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75 Cards in this Set
- Front
- Back
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What is the definition of capacitance?
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The capacitance of an object is the amount of charge it is able to store per unit potential difference across it.
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What is capacitance measured in?
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F (Farads)
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What is one farad equal to?
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C/V (one coulomb per volt)
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What is the equation for capacitance?
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C=Q/V
Where: • C is capacitance • Q is charge • V is potential difference |
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What is a capacitor?
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A capacitor is an electrical component that can store electrical charge.
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What are the components of a capacitor?
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Two electrical conducting plates separated by a dielectric.
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What is a dielectric?
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An electrical insulator.
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What happens when a capacitor is connected to a d.c. power source?
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• Charge builds up on the plates
• One plate becomes negatively charged the other positively • A potential difference builds up between the plates of the capacitor. |
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Why is a potential difference between the plates of the capacitor created?
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Because the plates are separated by an electrical insulator, so no charge can move between them.
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What is the voltage rating of a capacitor?
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It is the maximum potential difference that can be safely put across it.
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How much charge will a capacitor store?
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Up to the voltage of the power source it is connected to.
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What is the potential difference between two points?
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The work done in moving a unit charge between them.
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What is the value of a microfarad?
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x10-6
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What is the value of a nanofarad?
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x10-9
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What is the value of a picofarad?
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x10-12
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What does a current-time graph look like when charging a capacitor?
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• A straight line going horizontally across denoting a fixed current
• Current drops to zero as soon as the capacitor is fully charged. |
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What does the area under a current-time graph represent?
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The charge stored by the capacitor.
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What does a charge-potential difference graph look like when charging a capacitor?
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• Straight line through the origin
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How are Q and V related for a charging capacitor?
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They are directly proportional
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What does the gradient of a charge-potential difference graph represent?
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C: the capacitance of the capacitor.
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How long does it take a capacitor to discharge?
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A fraction of a second
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Why is having a short discharge time useful?
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Lots of energy is released all at once.
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What are three things capacitors can be used for?
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• Camera flashes
• Ultra-capacitors • For smoothing out variations in d.c. voltage supplies |
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What are ultra-capacitors used for?
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Back-up power supplies providing reliable energy over a short amount of time.
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How can a capacitor smooth variations in d.c power supplies?
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By absorbing the peaks and filling in the troughs.
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What effect do like charges have on each other?
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They repel
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What kind of energy does a capacitor store?
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Electric potential energy
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Where does a capacitors energy come from?
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The power source
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What happens when a capacitors charge is released?
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The electric potential energy is also released.
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What graph can be used to find the energy stored by a capacitor?
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A potential difference-charge graph.
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What is the formula for energy stored?
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E=½QV
Where: • E is energy stored • Q is charge on capacitor • V is the potential difference across the capacitor |
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How can a potential difference-charge graph be used to derive the equation for how much energy is stored?
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• Break the area under the graph into small segments whose area represents the energy change in that strip.
• E=QV gives the energy stored in each strip • Total energy stored by the capacitor is just the sum of all the energies stored in each increase, so it's the area under voltage-charge graph. • The area under the graph is a triangle. • The area of a triangle can be found using ½*base*height. • The base of the triangle is charge and the height is voltage. |
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Why is the energy stored by the capacitor only half of what is supplied by the power source?
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Energy is lost to resistance in the circuit and internal resistance of the battery.
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What equation do you get if you substitute C=QV into E=½QV?
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E=½CV²
Where: • E is energy stored • C is the capacitance of the capacitor • V is the potential difference across the capacitor |
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What do you need to substitute into E=½QV to get E=½Q²/C?
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V=Q/C
Where: • V is potential difference • Q is charge • C is capacitance |
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What happens to the current in a capacitor when it is connected to a d.c power supply?
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The current will flow in the circuit until the capacitor is fully charged, then stop.
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Which terminal do electrons flow from?
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The negative terminal
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How does a negative charge build up on one of the plates?
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Electrons flow from the negative terminal to the plate connected to it, causing a negative charge to build up on that plate.
