capacitance part 2 Flashcards
Define capacitance
the charge stored per unit p.d.
define 1 farad
one coulomb per unit volt
Describe the charging of capacitance in terms of electron flow (diagram)
- Draw a diagram
Electrons will flow from the negative terminal of the cell to the negative terminal of the capacitor. The excess electrons will move off the positive terminal of the capacitor and travel to the positive terminal of the cell. Overall, the plates will acquire an equal and opposite charges
Describe the discharging of capacitance in terms of electron flow
- Draw a diagram
We can use a charged capacitor and connect it to a light bulb. When a capacitor is connected to a circuit, the excess electrons are going to flow the negative plate to the positive plate. The charge on the capacitor will fall and the light bulb will turn off.
What are the characteristics of p.d., charge and capacitance when a capacitor is in series?
- charge stored by each capacitor is the same
- Kirchhoff’s 2nd law, total p.d. = sum of the individual p.d
- Total capacitance = 1/c + 1/c +1/c
What are the characteristics of p.d., charge and capacitance when a capacitor is in parallel?
- p.d. across each capacitor is the same
- electrical charge is conserved
- Total charge is equal to the sum of individual charges stored by the capacitors
- Total C is the sum of the individual ones
Describe how energy is stored by capacitors?
When the switch is off, the capacitor plates are neutral. When it is switched on, the current will flow around the circuit. The electrons will be pushed in one directions due to the voltage. This causes the plate with the most electron to be negative, whereas the other plate will be positive because it doesn’t have as many electrons
describe a practical of circuits containing capacitors?
- draw a diagram
When switch, S, is closed, the ammeter registers a current but very quickly settle down to a zero reading. This shows that electrons move in the circuit until the capacitors are fully charged. The p.d. V1 and V2 across each capacitor can be measured with the voltmeters.
what are multimeters used for?
Used to determine the individual capacitance by placing it across the combination of capacitors
This can used to calculate the capacitance in a much simple way as it doesn’t require an external battery, because multimeter uses it’s own battery to show the readings
What is the use of capacitors?
- back up memory devices
- emergency lighting
- alternative to battery pack
What is the time constant of a capacitor-resistant circuit?
the time taken for the p.d. to go to e^-1 (37%) of its initial value
What is the constant-ratio property for discharging called?
the exponential decay
Charging a capacitor practical?
-draw a diagram
Flick the two-way switch so that it forms part of a series circuit with the DC supply, the resistor, the microammeter, and the capacitor.
Record readings for the charge on the capacitor over time using the microammeter in conjunction with the equation Q = It. Take readings based on how quickly the capacitor charges.
Stop taking readings when the current gets to 0, as the plates will now be fully charged.
Plot a graph of Q against t for the capacitor charging, after you’re done flick the switch to the discharge circuit and take readings over time as the capacitor discharges.
Discharging a capacitor practical?
- draw a diagram
A high-resistance digital voltmeter may be used. Use the switch to charge then discharge the capacitor through the resistor. Use the voltmeter to measure the capacitor p.d. at regular intervals from the start of the discharge. Record your reading a table
Plot a graph of In V against time
Use the graph to determine the time constant RC. Hence, calculate the resistance of the resistor, given the capacitance of the capacitor.
describe in terms of electron how p.d. across the capacitor changes
Electrons move in an anticlockwise direction.
The charge on the plates decreases
Potential difference decreases exponentially
