Fields (3): Capacitors Flashcards
What is the definition of a capacitor?
A device designed for storing charge (controls the variable flow of charge)
Describe the set up of a capacitor
- Two conducting parallel plates connected to a battery
- Insulating material between them (sometimes a dielectric)
The charges stored on the plates of the capacitor are…?
Equal but opposite (+Q and -Q)
What is the circuit symbol of a capacitor?
Two separated parallel lines of the same length
- Like a battery but lines are the same length
Which side is the positive side of a cell when drawn as its circuit symbol?
Longer side - +ve -> electrons flow into this side
Shorter side - -ve -> electrons flow out of this side
What is the direction of conventional current?
+ve -> -ve (the opposite way of the flow of electrons)
How can you measure the potential difference across a capacitor in a circuit?
Attach a high resistance voltmeter in parallel
How can you investigate the change in capacitor pd with respect to time for a constant current?
In series place a:
- capacitor
- cell/battery/powerpack
- switch
- Ammeter
- Variable resistor
In parallel to the capacitor place a HIGH RESISTANCE VOLTMETER or a DATALOGGER (for more precise readings) in order to record voltage readings at measured times
NB: The variable resistor is used in order to keep the current constant
How do you calculate the charge stored by a capacitor in a measured time when current is constant?
Q = It
If you plotted a graph of charge against voltage for a capacitor of constant current, what relationship would you observe?
Directly proportional
What is the definition of Capacitance, C?
The charge stored per unit of potential difference
What is the general equation for capacitance?
C = Q/V
What are the units for capacitance?
Farads (normally micro x10^-6, nano x10^-9 or pico x10^-12)
What are 3 uses of capacitors?
Back-up power supplies - When the mains supply is interrupted
Smoothing circuits - Circuits that smooth out unwan ted variations in voltage
Timing circuits - Circuits that switch on or off automatically after a preset delay
Describe how a capacitor is charged
When connected to a DC power source there is a current as the supply draws electrons from one plate to the other. This leaves one plate with a +Q charge and the other with a -Q charge so that charge stored by the capacitor is Q. Current flows until the potential difference across the capacitor is the same as the emf across the power source.
What is formed between the two plates of a capacitor when it is charged?
A uniform electric field
How is energy stored in a capacitor?
As electric potential energy
Describe a general set up for charging and then discharging a capacitor?
In parallel place a:
- Voltage supply
- Capacitor
- torch bulb/joulemeter/high resistance resistor (whatever you are measuring or showing)
Place a switch in the lop of the capacitor that can switch between the voltage supply loop to charge and the other loop to discharge
Is work done when electrons are forced around a circuit and onto the capacitor plates?
YES
What does the graph of potential difference against charge look like for a capacitor? Why?
Directly proportional relationship
work done/energy transferred = VQ
- Q and V are proportional
What is the area under a Q against V graph of a capacitor representative of?
Total energy stored by the capacitor
What are the other equations for the energy stored by a capacitor, derived from E = 1/2QV?
C = Q/V
Q = VC
therefore, E = 1/2V^2C
V = Q/C
therefore, E = 1/2Q^2/C