Capacitance Flashcards
What is a capacitor?How is it made?
A capacitor is a device that stores charge.It consists of two parallel conducting plates separated by an insulating material.
How do electrons flow from a battery to the capacitor plates assuming actual current flow?
Electrons flow from the negative terminal of the battery to one of the plates giving this plate a negative charge.The other plate can be charged by induction and becomes positively charged as electrons are sucked out of it by the positive terminal of the battery.Each plate gains an equal and opposite charge as an equal number of electrons are entering and leaving the capacitor.
What does it mean when a capacitor stores a charge Q?
The positive plate stores a charge of +Q, negative plate stores a charge of -Q.
Describe a circuit arrangement to find the capacitance of a capacitor?
Connect a battery, microammeter, Switch, Capacitor, Voltmeter and variable resistor in series.Keep on varying the resistance to make sure the current in the circuit remains constant.Record the voltage across the capacitor at given time intervals.Then calculate the charge at these given intervals using q=it.Then plot a graph of Q against V.The gradient of the graph will give the capacitance.
What is the definition and formula for capacitance?
Capacitance is defined as the charge stored per unit volt.The formula is C=Q/V.The unit is farads.MicroQ/V gives microfarad.
How is energy stored in a capacitor?
Energy is stored as electrical potential energy in a capacitor as electrons are stored in one plate and taken off the other.
What happens as you discharge a charged capacitor across a bulb?
As long as the capacitor has been charged to the operating pd of the bulb it will release its energy as a brief flash of light from bulb.The charge flow is rapid enough to give a large current to light the bulb but only for a short period of time.
What is the relationship between the pd across the plates as the charge stored during the charging process.
The pd across the plates increases in proportion with the charge stored by the plates.
What is the formula for the energy stored by a capacitor?
E=1/2QV.
How much useful energy is transferred to the capacitor by the battery?
The battery moves charge q through a pd of V.Roughly 50 percent as the energy supplied by the battery =QV so the rest is wasted to the surroundings due to resistance in the circuit.
How can you measure the energy transferred by a capacitor when it is discharged?
A joulemeter can be used to measure the initial and final energy of the light bulb.Also measure the initial pd across the capacitor.The difference is equal to the energy transferred by the capacitor when discharged.
Explain differences in electric potential energy between a thundercloud and the ground?
We can model the thundercloud as a negatively charged plate and the ground as a positively charged plate.The electric potential between them equals V=ed.Where e is the electric field strength and d is the distance between them.Intially the energy stored is 1/2Qed.However,if the cloud rises due to the wind.The change in Ep = 1/2Qed-1/2Qed1.Work done is done by the wind as it has the overcome the force of attraction between the cloud and ground.Q is a charge being carried by the thundercloud.
What happens to the discharge current when a capacitor discharges through a fixed resistor?
The current will gradually decrease till 0
Why does the current decrease gradually?
It decreases gradually because the pd across the capacitor reduces as the charge stored falls.
What happens to current,pd and charge during discharge?
All of these quantities decrease exponentially with time as they are all proportional.
