An electric field is a region in space where a force is exerted on a positive charge.

The electric field is radially outward from a positive charge (+q) and radially inward on a negative point charge (-q).

Electric lines of force originate at the positive charge and terminate at the negative charge.

                                         

    

                                         

    

Electric field lines are not always straight. In the case of an electric dipole the lines are often curved. An electric dipole consists of two separated charges that have opposite signs.

See illustration at http://www.saskschools.ca/curr_content/physics30/elect/lessoniii_2_3.html

Field lines are used to provide a model for the electric field, they do not really exist.

Electric fields do exist and are produced by one or more charges.

f.y.i: Sharks have electrically sensitive cells that allow them to detect weak electric fields generated by living organisms, specifically their prey.

Recall the change in potential energy of a ball when you lift it from point A to B. Gravitational force is acting on the ball. If you lift the ball against the force of gravity from A to B work is done, and thus it's potential energy is increased. Work done by the hand to lift the ball from A to B equals the difference between the electric potential energy at B

and at A:

                                                    

a positive test charge +q is situated between two oppositely charged plates.

The test charge will undergo an electric force from the + plate (directed to the - plate).

An external force is required to move the test charge towards the + plate because of the electric force (like charges repel).

At locations A and B, the test charge q has electric potential energies. The electric potential difference (V) is the work done per unit charge as a charge is moved between two points in an electric field.

                                

                    

The SI unit of electric potential: joule/coulomb = volt (V).

Note: Electric potential difference uses the variable V as well as the unit volt. Do not refer to electric potential as Volts (even though the units are volts). When you use V in this situation it stands for electric potential.

What other equations can we substitute in for Q, and W?

New equations?

The work done to move the test charge (q = +3.0 x 10-5 C) at a constant speed from A to B is + 4.5 x 10-6J.

Find the difference in the electric potential energies of the charge between A and B.

Determine the potential difference between A and B.

The difference in potential energy between points A and B is equivalent to the work done in moving the charge from A to B. Therefore, + 4.5 x 10-6J is the difference in potential energy.

The potential difference between A and B is equal to the difference in potential energy divided by the charge:

A positively charged object with a charge of 4.0 x 10-7 C is moved from it's original position A 90 cm closer to a positively charged plate. The electric force of repulsion is 2.5 x 10-2 N.

1. Calculate the work done to move the charge from A to B.

2. Calculate the work done per unit charge in moving from A to B.

                                                                             

 

a.

b.

The earth has an endless supply of electrons. Electrons can be added or taken away without its potential being changed. The potential of the earth is arbitrarily said to be zero. An object connected directly to the ground can be described as being earthed or grounded.

Why are objects grounded?

In electrical circuits the ground means zero voltage when compared with the rest of the circuit. This is sometimes called a chassis ground.

The electrical chord that connects to the wall has three wires in it. One of the wires is a ground wire which is connected to the third prong on the electrical cord, which is then plugged into an electrical receptacle.

The receptacle ground is connected to a copper wire which leads to the breaker box. This copper wire is connected to another very thick copper wire which is connected to a copper rod buried into the ground.

This provides a path for electrons to flow if a "short" occurs inside any other device as a safety measure.

This meter can measure current AC or DC, voltage, resistance, etc. This voltmeter can be used to measure the electric potential difference between two points in an electric circuit.

The internal resistance of the Voltmeter is very high so a negligible amount of current can flow through the meter and will not disturb the circuit.

                                                                                                                         

    

Electric Potential Questions

A.