# physics capacitors

 capacitor device that stores electrical charge and therefor electrical energy capacitor consists of two conductors placed near each other but not touching parallel plate capacitors are separated by an electrically charged insulating material called a dielectric one capacitor plate is positively charged the other is negatively charged electric potential of positive plate exceeds that of the negative plate by an amount V when the magnitude of the charge on each plate is doubled the electric potential is doubled Q is proportional to VQ=CV where c is capacitance and is a constant units of capacitance CV-1 F 1uF = 1*10-6F 1nF 1*10-9F 1pF 1*10-12F capacitance the charge stored per unit potential difference across a capacitor initially the flow of charge , current , to an uncharged capacitor is high as time passes the rate reduces as the charge from the cell must overcome the build of of charge on the plate on charging electrons flow from negative terminal of cell to one end of plate (which becomes negative) and then from the other end of the plate (which becomes positive) to positive terminal of cell it isn't possible to store more and more charge by applying an ever increasing pd across the capacitor eventually the pd becomes too large for the insulating material between the plates . electrical breakdown then occurs and conduction takes place across the gap various dielectrics are used to increase the capacitance , the material used and its thickness determine the max working pd of the capacitor for a Q VS pd graph the gradient is C .... must be done to charge up a capacitor therefor workelectrical potential energy is stored as a result the amount of energy stored depends on charge stored by the capacitor and the pd across it when a capacitor is being charged charge flows on to the capacitor the battery has to transfer energy to do work against the repulsive force from the charge already on the capacitor work done in moving charge through a pd is W=QV ... across a capacitor isn't constant pd average pd is 0.5pd (pd increases from 0 to max pd when capacitor is fully charged) work done in charging a capacitor is therefor 0.5QV energy stored by the capacitor is therefor 0.5QV Q=CV so energy stored also = 0.5CV2 or 0.5Q2/V draw a Q vs V graph draw a V vs Q graph what's the gradient what does the area under the graph represent 1/C work done in charging a capacitor , energy stored by a capacitor 0.5QV capacitor discharge circuit charging through a fixed resistor  when an uncharged capacitor is placed in a circuit to be charged the initial current I0 depends on the emf of the battery and the resistance of the circuit I0 = V0/R as charge accumulates on the capacitor the pd across it increases . the pd opposes that of the battery and it becomes increasingly difficult to push charge on to the capacitor and the charging current drops eventually the pd across the capacitor is equal but opposite to that of the battery and the charging current drops to zero as the capacitor charges the pd across the resistor gets smaller because the pd across the capacitor gets bigger and so the current drops Q ∞ V sp Q vs t graph is the same as V vs T graph  draw these graphs for charging through a fixed resistor the rate at which the current drops depends on resistance and on the capacitance of the circuit the larger the resistance the lower the current the capacitor therefor takes longer to charge a large value of capacitance also increases the charging time since it takes more charge to fill the capacitor the product RC is known as the time constant for the circuit draw an I vs T graph for charging through a fixed resistor time constant is the time it takes for current to drop to 1/e of its original value draw a V vs T graph for charging across a fixed resistor showing curves for pd across capacitor and pd across resistor where : A = across capacitor and B = across resistor Vc + Vr = EMF exponential decay curves have the property that they always take the same time to decrease by a given fraction to discharge a capacitor take out the battery and reconnect the circuit when a charged capacitor is connected across a resistor the pd drives a current through the circuit the current flows in the opposite direction from the charging current the capacitor is fully discharged when the pd across the plates and the current in the circuit are both zero draw the three  vs T graph for discharging through a fixed resistor the I vs T graph is the same as the one for charging the current starts off relatively high and gradually decreases to zero . this is because  initially when a charged capacitor is connected across a resistor the pd drives a current through the circuit but the pd across the resistor decreases as the charge on the capacitor decreases Q = Q0e-t/RC  I = I0e-t/RC V = V0e-t/RC time constant is the time taken for charge/voltage/current on a discharging capacitor to fall to 0.37Q0/V0/I0its's also the time taken for voltage/charge on a charging capacitor to rise to 0.63V0/Q0it's also the time taken for current on a charging capacitor to fall to 0.37I0 Q = It capacitors discharge energy quickly so they can be used in applications where a lot of energy is required rapidly , a taser is an example . Filters also use capacitors to smooth electrical signals if we have two capacitors in parallel the voltage of each capacitor has to be the same because the capacitors are in parallel if we have two capacitors in parallel the final charge in each capcitor is capacitor A------------         x total charge total capacitance and the same goes for B total charge is Q=CV for capacitor that is being charged voltage can be worked out in the same way but you will find the voltage is the same for both capacitors as discussed in previous question if we have two capacitors in parallel the final energy stored in each capacitors is E = 0.5CV2 where v is the final pd across the capacitor why is there a loss of energy stored because battery has to transfer energy to do work against the repulsive force from the charge already on the capacitor so energy is wasted and hence less is stored battery has more energy because pd in battery is constant so E=QV Authorghoran ID292491 Card Setphysics capacitors Descriptionrevision Updated2015-01-07T17:33:26Z Show Answers