Electric FieldsChapter 21

What image do you imagine when you hear the term electric field?

What is an electric field?• Electric field is defined as the electric force per unit

charge. • The direction of the field is taken to be the direction

of the force it would exert on a positive test charge. • The electric field is radially outward from a positive

charge and radially in toward a negative point charge.

Electric Field strength

• The Magnitude of the field is measured in Newtons per coulomb., N/C

ANSWERS TO THE PROBLEM SET

ANSWERS TO THE PROBLEM SET

HOMEWORKChoose 1 of the following:1. Electric field mill2. Research Conducted by Human Radiation

Group in Bristol University Make a brief research on your chosen topic and write /encode your output on an A4 size white paper. Minimum of 150 words and maximum of 250 words. To be submitted next meeting.

Electric Field Lines• Electric field lines are imaginary lines. • The spacing, density, of lines is related to the

magnitude of the electric field at that point• At any given point, there can be only one field line

(The lines don’t cross each other!)• Electric Field Lines Begin on Positive Charges, or at infinity End on Negative Charges, or at infinity

Electric Field Lines

A positive charge sets up an electric field pointing away from the charge

A negative charge sets up an electric field pointing towards the charge

Drawing the electric field

Electric Field LinesThe charge on the right is twice the magnitude of the charge on the left (and opposite in sign), so there are twice as many field lines, and they point towards the charge rather than away from it.

Conductors in electrostatic equilibrium

Since excess charge on a conductor is free to move, the charges will move so that they are as far apart as possible. This means that excess charge on a conductor resides on its surface, as in the upper diagram.

When electric charges are at rest, the electric field inside a conductor is zero.

The electric field is always perpendicular to the surface of a conductor (Spiky haircut!).

The electric field is always perpendicular to the surface of a conductor (Spiky haircut!).

Net charge = q

Net charge = 2q

Net charge = 3q

Remember: The spacing, density, of lines is related to the magnitude of the electric field at that point. (charge number of E lines )

The electric field is stronger

where the surface

is more sharply

curved.

VAN DE GRAAF GENERATORS

APPLICATIONS OF ELECTRIC FIELDS

Can you think of any practical applications of electric fields?

Energy and Electric PotentialElectronic devices are not only easily damaged, they can do damage as well. Do you agree? Explain.because of the interacting electric fields causing interference. Example: Modern pacemakers have built-in features to protect them from most types of interference produced by other electrical devices you might encounter in your daily routine.

Energy and Electric PotentialDo you still get shocked even if the power is turned off? Why is that so? - Because the capacitors can remain charged. capacitor - a passive two-terminal electrical component used to store electrical energy temporarily in an electric field.

Energy and Electric PotentialWhy is it that if you touch a charged electrode, you could get a significant shock? - because not all electrodes are grounded. Electrode - an electrical conductor used to make contact with a nonmetallic part of a circuit.

Electric potential analogy

Electric Potential Difference

The work done on a charge (potential difference) or work needed to move a positive test charge from one point to another.

Infer: How would you increase the potential energy of a system that contains two like charges?

Electric potential difference

Determining Electric potential Difference

SAMPLE PROBLEM

What work is done on a 8.0 C charge when you move that charge through a 5 V electric potential difference?

= (8 C) (5V) W = 40 C.V or Joules

Equipotential or zero electric potential difference

• If you move the test charge in a circle around the negative charge. The force the electric field exerts on the test charge is always perpendicular to the direction in which you moved it, so you do not work on the test charge. Therefore, the electric difference is zero

Electric Potential in a uniform field

• The magnitude and direction of the electric field are the same at all points between the plates, except at the edges of the plates, and the electric field prom the positive plate to the negative plate.

SAMPLE PROBLEM• What electric potential difference is between

two metal plates that are 3m apart if the electric field between those plates is 5 N/C?

= (5N/C) (3m)= 15 N/C.m or N.m/C or Volts

PROBLEM SOLVING EXERCISES

1. The electric field intensity between two large charged parallel metal plates is 6000N/C. The plates are 0.05m apart. What is the electric potential difference between them?

2. A voltmeter reads 400V across two charged parallel plates that are 0.020 m apart. What is the magnitude of the electric field between them?

3. What electric potential difference is between two metal plates that are 0.200m apart if the electric field between those plates is 2.50 N/C?

PROBLEM SOLVING EXERCISES

4. When you apply a potential difference of 125 V between two parallel plates, the field between them is 4.25 N/C. How far afar are the plates?5. What is the work done on a 3.0 C charge when you move that charge through a 1.5 V electric potential difference?