charging and discharging

7/29/2019 Charging and Discharging

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PRESENTATIONON

DIVYA PORWAL JAIN (13)GAJENDRA SINGH SOLANKI(14)GARIMA GARG (15)

GOVIND (16)ON : 15thMAY-2012

FOR : Mrs. MANISHA


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To study the charge and discharge

of a condenser and hence determinetime constant (both current and

voltage graphs are to be plotted ).


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Resistances of variable values, condensers

of different capacities , milliammeter (inwhich zero is in the middle so as to

read current in both the directions),

voltmeter, keys, battery, stop watch etc.


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When resistance R and condenser C are connected in series with a

battery of constant e.m.f. through a key K , then on closing key K thebattery gets connected to the circuit . Charging of the condenser takes

place. But if K is released and K is pressed, then the battery gets

disconnected and discharging of the condenser starts through the resistanceR.

Both these processes depend

upon time.

1

1 2

1


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When key K is closed then resistance R, condenser Cand battery Eget connected in series and a current flows for a short interval of time

which charges the condenser. Let at any instant of time t, a current I

flowing through resistance R accumulates a charge Q on the condenser.Due to this a potential difference Q/C appears across the condenser.

Hence at time t

:.Total Potential differenceE= V + V= RI + Q/C

R C


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When the capacitor is connected across abattery, the pd across it increases from zeroto a pd equal to the emf of the battery.When a small amount of chargeQ, istransferred from one plate of the capacitorand removed from the other, the battery doeswork, equal to the area of the shaded stripin the diagram.

E=VoQ = the area of the shaded strip in the diagram.

The total energy transferred to a capacitor, E= total area under graph.E= QV

As Q = CV, we also have

E = CV

2

and E = Q

2

/ C


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If keyK is released and keyK is pressed then battery is disconnectedfrom the circuit and the charged condenser tarts discharging through

resistanceR.

1

2

InitiallyV =Eand chargeon the condenser

Q = Q =Ec.After full dischargeE=0.Thus during discharge at

time t=0,

Q= Q = EC

C

0

0


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This circuit opposite can be used toshow the discharge of a capacitor.

It is charged by connecting theswitch to A. It discharges throughthe resistor when the switch is atB.

Io is approx80A.Discharge time is approx 4 mins.

Plot graphs ofI, VandQAgainst

t.


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Variation of current with

discharging of condenser.

Variation of charge with


Variation of voltage with



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During charging of condenser, current decreases

exponentially and during an interval equal to timeconstant the value of current falls to 1/e of its

maximum valueI , i.e. it falls to 37% of its

maximum value .This is known asDECAY OF CURRENT.

0


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The value of charge and voltage on the condenser at any time t is given by

Q= Q (1-e )

V =V (1-e )

-t/RC

-t/RC

C C

0The current flowing through the condenser is :

I=I e0

-t/RC

CHARGING OF THE CONDENSER

DISCHARGING OF THE CONDENSER

The value of charge and voltage on the condenser at any time t is given by

Q= Q e

V =V e-t/RC

C0

-t/RC

The current flowing through the condenser

is :-I=I

0

e-t/RC


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Resistance R, condenser C , two key K & K, milliammeter and battery E areconnected . The value of resistance R and condenser C are selected such that the value o ftime constant RC is large. By connecting various resistances in series proper valueof resistance can be selected. Similarly connecting capacitors in parallel we canincrease the value of capacitance. It is convinient to choose proper value of timeconstant RC=100 sec. so that proper nu mber of observations can be taken. Keep the key K open , now close the key K and simultaneously start the stopwatch. The reading of the voltmeter and amm eter are noted every 10 seconds till thereading becomes constant. Now the condenser is allowed to discharge by releasing key K and pressing keyK . Simultaneously start the stop watch and note the readings of ammeter andvoltmeter after every 10 seconds.

charging and discharging

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