Module 3: Electrical Fundamentals - Capacitance

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Questions and Answers

What happens to a capacitor when the resistance is high and remains relatively the same?

  • The capacitor discharges rapidly.
  • The capacitor charges quickly.
  • The capacitor begins to short circuit.
  • The capacitor is considered open. (correct)

Which factor does NOT affect capacitance?

  • Area of the plates
  • Temperature of the environment (correct)
  • Distance between the plates
  • Dielectric material used

Which of the following is a characteristic of capacitors?

  • They can only be used in parallel circuits.
  • They can maintain a constant voltage indefinitely.
  • They cannot store energy.
  • They exhibit exponential charge and discharge behavior. (correct)

What is the purpose of the dielectric in a capacitor?

<p>To ensure electrical isolation between the plates. (A)</p> Signup and view all the answers

In capacitor testing, what does a high resistance value signify?

<p>The capacitor is functioning normally. (A)</p> Signup and view all the answers

What is the primary function of a capacitor?

<p>To store electrical charge (C)</p> Signup and view all the answers

What happens to current flow across a fully charged capacitor?

<p>Current flow ceases (A)</p> Signup and view all the answers

How does a capacitor behave when a DC voltage is applied?

<p>It blocks DC after charging (C)</p> Signup and view all the answers

What occurs when the battery connected to a charged capacitor is reversed?

<p>The capacitor discharges and then recharges in the opposite direction (D)</p> Signup and view all the answers

What is a result of applying alternating current (AC) to a capacitor?

<p>Current flows into and out of the plates, but no electron transfer occurs (B)</p> Signup and view all the answers

What is necessary for a capacitor to hold a large charge for a given voltage?

<p>Having a dielectric and greater surface area (A)</p> Signup and view all the answers

Which phenomenon occurs during the initial connection of a voltage to a capacitor?

<p>A rush of current flows until a potential difference is established (B)</p> Signup and view all the answers

What is the capacitance value of a capacitor that is rated at 2.2 µF?

<p>2200 pF (C)</p> Signup and view all the answers

What tolerance percentage applies to a 0.1 µF capacitor?

<p>+/- 20% (B)</p> Signup and view all the answers

Which type of variable capacitor is best suited for small adjustments?

<p>Trimmer capacitor (B)</p> Signup and view all the answers

In an RC circuit, what determines the charge time of the capacitor?

<p>The resistance and capacitance values together (C)</p> Signup and view all the answers

What happens to the voltage across a capacitor's plates when power is applied?

<p>It rises exponentially. (D)</p> Signup and view all the answers

Which letter denotes a capacitor with a tolerance of +/- 0.5pF?

<p>D (C)</p> Signup and view all the answers

What component is used for tuning most radio receivers?

<p>Rotary variable capacitor (C)</p> Signup and view all the answers

If a capacitor's capacitance is above 10pF, what is the tolerance for a 10,000pF capacitor?

<p>+/- 2% (C)</p> Signup and view all the answers

What is the time constant for the RC circuit described with a 10 kΩ resistor and a 100 µF capacitor?

<p>1 second (C)</p> Signup and view all the answers

How much time is required for the voltage across the capacitor to reach 63.2% of the source voltage in the given RC circuit?

<p>1 second (B)</p> Signup and view all the answers

What is the time required for complete discharge for the first circuit?

<p>9.4 seconds (C)</p> Signup and view all the answers

In a parallel configuration of capacitors, how is the voltage drop across the capacitors described?

<p>It is equal across all capacitors. (B)</p> Signup and view all the answers

When discharging a capacitor with a time constant of 0.4 seconds, how much voltage will remain after one time constant has elapsed?

<p>36.8 volts (B)</p> Signup and view all the answers

How is the voltage drop across capacitors in series related to their capacitance?

<p>Inversely proportional to capacitance. (B)</p> Signup and view all the answers

What is the total time it takes for a capacitor to fully discharge in the described RC circuit with a time constant of 0.4 seconds?

<p>5.0 seconds (B)</p> Signup and view all the answers

If the resistance in a timing circuit is increased from 10 kΩ to 20 kΩ while keeping capacitance constant, what will happen to the time constant?

<p>It will double (B)</p> Signup and view all the answers

What is the voltage across the capacitor after 1 time constant during charging in the first circuit?

<p>17.7 V (D)</p> Signup and view all the answers

What voltage level is reached across a capacitor after 5 time constants in the described circuit?

<p>100 volts (C)</p> Signup and view all the answers

What happens to the total capacitance when capacitors are connected in parallel?

<p>It increases, calculated by the sum of individual capacitances. (A)</p> Signup and view all the answers

If a capacitor is charged to 28 VDC, what will be the voltage across each capacitor if they are in parallel?

