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VOLTAGE, ELECTRIC
ENERGY AND
CAPACITANCE
Engr. Mehran
Mamonai
Applied Physics
1 st Term
1 st Year
SUPPOSE
THIS TECHNOLOGY
DEFIBRILLATOR
WHAT IS CAPACITOR
FLOW OF ENERGY
WHY FLOW OF ENERGY IN PARTICULAR
DIRECTION
 If we follow Potential Energy in Gravity
 The object which is raised from the surface will follow the
downward path
 In order to get equilibrium state
WHAT IS WORK PERFORMED
HOW MUCH AN ELECTRIC ENERGY
SYSTEM HAVE?
 Everything sense energy
 Question arises, How much an Electric Energy a system have
 And
 How much an Work it can perform
DID YOU KNOW ??
DEFIBRILLATOR PRINCIPLE
 The device which we discuss before Defibrillator,
 Provides enough charge to heart to make it stop
 But energy is not so strong that it damages it permanently
 So do we know, how much energy is required to charge up
POINT CHARGE IN CAPACITOR
WORK DONE CONCEPT
MOVING POINT CHARGE ACROSS THE
CAPACITOR
 Now we can apply equal and opposite force to move the point
charge across the capacitor slowly as we like
 So the Kinetic Energy of particle remains negligible
 From Law of Conversation of Energy and Work Energy
Theorem
 Change in PE is equal to negative work done (Lets understand
Work Energy Theorem first)
WORK-ENERGY THEOREM
 The principle of work and kinetic energy (also known as the
work-energy theorem) states that the work done by the sum of
all forces acting on a particle equals the change in the kinetic
energy of the particle.
 Work can change the potential energy of a mechanical device,
the heat energy in a thermal system, or the electrical energy
in an electrical device.
The work-energy theorem can be derived from Newton's
second law.
 KE =
WORK DONE BY EXTERNAL FORCE
ELECTRIC FIELD ON POINT CHARGE
So We conclude that
Potential Energy Decreases
on a point charge in Uniform
Electric Field
POTENTIAL ENERGY VS MAGNITUDE OF
POINT CHARGE
REARRANGING THE EQUATION
ELECTRIC POTENTIAL
VS
ELECTRIC ENERGY
• Electric Potential depends on
Electric Field difference and the
distance at which Point charge is
Placed
• It doesn’t depends on
magnitude of point charge
UNIT OF ELECTRIC POTENTIAL
EFFECT OF POINT CHARGE
 Electric Potential decreases as the point move away in
distances
MATHEMATICAL EXPRESSION
OF VOLTAGE
HYPNOTICAL POINT CHARGE
VS REAL LIFE
 We discussed all the phenomenon is happen on a point test
charge
 We calculated the Electric Potential on this charge across a
charged Capacitor.
 Let us formulate, how can we measure Electric potential in
real world components
SUPPOSE A CAPACITOR
ELECTRIC FIELD CAN BE FOUND BY
SO WE CAN EXPRESS EQUATION AS
ELECTRICAL POTENTIAL AND DISTANCE
 Since a Capacitor creates an uniform electric field
 We can assume
 Electric Field is Constant
 Acts in the direction, where the test charge moves
 So As charge moves, Electric Potential decreases more and
more with distance
EFFECT DUE TO EQUIPOTENTIAL LINES
IN CAPACITOR
 The Equipotential line are parallel in Capacitor
 These Lines are always perpendicular to Electric Field Lines
ELECTRIC POTENTIAL IN CAPACITOR
VS
ELECTRIC POTENTIAL IN POINT CHARGES
 Now we have calculated the voltage in Capacitor
 Can we apply the same principal on point charges?
 As there is no uniform Electric Field available on point charge,
so we can’t use these equations on point charge
 Because we can’t measure the ef fect on the point charge
infinitely far away
CONCLUSION
 We find that point charge with same charge (positive) is place
near to charge Q
 it will experience high Electric potential energy
WE CAN CALCULATE AS
EQUIPOTENTIAL LINES BY POINT CHARGE
EQUIPOTENTIAL LINE IN ELECTRICAL
DIPOLE
POINT CHARGE ON DIFFERENT
POSITIONS
LETS DISCUSS WHAT WE HAVE STUDIED
 Now let’s go back to potential energy in capacitors
 Why Capacitor?
 These are very useful – from defibrillators to electronic component
 When a capacitor’s plates store electric charge, they’re
actually storing energy
LETS APPLY CURRENT IN ONE DIRECTION
WILL CAPACITOR GET ANY CHARGE?
 Capacitor hasn’t gained any net charge
 There’s just as much positive charge on the positive plate as
there is negative charge on the negative one
 So How does Capacitor gets charged up??
 Answer: Battery uses its own electric potential to generate a
current that transfers voltage in the capacitor, giving the
capacitor a certain amount of potential energy.
ENERGY OUT OF THE CAPACITOR
 Now if you’re trying to save a life, you want to make sure
you’re getting just the right amount.
 how much charge a capacitor can store?
 We can use a battery in our circuit to create a voltage
between the plates, and then divide the charge in each plate
by that voltage. This value is known as capacitance
UNIT OF CAPACITANCE
 Capacitance uses units of Farads,
 One Farad equal to one Coulomb per one Volt
SMALL VALUES OF CAPACITANCE
 Typically, capacitance values are very small (Why ??)
 so we often talk about capacitors in terms of microFarads or
nanoFarads.
 Capacitance is actually determined by the size and shape of a
capacitor
ANOTHER WAY TO REPRESENT
CAPACITANCE
HOW CAN WE CHANGE CAPACITANCE
Make the plates larger
or
Move them closer together
 This way there’s room to fit more charge, creating a stronger
electric field
 once you’ve established the geometry of the plates, the
capacitance does not change
 unless you stick something between them, which actually
increases the capacitance
ADDING SOMETHING TO CAPACITOR
 This something is called a dielectric
WHAT IS DIALECTIC?
 Typically an insulating material like plastic or glass
 A dielectric is used to increase capacitance while preventing
any charge from jumping from plate to plate.
 But why would charges jump?
 Sometimes, as the plates get hotter or the voltage gets
higher, some electrons naturally jump between the plates,
decreasing the amount of stored charge
PURPOSE OF DIELECTRIC
 So an insulator prevents electrons from crossing the gap
 you usually want the plates to be as close together as
possible without touching
 Because a smaller distance equals a larger capacitance,
 By using a very thin dielectric, the distance is decreased while
the plates remain separate.
 Dielectrics are polar, which means one side of the molecule is
slightly positive while the other side is slightly negative
EFFECT OF DIELECTRIC
SO EQUATION OF CAPACITANCE WILL
REFORM AS
POTENTIAL ENERGY STORED IN THIS
FIELD
 By integrating the voltage over the charge in the plates, which
reduces down to one half charge time voltage
ENERGY DENSIT Y IN CAPACITOR
 It is useful to know how much energy is stored in an electric
field per unit volume
 To know how much energy is in a certain location, like
between capacitor plates
 The amount of energy stored in the electric field per unit
volume is called Energy Density
 We can calculate the energy density associated with an
electric field (at any point space not only in Capacitor)
POTENTIAL ENERGY BY THE VOLUME
 With some algebra
CONCLUSION
 Now that we know all this, a medic can make sure those
defibrillator paddles have the right capacitance, have been
given enough charge to create a high voltage
 we learned a lot things (dif ferently)
 We talked about electric potential energy and how it dif fers
from electric potential, or voltage.
 We discussed how capacitors function and the factors that
determine how much charge they hold.
 We also learned how to maximize energy storage and how to
calculate the potential energy held by any capacitor.