Capacitors Flashcards
What is the Capacitor Symbol?
What happens when the switch in this is closed, ignore the resistor?
1 - Electrons flow from negative terminal of the cell to the right hand plate
2 - Plates become negatively charged
3 - At the same time electrons flow from the left hand plate towards the positive terminal of the cell
4 - This flow of charge is an electric current
5 - There is an exponential change in charge build up
6 - Discharges when the P.D of the capacitor = the P.D of supply
What is the Time Constant of a Capacitor?
- 37% - time taken for the charge to drop to this value
- Time Constant = CR
Why do we use LN with Capacitors?
We use ln to turn the exponential in to a linear graph
What is a Capacitor?
Is an electrical Component that can store electrical charge.
What are Capacitors made up of?
Made of…
- Two conducting plates separated by an air gap or an insulating material
What happens when a Capacitor is connected to a Power Source?
- Positive and Negative Charge build up on opposite plates
- Insulating material stops charge moving between the two plates.
- So a P.D is created
- This creates a uniform electric field
What kind of Electric Field does a Capacitor create?
Uniform Electric Field between the plates
Define Capacitance.
Is the amount of charge per unit voltage stored by a Capacitor
What is the Capacitance equation?
C = Q / V
C - Capacitance in Farads (1 F)
Q - Charge (C)
V - Potential Difference (V)
What is Capacitance measured in?
Farads (F)
What units do we usually have to express Capacitance in?
μF - Microfarads ( x 10*-6)
nF - nanofarads ( x 10*-9)
pF - picofarads ( x 10*-12)
Capacitors store…
Energy
What happens in this circuit when the switch is flicked to the LEFT?
- Charge builds up on the plates of the Capacitor
- Electrical Energy, provided by the battery, is stored by the Capacitor.
What happens in this circuit when the switch is flicked to the RIGHT?
- The charge stored on the plates will discharge through the bulb, converting electrical energy into light and heat.
How do you calculate the electrical energy from the battery needed to charge the Capacitor?
Charge X Average P.D
What is done when charging the capacitor?
WORK is done removing negative charge from one plate and depositing it into the other plates
What is the energy stored by a Capacitor equal to?
The energy stored by a Capacitor is equal to the work done by the battery
How do you find out the energy stored in a Capacitor graphically?
It’s the area underneath the graph of P.D against charge stored.
Why is the P.D over Charge graph linear?
P.D across the Capacitor is PROPORTIONAL to the charge stored on it, so the graph is a straight line through the origin.
Where does the Equation W = 0.5 x Q x V come from?
W = 0.5 x Q x V comes from this graph
Investigate what happens when you CHARGE a Capacitor.
- Set up the circuit
- Close the switch to connect the uncharged Capacitor to the power supply
- Let the Capacitor charge whilst the data logger Records both the potential difference (from the voltmeter) and the current (from the ammeter) over time.
- You can use a computer to plot a graph of Charge, P.D or Current against Time. Like these…
What does Q0 & V0 & I0 stand for?
Q0 - Charge on the Capacitor when it is fully charged
V0 - Potential Difference across the capacitor when it is fully charged
I0 - Maximum Current flowing through the circuit
Explain the shape of this Capacitor Graph.
CHARGING a Capacitor Experiment. (LOOK AT THE AXIS)
- When the switches close, Current starts to flow. Electrons flow onto the plate connected to the negative terminal of the power supply, there is a build up of negative charge.
- The negative charge on the plate repels electrons off the plate connected to the positive terminal of the power supply, making that plate positive. These electrons are attracted to the positive plate.
Explain the shape of this Capacitor Graph.
CHARGING a Capacitor Experiment. (LOOK AT THE AXIS)
An EQUAL but OPPOSITE charge builds up on each plate, causing Potential Difference between the plates. Remember that no charge can flow between the plates because they are separated by an insulator.
Explain the shape of this Capacitor Graph. CHARGING a Capacitor Experiment.
- At first the Current flowing through the circuit is high.
- But as charge builds up the electrostatic repulsion (of the electrons) makes it harder for electrons to be deposited.
- When the P.D across the capacitor is equal to the P.D across the power supply, the current falls to zero
- The capacitor is fully charged
Investigate what happens when you DISCHARGE a Capacitor.
- Let the Capacitor Discharge whilst the data logged Records Potential Difference and current over time.
- When the current through the ammeter and the Potential Difference across the plates are zero the Capacitor is fully discharged
- Then plot these graphs
Explain the shape of these graphs. DISCHARGING a Capacitor Experiment.
When a Capacitor is discharging, the amount of charge and P.D between the plates falls exponentially with time. This is also true for current.
What is the equation for the charge left on the plates at a given time after a Capacitor begins discharging from being fully charged?
Q0 - Charge of the Capacitor when it’s fully charged (C)
t - time since charging began (s)
R - Resistance (Ohms)
C - Capacitance (F)
Give the equations for Potential Difference and Current as a Capacitor discharges.
V0 - P.D of the Capacitor when it’s fully charged (V)
I0 - Current of the Capacitor when it’s fully charged (A)
t - time since charging began (s)
R - Resistance (Ohms)
C - Capacitance (F)
Because you have the equation Ln(Q) = Ln(Q0) - t / RC in terms of natural logs and y = mx + c how do you plot a graph to get the TIME CONSTANT?
- Plot ln(Q) over time and you get a straight line
- Gradient of this line is - 1 / RC
- Y-Intercept is ln(Q0)
- To get the time constant divide the gradient by -1
Can this equation, Ln(Q) = Ln(Q0) - t / RC be expressed in terms of V and I?
YES!
