Capacitance - MCFLY! Flashcards
what is a capacitor
an electric component in which charge is separated. consists of 2 metalic plates seperated from eachother by an insulator (dielectric).
What does dielectric mean?
The material will polarise when in contact with an electric field.
Define capacitance:
The charge stored per unit pd across it
What is the relationship between charge and pd?
Charge on the capacitor is directly proportional to the pd.
How to work out total capacitance in parallel?
Add them up. Together capacitance is greater than individual charges stored by the capacitors.
Where does the energy in a capacitor come from in a capacitor and how Is it stored?
Energy stored comes from energy of battery or power supply. Work must be done in order to deposit negatively charged electrons onto the negative plate (like charges repel coulombs law) . Same way work is done to remove electrons from the positive plate since negative attracted to positive.
How to calculate the work done on a capacitor?
Area under a pd - charge graph
What is the time constant for a discharging capacitor?
The time taken for the pd to decrease to e^-1 (37%) of its initial value.
How do capacitors work?
- When the capacitor is connected to the cell , electrons flow from the cell for a short time.
- The brief current means electrons are removed from plate A and at the same time electrons are deposited onto plate B.
- Plate A gets positive net charge and plate B gets negative net charge so there is no net overall charge
- Current in the circuit falls to 0 when the pd across the plates is equal to the emf of the cell, capacitor is then fully charged
The build of the electrons on one side means there is stored energy which can be used to do work
Total capacitance In series?
Use kirchoffs second law. 1/c = 1/c1 + 1/c2 . Together their capacitance is less than their individual capacitances so the combination will store less charge for a given pd. All capacitors in series store the same charge even when they have different capacitances.
Capacitor circuit practical:
- Ammeter, resistor, 2 capacitors and a resistor are all connected in series to a power supply
- When switch is closed ammeter regsiters a current but quickly goes to 0 again which shows that electrons move only until capacitors are fully charged.
- Pd across each capacitor can be measured using a voltmeter
- Using c = q/v work out the charge stored by the capacitors which should be the same
Any differences are because of uncertainties in the voltmeter readings and manufacturer values
What is the equation for the energy stored in a capacitor?
W = 1/2QV
W = 1/2V^2C
W = 1/2 Q^2/C
What graphs show how the capacitor discharges over time?
V-t, i-t, and q-t all show exponential decay graph
What is the time constant of a capacitor resistor circuit?
Product of the capacitance and resistance.
Time constant definition
Time constant t for a discharging capacitor is equal to the time taken for the pd charge and currentto decrease to about 37% of its initial value
