wrong polarity?
It will destroy the dielectric and permanently damage the capacitor
What are electrolytic capacitors commonly used for?
Providing high capacitance in a small space (e.g., flashguns, DC voltage filtering)
What is the purpose of variable capacitors in circuits?
To accept or reject certain frequencies of AC input
How can the capacitance of a variable capacitor be altered?
By turning a shaft to vary the area of plate overlap
What are trimmer capacitors used for?
Fine modification of the capacitance in tuned circuits
What do the colour bands on a capacitor represent?
The capacitance value and tolerance
How many colour bands are typically used to mark a capacitor?
Four or more colour bands
In a four-colour band capacitor, what do the first two colour bands represent?
The first and second digits of its capacitance
What does the third colour band in a four-colour band capacitor represent?
The decimal multiplier
What does the fourth colour band represent in a four-colour band capacitor?
Tolerance
What unit is capacitance value given in?
Picofarads (10^-12 F)
What does a fifth colour band (Type K) denote on a capacitor?
The voltage rating
What is the consequence of subjecting a capacitor to a voltage greater than its voltage rating?
It may permanently damage the capacitor
What is the voltage rating rule of thumb for selecting capacitors?
Voltage rating = 2 x working voltage
Fill in the blank: The colour code for Black is ______.
0
Fill in the blank: The tolerance for a Brown colour band is ______.
± 1%
True or False: The colour Blue in a capacitor colour code is used for a decimal multiplier.
False
List the digits represented by the colour bands for Red.
- Digit 1: 2
- Digit 2: 2
- Multiplier: 100
- Tolerance: ± 2%
What is the maximum voltage for a capacitor with a voltage rating of 500 V?
500 V
What is the capacitance value for a capacitor with colour bands Yellow, Brown, Red, and Green?
4100 pF ± 5%
Which colour band indicates a tolerance of ± 0.1 pF?
Brown
For capacitors with a capacitance greater than 10 pF, what is the tolerance for Yellow?
± 4%
What does the Grey colour band represent in terms of tolerance?
+ 80%, - 20%
What is the voltage rating for a capacitor with a Blue colour band?
700 V
What happens to capacitance when capacitors are connected in series?
Capacitance is reduced
The total capacitance CT of n capacitors connected in series is _______.
[calculated using the appropriate formula]
In a series connection, the sum of the capacitances is _______.
smaller than that of the smallest individual capacitor
What is the relationship between the total voltage and the voltages across series capacitors?
The sum of the voltages across series capacitors equals the total voltage across the equivalent capacitor CT
In a series connection, how are the voltages distributed across capacitors?
In inverse proportions to the capacitances
When capacitors are connected in parallel, their capacitances must be _______.
added together
The total capacitance CT for n capacitors connected in parallel is _______.
[calculated using the appropriate formula]
What is the relationship between the voltages across parallel capacitors?
The individual voltages are equal to the total voltage across the equivalent capacitor CT
In a parallel connection, how do the voltages across the capacitors compare?
All the voltages are the same
What governs the time taken for a capacitor to charge or discharge?
The mathematical law of exponential decay, capacitance, resistance, and Potential Difference (PD)
The time taken for a capacitor to charge or discharge is influenced by the values of capacitance (C), resistance (R), and the potential difference (PD) across the capacitor.
What happens to the initial current when a completely discharged capacitor is connected to a battery?
The initial current will be extremely high
This occurs because the capacitor initially has no potential difference (PD) across it.
Why do we use a resistor in series with a capacitor?
To limit the initial current
The initial current depends only on the resistor when a capacitor is connected to a battery.
What is the formula for the time constant of a capacitor?
Time constant = RC
Where ‘R’ is the resistance in ohms (Ω) and ‘C’ is the capacitance in farads (F).
How does the voltage across the resistor change as the capacitor charges?
The higher the capacitor voltage, the lower the resistor voltage
This results in a lower current as the capacitor charges.
After one time constant, what percentage of the initial current remains?
37%
After one time constant, the capacitor voltage will rise to 63% of the final supply voltage.
What percentage of the initial charge does the capacitor hold after one time constant when discharging?
37%
After t=RC, the capacitor holds 37% of its initial charge.
Fill in the blank: The product of resistance and capacitance is known as the _______.
time constant
What happens to the current after five time constants have passed during charging?
The current will have fallen to (37%)^5, which is equal to 0.7% of its initial value
After five time constants, the capacitor is considered to be fully charged.
True or False: The voltage of a capacitor shows exponential growth during charging.
True
At what time does a capacitor reach 63% of its charge?
t=RC
After t=RC, the capacitor is 63% charged.
What is the effect of increasing resistance and capacitance on the charging time of a capacitor?
The charging time will be longer
Higher values of resistance and capacitance result in a longer time constant.
What law governs the charging and discharging process of a capacitor?
Ohm’s Law
Ohm’s Law explains how the resistor limits the current in the circuit.
What are the procedures used for testing capacitors?
Visual inspection, using an ohmmeter, and dedicated LCR tester for small capacitors
Visual inspection can reveal bulging and electrolyte leakage.
What types of capacitor failure can be identified during testing?
Catastrophic Failures and Degradation Failures
Catastrophic Failures are due to short circuits or open circuits, while Degradation Failures involve gradual decreases in leakage resistance.
What can visual inspection of capacitors reveal?
Bulging of the casing and electrolyte leakage
How does an ohmmeter test a capacitor?
The capacitor charges from the internal battery of the ohmmeter
Select [Rx1] or [Rx10] for small values (<1uF) and [Rx100] or above for large values (>1uF).
What should be done before testing a capacitor with an ohmmeter?
Discharge the capacitor by touching the capacitor’s leads together.
What indicates a properly functioning capacitor during the ohmmeter test?
The meter pointer moves to zero resistance and then swings to infinity.
What does an infinite resistance reading indicate in a capacitor test?
The capacitor is probably working and has no current flow after being fully charged.
What does a shorted capacitor show on an ohmmeter?
A zero or near-zero reading.
What percentage of defective capacitors have excessive leakage current?
More than 40%.
What is dielectric absorption in capacitors?
A condition where capacitors retain a residual charge after discharge.
What can excessive equivalent series resistance in capacitors be caused by?
Defective lead to plate contact, resistive leads, or resistive plates.
True or False: A capacitor with an open circuit fault will show infinite resistance on both readings.
True.
Fill in the blank: Capacitor failures can be categorized into _______ and _______.
[Catastrophic Failures], [Degradation Failures]
What is a common symptom of a capacitor failure due to dielectric absorption?
Capacitor does not completely discharge during use.
What is the test method for small capacitors that charge extremely quickly?
Dedicated LCR tester.
