1. Electric Fields and Potential
David Scott

2. Electric Fields
A force field is a region of space around an object that exerts a force on another object
Examples:
Gravitational fields, magnetic fields
An electrical field is the region of space around an electric charge that exerts and electric force on another charge

3. Electric Fields
The magnitude (strength) of an electric field can be measured by its effect on charges located in the field.
A small positive “test charge” q0 is used to detect and measure electric fields.

4. Calculating Electric Field Strength
The strength of an electric field is the amount of electric force applied per unit charge
What are the units of electrical field strength?

5. Direction of Electric Fields
By convention, electric fields point away from positive and toward negative.
Why might this be?

6. Direction of Electric Fields
The direction of an electric field at any point is the direction of an electric force acting on a small positive test charge.

7. Electric Field Lines
Electric field lines are used to represent the direction and relative strength of an electric field.
The relative strength of a field is indicated by the closeness of the field lines.
Electric field lines for an isolated charge extend to infinity

8. Electric Field Lines
Electric field lines point away from positive and toward negative.

9. Electric Field Lines
Electric field lines point away from positive and toward negative.

10. Electric Field Lines Can you identify these charges or fields?

11. Electric Shielding
When a conductor is in electrostatic equilibrium, the electric field inside the conductor is zero.
Lightning striking aircraft

12. Electrical Shielding

13. Electric Field Inside a Conductor

14. Electric Shielding
Electric shielding prevents formation of an electrical field inside the shielded object, thereby preventing electrical conduction or interference inside the object.

15. Faraday Cage

16. Electric Potential Energy
A charged particle has electric potential energy by virtue of its position in an electrical field.

17. Electric Potential Energy
When a brick is raised from the floor, work was done on the brick (W = Fd)
Consequently, the brick now has gravitational PE (PE = mgd) due to its postion in the gravitational field
PE was gained because work was done against the gravitational field
When the brick returns to the floor, PE is converted to KE

18. Electric Potential Energy
The same is true when a charge moves through an electrical field
Work must be done to move a charge against an electrical field
So, the energy a charge has due to its location in an electric field is called electric potential energy

19. Electric Potential Energy
PEelect depends on the charge, the electric field, and the location of the charge in the field

20. Electric Potential
Electric potential is different from electric potential energy.
Electric potential (V) is the electric potential energy per unit charge

21. Electric Potential
It’s not the amount of volts that is dangerous. It’s the amount of charge and energy!

22. Electrical Energy Storage
Capacitors store electric energy.
A capacitor is made of parallel plates separated by a very small distance.
Plates are charged by hooking them to a voltage source, e.g. a battery

23. Capacitors
Provide temporary storage of electric potential energy, through the separation of charges
Fully charged when the potential difference (V) of the capacitor equals the potential difference of the battery.

24. Capacitor Design
Usually the parallel plates are separated by an insulator, called a dieletric, such as paper.
This helps keep the charges separated.
Capacitance, the ability to store charge, depends upon the distance between the plates and the area of the plates

25. Capacitor Design
Since C is directly proportional to the Area of the plates, increasing area increases C
Larger areas can be gained by a “jelly roll” design.

26. Capacitance Calculations
The unit of capacitance is the farad (F), named for Michael Faraday
1 Farad is a large amount, so often capacitance is reported I pF, nF, or µF

27. Van de Graaff Generator

28. Van de Graaff Generator
Generate high static voltages (thousands to millions of volts)
Voltage can be increased by increasing diameter of the globe or by operating in pressurized gas.
Used in nuclear research to accelerate charged particles