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How does one plate become positive?
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• Electrons are repelled by the negative charge from the negative plate.
• They flow from the now positive plate to the positive terminal of the supply. |
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What quantity of electrons are repelled from the positive plate?
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The same number of electrons that have built up on the negative plate.
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How are the two charges on each plate related?
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There is an equal but opposite charge on each plate.
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What does an equal yet opposite charge cause between the plates?
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A potential difference
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Why can no charge flow between the plates?
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They are separated by a dielectric.
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What is the current like initially?
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High
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What does electrostatic repulsion do?
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Makes it harder for more electrons to be deposited.
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When does the current fall to zero?
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When the potential difference across the capacitor equals the potential difference across the supply.
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When is the capacitor fully charged?
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When the current drops to zero
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What has an effect on the time taken to charge a capacitor?
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Charging through a resistor
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What is the initial potential difference across a capacitor charging through a resistor equal to?
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Zero
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What causes an initially high current to flow through the circuit?
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The potential difference of the battery
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What is the initial current in a capacitor charging through a resistor equal to?
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V/R
Where: • V is voltage of the power supply • R is the resistance of the resistor |
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What happens to the potential difference across the resistor as the capacitor charges?
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It gets smaller
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Why does the potential difference across the resistor decrease?
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The potential difference across the capacitor is increasing
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What happen to the current in the circuit as the potential difference across the resistor decreases?
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The current decreeses
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What is charge proportional to?
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Potential difference
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Which two charging graphs look the same for a capacitor being charged through a resistor?
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Charge-time and Voltage-time
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What does the Current-time graph look like for a capacitor charging through a resistor?
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• Starts high
• Decreases in a curve as time increases |
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What does the Charge-time graph look like for a capacitor charging through a resistor?
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• Starts at the origin
• Increases in a curve as time increases |
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What does the Voltage-time graph look like for a capacitor charging through a resistor?
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• Starts at the origin
• Increases in a curve as time increases |
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Which two factors does the time taken to charge a capacitor through resistor depend upon?
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• The capacitance the capacitor (C)
• The resistance of the circuit (R) |
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How does the capacitance of the capacitor affect the time taken to charge a capacitor through a resistor?
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Capacitance affects the amount of charge can be transferred at a given voltage.
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How does the resistance of the capacitor affect the time taken to charge a capacitor through a resistor?
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Resistance affects the current in the circuit.
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What happens when a charged capacitor is connected across a resistor?
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The potential difference drives a current through the circuit.
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Which direction does the discharging current flow in?
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The opposite direction from the charging current
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When is the capacitor fully discharged?
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• When the potential difference across the plates is zero
• When the current in the circuit is zero |
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What equipment could you use to record the voltage across a discharging capacitor?
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A voltage sensor attached to a datalogger
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What does the Current-time graph look like for a capacitor discharging through a resistor?
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• The same as the graph for charging
• Starts high • Gradually decreases to zero |
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What happens to the potential difference across the resistor as the charge on the capacitor decreases?
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The potential difference decreases
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What does the Charge-time graph look like for a capacitor discharging through a resistor?
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• Reverse of charging
• Starts high • Gradually decreases to zero |
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What does the Voltage-time graph look like for a capacitor discharging through a resistor?
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• Reverse of charging
• Starts high • Gradually decreases to zero |
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How does the charge left on the plates of a capacitor reduce with time?
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Exponetially
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What is the equation for how much charge is left on the plates of a discharging capacitor?
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Q=Q0e^(-1/RC)
Where: • Q is the charge of the capacitor at time t • Q0 is the charge of the capacitor when fully charged • R is the resistance of the fixed resistor • t is the time since discharging began • C is the capacitance of the capacitor |
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What is the time constant?
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t=RC
• The time taken for the charge on a discharging capacitor to fall to 37% of Q0. or for a charging capacitor to rise to 63% of Q0. |
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How long does a capacitor take to charge or discharge in practice?
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About 5RC
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How does a larger resistor affect the time it takes to charge or discharge a capacitor?
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Makes it take longer
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