<p>28 VDC for each capacitor. (A)</p> Signup and view all the answers

In an RC circuit with a capacitor charged to 100 volts, how much voltage is left after 2 time constants if the time constant is 0.4 seconds?

<p>54.6 volts (D)</p> Signup and view all the answers

What is the time required for complete charge in the second circuit?

<p>1 second (D)</p> Signup and view all the answers

Which of the following describes the behavior of voltage across the capacitor during the charging phase?

<p>It increases exponentially (C)</p> Signup and view all the answers

After 1 time constant during discharge of the second circuit, what is the voltage across the capacitor?

<p>4.42 V (C)</p> Signup and view all the answers

What effect does increasing the area of the plates have on the capacitance of a capacitor?

<p>Capacitance increases with larger plate area. (C)</p> Signup and view all the answers

How does the distance between capacitor plates affect capacitance?

<p>Capacitance decreases as the distance decreases. (C)</p> Signup and view all the answers

What is the effect of connecting capacitors in parallel?

<p>Increases overall capacitance by effectively enlarging plate area. (C)</p> Signup and view all the answers

Which formula is used to calculate total capacitance for capacitors in series?

<p>1/Ctotal = 1/C1 + 1/C2 + 1/C3... (B)</p> Signup and view all the answers

What characterizes a dielectric material?

<p>An effective supporter of electrostatic fields. (A)</p> Signup and view all the answers

What happens to capacitance when the distance between plates is increased?

<p>Capacitance decreases. (A)</p> Signup and view all the answers

What is NOT a factor that affects capacitance?

<p>Voltage applied to the capacitor. (A)</p> Signup and view all the answers

How does the use of a dielectric material impact a capacitor's performance?

<p>It enhances energy storage capacity by increasing capacitance. (C)</p> Signup and view all the answers

Flashcards

Capacitance

The amount of charge a capacitor can store at a given voltage. It depends on the physical characteristics of the capacitor, including the area of the plates, the distance between them, and the dielectric material used.

How does plate area affect capacitance?

The larger the area of the plates, the greater the capacitance. This is because more charge can be stored on larger plates.

How does distance between plates affect capacitance?

Capacitance increases as the distance between the plates decreases. This is because the electric field between the plates is stronger when they are closer together.

What is a dielectric material?

A material that is a poor conductor of electricity but can support an electrostatic field. It's like an insulator for electricity.

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How does a dielectric material affect capacitance?

A non-conducting material that reduces the electric field between the capacitor plates, increasing capacitance.

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How does connecting capacitors in parallel affect capacitance?

Connecting capacitors in parallel increases the total plate area, leading to a larger total capacitance. You simply add up the individual capacitances.

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Connecting capacitors in series

Connecting capacitors in series increases the effective distance between the plates, decreasing the total capacitance.

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Formula: Ctotal in parallel

The formula for calculating total capacitance of capacitors connected in parallel is to simply add up the individual capacitances.

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What is a capacitor?

A capacitor is an electrical component that stores electrical energy in an electric field. It consists of two conductive plates separated by an insulating material called a dielectric.

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Why use plates in a capacitor?

Capacitors are designed to increase charge storage by using plates instead of single conductors. This allows for a greater surface area, which in turn leads to greater charge capacity.

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What is the role of a dielectric in a capacitor?

The dielectric material between the plates of a capacitor prevents the flow of direct current (DC) while allowing the electric field to build up to full potential.

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How does a capacitor charge?

When a voltage is applied to a capacitor, electrons flow from one plate to the other, creating an electric field between the plates. This process is called charging the capacitor.

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What happens when a capacitor is fully charged?

Once a capacitor is fully charged, the voltage across the plates equals the applied voltage. At this point, the current flow ceases, and the capacitor acts as an open circuit for DC.

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What happens when a capacitor is discharged and recharged?

When the voltage across a capacitor reverses, the plates discharge and then re-charge with the opposite polarity. Again, current flow stops once the capacitor reaches the applied voltage.

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How does a capacitor react to alternating current (AC)?

Unlike DC, alternating current (AC) causes a capacitor to continuously charge and discharge, resulting in a constant current flow through the wires connected to the plates. However, electrons do not actually transfer across the dielectric.

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Why is it said that a capacitor 'passes' AC?

Since a capacitor allows for a continuous flow of current in the wires connecting to its plates when AC is applied, it is said to 'pass' AC.

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Time Constant (TC)

The time it takes for the voltage across a capacitor to reach 63.2% of the source voltage in an RC circuit.

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Time Constant Formula

TC = R x C, where:

  • TC is the time constant in seconds.
  • R is the resistance in ohms.
  • C is the capacitance in farads.
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Full Charge Time

In order for a capacitor to charge to 100% of the source voltage in an RC circuit, it takes 5 times the time constant (5 x TC).

