ELECTRIC POTENTIAL DIFFERENCE VOLTAGE ELECTRIC POTENTIAL DIFFERENCE

ELECTRIC FIELDS AND THE MOVEMENT OF CHARGE THE ELECTRIC FORCE IS A FIELD FORCE, IT OCCURS IN THE ABSENCE OF PHYSICAL CONTACT THE ELECTRIC FIELD IS A VECTOR QUANTITY, AND THE DIRECTION IS THAT WHICH A POSITIVE TEST CHARGE WOULD MOVE WHEN PLACED IN THE FIELD

The Electric field, Work, and PE Electric fields are similar to gravitational fields Work must be done by an external force to move an object against nature, from low to high PE Objects naturally go from high to low PE

The Electric field, Work, and PE To move a charge in an electric field against its natural direction of motion would require work and would add PE to the object To move a charge in an electric field in its natural direction of motion causes the object to go from high energy to low energy

The Electric field, Work, and PE The previous example had a + source charge, in this instance the source charge is negative Predict what will happen in terms of the test charge moving to the given location in the presence of this negative source

Electric Potential The gravitational analogy: Gravitational Potential Energy or an object is due to its vertical position about the Earth, it depends on the objects mass and height Gravitational Potential is defined as PE/mass, and becomes mass independent Gravitational Potential is a location dependent quantity but is independent of the mass of the object experiencing the field Gravitational Potential describes the affects a gravitational field has upon objects placed within it

Electric Potential Electric Potential Energy is dependent upon the charge and the location of the object in the electric field (EPE= -qEd) Electric Potential is only location dependent (Electric Potential = PE/q) Electric Potential is used to express the affect of an electric field of a source in terms of the location within the electric field A test charge with 2x the charge of another would possess 2x the potential energy at a given location; yet its electric potential at that location would be the same as any other test charge

ELECTRIC POTENTIAL In a Circuit The voltage source in a circuit establishes an electric field between its two terminals A positive test charge would move naturally from high PE to low PE, thus the + terminal is described as the high potential terminal A positive test charge will move through a circuit and do work upon the components, the electric potential energy it started with is transformed into other forms of energy The positive test charge returns to the negative terminal at a low energy and low potential, ready to repeat the cycle

Electric Potential Difference (aka Voltage) Say you are moving a + test charge in a uniform electric field from location A to location B In moving the charge against the electric field from location A to location B, work will have to be done on the charge by an external force The work done on the charge changes its potential energy to a higher value; and the amount of work which is done is equal to the change in the potential energy As a result of this change in potential energy, there is also a difference in electric potential between locations A and B Electric potential difference is the difference in electric potential (V) between the final and the initial location when work is done upon a charge to change its potential energy. In equation form, the electric potential difference is:

VOLTAGE The SI unit of electric potential difference is the volt (V) one Volt = one Joule per Coulomb If the electric potential difference between two locations is one volt, then one Coulomb of charge will gain one joule of potential energy when moved between those two locations If the electric potential difference between two locations is 12 volts, then one coulomb of charge will gain 12 joules of potential energy when moved between those two locations

Electric Potential Difference: Circuits As a charge moves through a circuit, it encounters a variety of types of circuit elements which are energy-transforming devices. In these devices, the electrical PE of the charge is transformed into other forms of energy. In a light bulb, the electric PE of the charge is transformed into light energy and thermal energy. The moving charge is doing work upon the light bulb to produce the other forms of energy and thus is losing its electric PE. Upon leaving the circuit element, the charge is less energized. Prior to entering the light bulb (or any circuit element) is a high electric potential location; just after leaving the light bulb is a low electric potential location. The loss in electric potential while passing through a circuit element is often referred to as a voltage drop. By the time the charge has returned to the (-) terminal, it is at 0 volts and is ready to be re-energized and pumped back up to the high voltage, (+) terminal.

Electric Potential Diagrams The battery energizes the charge to pump it from the low voltage terminal to the high voltage terminal and establishes an electric potential difference across the two ends of the external circuit Being under electric pressure, the charge will now move through the circuit. As its electric potential energy is transformed into light energy and heat energy at the light bulb locations, the charge decreases its electric potential The total voltage drop across the circuit equals the battery voltage as the charge moves from the positive terminal back to 0 volts at the negative terminal In Circuit A, there is a 1.5 volt D-cell and a single light bulb. In Circuit B, there is a 6-volt battery (four 1.5 volt D-cells) and two light bulbs. In each case, the negative terminal of the battery is the 0 volt location. The positive terminal of the battery has an electric potential which is equal to the voltage rating of the battery. The battery energizes the charge to pump it from the low voltage terminal to the high voltage terminal. By so doing the battery establishes an electric potential difference across the two ends of the external circuit. Being under electric pressure, the charge will now move through the external circuit. As its electric potential energy is transformed into light energy and heat energy at the light bulb locations, the charge decreases its electric potential. The total voltage drop across the external circuit equals the battery voltage as the charge moves from the positive terminal back to 0 volts at the negative terminal. In the case of Circuit B, there are two voltage drops in the external circuit, one for each light bulb. While the amount of voltage drop in an individual bulb depends upon various factors.

The water analogy The power source is like a pump in a water system. It takes in water at low pressure and does work on it, ejecting it at high pressure A battery takes in charge at a low voltage, does work on it, and ejects it at a high voltage Voltage is like water pressure (and is often called electric pressure)