1. 17.1 Electric potential Energy
Warm-up:
A load weighing 400 N is raised a height of 5m. Calculate the work done. What is the gravitational potential energy?

2. Electric potential Energy
Consider a charged particle being moved towards a positively charged object.
Separation distance between charged objects + + +
Push + + + + + + +
+ qo + + + +q

3. Question:
Why is it not easy to reduce the separation distance between two charged objects?
Answer:
The small charged particle will be repelled by the large object.
In order to move it, External work has to be done.

4. Electric potential Energy
When work is done on a charge, it acquires Electric potential Energy. (energy of position)
Think of charging your cell phone before using it.
Work Done = Electrical potential Energy

5. Electric kinetic Energy
When a positive charge close to a large positively charged object is released, all the electrical potential Energy becomes Kinetic Energy.
(Think of using your already charged cell phone).
Motion due to Repelling force + + +
+ qo +q + + + + +
Work done to move charge = Electric potential energy = Kinetic Energy
Charging cell phone → Charged cell phone → using cell phone
( law of conservation of energy)

6. Electric potential energy Vs Gravitational-PE
Pushing charges close to a large charged object is like raising water up the cliff. + + + + + + + + + + + + + + + + +

7. Electric potential
It is the electric potential energy per unit charge.
Also called potential difference or voltage (V)
The units of electric potential is Volts

8. Electric potential is constant
Pushing more charges requires more work, but more electrical potential energy is generated
Does the ratio of electric potential energy acquired, to the number of charge moved, change?
_____________
______________
Push
+ qo +q

9. Electric potential
Any charged body has an electric potential within its electric field.
Charge must not be present for there to be electric potential.
When a charge is introduced in the field, that charge acquires;
Electric __________ __________

10. Example
Suppose an electron in the picture tube of a TV set is accelerated from rest through a potential difference of 5000 V, What is the change in electric potential energy of the electron?
Solution:

11. Class work
Textbook pg 488 # 1-8

12. 17-2: Electric potential, electric field
For any uniform electric field such as that between parallel plates, the Electric potential is proportional to the product of electric field between the plates and the distance of separation of the plates.

14. Proof Given then Finally, By solving for E, the formula becomes:
E = Electric field, F = Force, V = Electric potential, d= Distance

15. Example
Two parallel charged plates have a potential difference of about 50 V. If the separation between the plates is 0.05 m, Calculate the magnitude of the electric field in the space between the plates.

16. 17.4 Electron Volt From We get 1 eV = 1.6x10 -19J
The energy acquired by a particle whose charge equals that of an electron (1.6x10 -19C) as a result of moving through a potential difference of 1 volt is called Electron Volt (eV).
1 eV = 1.6x10 -19J

17. Example
Suppose an electron in the picture tube of a TV set is accelerated from rest through a potential difference of 5000 V. How much kinetic energy does the electron acquire? (Give your answer in eV).

18. Exercise
Class work page 489, #1-13

19. Video on Demand - The Mechanical Universe
Video on Demand - The Mechanical Universe...and Beyond - The Millikan Experiment

20. Class work:
workbooks, Pg 203, Exercises 11-14, A10- A13.

21. Capacitors A capacitor is a device for storing charge.
made up of two parallel plates with a space between them .
The plates have an equal and opposite charge on them, creating a potential difference (Voltage) between the plates. But, the net charge on a capacitor is zero.
Two plate capacitor

22. capacitors Electronic capacitors Capacitors in a circuit
In circuit diagrams, a capacitor is represented by two equal parallel lines.

23. Charging a capacitor
If a voltage is applied across the capacitor by connecting the capacitor to a battery (Voltage source), the two plates quickly becomes charged as shown below.

24. Formula of Capacitance
The constant of proportionality C is called Capacitance of the capacitor.
For any given capacitor, the amount of charge q acquired by each plate is proportional to the magnitude of the potential difference V between them.
The unit of capacitance is coulomb per Volt. It is called Farad (F).

25. Example
If the capacitor has been charged to 5.8x10-6 C at a location where it has a potential difference with Earth of 60 V, what is its capacitance?

26. Parallel plate capacitor
Capacitance does not depend on the charge and potential difference.
Capacitance depends only on geometric factors of the capacitor such as:
Area (A) of the plates, and;
Distance (d) between the two plates.

27. Capacitance
Therefore, the capacitance of a parallel plate capacitor is given by:
Where is permittivity of free space. It is
=8.85 x C2/Nm2

28. Example
Calculate the capacitance of a parallel plate capacitor whose plates are 20 cm x 30 cm and are separated by 1.0 mm air gap.

29. Dielectric
For such capacitors, Capacitance is given by the formula:
some capacitors have material in between the plates. That material is known as Dielectric.
Where:
K = Dielectric constant,
kЄ0 = permittivity of material

30. Example
What is the capacitance of two square parallel plates 5.5 cm on a side that are separated by 1.8 mm of paraffin?

31. Class work
Textbook pg 490 # 31-38, #

32. Storage of charge
A charged capacitor stores electric charge by separating (+) and (-) charges.
The net effect of charging a capacitor is to remove charge from one plate and to add it to the other plate.

33. Work done in charging = potential Energy stored
The process of charging requires energy, or work input.
Work done in charging = potential Energy stored
From these two formulas, average potential energy can be derived.
Where
C = capacitance
V = Voltage (potential difference

34. Example
A camera flash unit stores energy in 150μF capacitor at 200V. How much electric energy can be stored by that capacitor?

35. Changes in plate separation (d)
If we increase the separation distance (d), by a factor of 2, Capacitance decreases by ½ .

36. Changes in plate separation (d)
If C decreases by ½, and q doesn’t change, the V double.
If V double and c is halved, energy is doubled!
Therefore: doubling (d), doubles Energy!

37. Example
How does the energy stored in capacitor change if the separation between plates is tripled while the charge remains the same?

38. Class work
Textbook pg 490 #