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Discharge Time (63.2%)

It takes 1 time constant (TC) for a capacitor to discharge by 63.2% of its initial voltage in an RC circuit.

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Full Discharge Time

In order for a capacitor to completely discharge in an RC circuit, it takes 5 times the time constant (5 x TC).

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RC Circuit Time Constant Dependence

The value of resistance (R) and capacitance (C) determines the time constant (TC) in an RC circuit.

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Higher Time Constant

An RC circuit with higher resistance and capacitance values will have a longer time constant.

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Lower Time Constant

An RC circuit with lower resistance and capacitance values will have a shorter time constant.

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Capacitor Tolerance

The tolerance of a capacitor indicates the percentage deviation from the nominal value that is acceptable. It signifies how much the actual capacitance can vary from the stated value.

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Capacitance Units: Picofarad (pF)

The value of capacitance in a capacitor can be expressed in various units, with picofarads (pF) being a common one. 1 picofarad is equal to 10^-12 farads.

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Calculation of Tolerance

The tolerance value is expressed as a percentage of the nominal value of the capacitor. For example, a capacitor with a 10% tolerance can have a capacitance value that is 10% higher or lower than the stated value.

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What is a Variable Capacitor?

A variable capacitor is a type of capacitor where its capacitance can be adjusted by changing the distance between its plates.

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Application of Variable Capacitors

Variable capacitors are primarily used for tuning radio receivers and other electronic circuits.

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What is a Trimmer Capacitor?

A trimmer capacitor is a type of variable capacitor that allows for fine-tuning adjustments.

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Exponential Capacitor Charge

The charging of a capacitor in an RC circuit follows an exponential curve. This means the voltage across capacitor plates increases gradually over time, eventually reaching a steady state.

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Factors Affecting Charge Time

The amount of time it takes for a capacitor to charge is influenced by the values of resistance and capacitance in the circuit.

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Capacitor Charging

The process where a capacitor stores electrical energy in an electric field.

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Capacitor Discharging

The process where a capacitor releases stored electrical energy.

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Voltage after 1 TC (Charging)

The voltage across a capacitor after one time constant (TC) during the charging process.

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Voltage after 1 TC (Discharging)

The voltage across a capacitor after one time constant (TC) during the discharging process.

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Capacitors in Parallel

Connecting capacitors side-by-side increases their total capacitance.

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Capacitors in Series

Connecting capacitors end-to-end decreases their total capacitance.

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Voltage Drop (Parallel)

The voltage across each capacitor in a parallel connection is the same.

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What is capacitance?

The capacitance of a capacitor is measured in Farads (F). It represents the amount of electrical charge a capacitor can store at a given voltage. Basically, how much energy it can hold.

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How does the area of the plates affect a capacitor's capacitance?

Increased plate area allows for more charge to accumulate on the plates, leading to a higher capacitance. Think of it as having more space to store charge.

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How does the distance between the plates affect a capacitor's capacitance?

The distance between the plates is inversely proportional to capacitance. That means decreasing the distance between the plates will increase the capacitance. Imagine squeezing the container to make it smaller, it holds less water.

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Study Notes

Module 3: Electrical Fundamentals, Topic 3.9: Capacitance/Capacitor

  • A capacitor is a device used to store electrical charge.
  • Capacitance describes a capacitor's ability to store charge.
  • Factors affecting capacitance include the distance between plates, area of plates, number of plates, dielectric material and its constant, working voltage, voltage rating.
  • Different capacitor types exist, each with unique construction and functions.
  • Calculations involving capacitance and voltage in series and parallel circuits are essential.
  • Capacitors exhibit exponential charge and discharge behaviour governed by time constants.
  • Capacitor testing involves visual inspection for swelling and ensuring the capacitor is fully discharged before handling.
  • Capacitance measurement techniques and tools are used to determine capacitance values (e.g., capacitance meter, multimeter).
  • Important considerations for capacitor use include polarity, voltage ratings, and dielectric strength.
  • Different dielectric materials exist with varying dielectric strengths and constants (e.g., air, mica, tantalum oxide, etc.).
  • Basic capacitor operation involves the build-up of charge on plates as current flows until it reaches its maximum, and then ceases.
  • Capacitors block DC current but allow AC current to flow.
  • Common capacitor types include paper, mica, ceramic, and electrolytic.
  • Tantalum capacitors are a newer type of electrolytic capacitor with higher costs and are polarity sensitive.
  • Understanding capacitor behavior in parallel and series configurations is crucial for circuit analysis and design.
  • Time constant calculations are important for determining the charging and discharging time of capacitors in circuits.

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