Capacitors DONE Flashcards
Describe what a capacitor is in terms of what it does and its structure.
An electrical component that can store electrical charge
Made up of 2 conducting plates separated by an air gap or an insulating material
Define capacitance and its unit.
The amount of charge per unit voltage stored by a capacitor
Measured in Farads
Explain the problem if the p.d. is too high.
It is dangerous
Because a spark could pass across the plates if the high p.d. causes the air/ insulating material to break down into charge-carrying ions
Explain in terms of charge what happens when a capacitor is connected to a power supply and becomes charged and what 2 things are created across/ between the plates.
Negative charge is removed from one plate and deposited on the other plate
A potential difference is created across the plates- due to the difference
A uniform electric field is created between the plates
Explain why a potential difference is created across the plates.
There is a difference in charge between the 2 plates
The charge can’t move between the 2 plates because of the insulating material
Describe the energy stored by the capacitor in terms of energy transferred/ work done. State the type of energy stored.
Energy stored is the energy transferred/ work done when negative charge is removed from one plate and deposited onto the other plate
Electrical potential energy is stored
State the energy stored by the battery, the energy supplied by the battery and the energy stored by the capacitor.
Chemical energy stored by battery
Electrical energy is supplied by the battery
Electrical potential energy is stored by the capacitor
State what the area under a V-Q graph represents.
Energy transferred/ work done removing negative charge from one plate and depositing it onto the other plate- W= 0.5 V Q
Explain why the energy stored by the capacitor is half the energy supplied by the battery referring to what happens to the apparent “lost” energy and how it affects the surroundings. Refer to internal energy.
Half of the energy supplied is transferred to internal energy in the connecting wires
As work is done in pushing the electrons along from one plate to the other
Internal energy results in the transfer of thermal energy to the surroundings
State the relationship between the potential difference across the plates and the charge on the plates.
V is directly proportional to Q
Explain why you have to use the average p.d. instead of one value of p.d. in the equation for energy stored (W= 1/2 V Q)
Potential difference is not constant as you charge the capacitor
Th p.d. across the capacitor increases as you charge it
Thus you have to take the average p.d. instead
Explain how to keep current constant in a circuit where a capacitor is charging.
A variable resistor
Starts at max. resistance and is decreased to keep current constant
As capacitor charges, current decreases so resistance is decreased to increase current at same rate it is decreasing
Name the type of graph created when a capacitor discharges, describe what this means in terms of charge reduction and describe what happens to the rate of discharge as the charge left on the capacitor decreases.
Exponential decay
Charge reduces by same fraction each equal time interval
As the charge left on capacitor decreases, rate of discharge decreases
State the direction of the current of a discharging capacitor.
The opposite direction it was flowing when it was charging
Describe what is meant by the time constant,τ and how it affects the time a capacitor takes to discharge.
τ = RC for a capacitor discharging through a resistance, R
It is the time taken for charge/ p.d./ current of a discharging capacitor to fall to 37% of its value when fully charged
The larger the time constant, the longer the decay takes
