ELECTRIC ENERGY AND ELECTRIC POTENTIAL

ENERGY REVIEW

• When we studied energy a couple of months ago there were a few major points of interest:

• Work is a change in energy that one object gave to another object.

• Kinetic Energy is moving energy.

• Potential Energy is stored energy.

• Kinetic energy can turn into Potential Energy and vice versa.

ELECTRIC POTENTIAL ENERGY• Earlier in the year we looked at two different types of

Potential Energy: Gravitational and Elastic/Spring.

• For Gravitational Potential Energy, the work done by the gravity field on the object was equal to the negative work done by the external force causing the displacement of the object.

• Example: Lifting a box in a gravity field

• Work done by external force: W = mgh

• Work done by gravity field: W = -mgh

ELECTRIC POTENTIAL ENERGY• The same concept holds true for Electric Fields!

• When a test charged is moved by an external force in an Electric Field, the test charges gains/loses Electric Potential Energy.

POTENTIAL ENERGY DUE TO A FIELD

• A test charge moving through an electric field can be treated just like a ball moving through a gravity field.

Positive Charge

Negative Charge

Positive Charge

+-

POTENTIAL ENERGY DUE TO CHARGES

• If two charges have the SAME sign, the Electric Potential Energy is POSITIVE since it takes POSITIVE WORK by an external force to bring them together.

• If two charges have DIFFERENT signs, the Electric Potential Energy is NEGATIVE since it takes NEGATIVE WORK by an external force to bring them together.

• Think of this like you would a spring. If these two charges were released from their positions, they would fly apart due to the stored potential energy.

+ +

ANOTHER WAY TO THINK OF IT…• Since the charge would naturally want to go down (towards the negative charge) it would

require work to move it up.

• If I do POSITIVE work to the charge, it would GAIN potential energy.

ELECTRIC POTENTIAL

• Electric Potential – A scalar characteristic of an electric field, independent of any charges that may be placed in the field.

• Essentially, Electric Potential (or Potential for short) is Electric Potential Energy without the charge. Electric Potential only describes the field.Only Source Charge Source and Test ChargeElectric Field Electric Force

Electric Potential Electric Potential Energy

ELECTRIC POTENTIAL

• Units: Volts Scalar Quantity

Electric Potential

Test Charge

Source Charge

Distance

Electric Field

Work

THE ELECTRON VOLT

• Electron Volt (eV) – the energy a charge-field system gains or loses when a charge of 1 electron is moved through a potential difference of 1 Volt.

• 1 eV = 1.6 x 10-19 J

POTENTIAL DIFFERENCE IN A FIELD

• Potential Difference is the change in the amount of Electric Potential between two different locations.

• Another name for Potential Difference is Voltage.

Initial Potential

Final Potential

Potential Difference = Final Potential – Initial PotentialArrows ALWAYS point toward the area with a lower potential.

POTENTIAL DIFFERENCE

• This is how a battery makes a circuit work! The battery creates a potential difference in the wires, therefore moving the electrons through the wire…

POTENTIAL DUE TO A SOURCE CHARGE

• If you don’t have a uniform field as in the previous slide, the can calculate the Potential at a specific distance from source charges using the following set up.

• Unlike Electric Force and Electric Field problems, you DO include the negative sign if the charge has a negative value. (A negative V is OK)

r1

r2q1

q2

PRACTICE PROBLEM

• A charge of 3 x 10-6 C is located 0.1 m to the left of the origin and a charge of - 4.0 x 10-6 C is located 0.1 m to the right of the origin. Where is the electric potential equal to zero between them?