Energy and Electrostatics

A new definition of potential energy An object has potential energy due to its location within a force field. To change the object’s location (and therefore potential energy), work has to be done by applying a force on the object.

Mechanical example Work is done to lift an object against the gravitational force field of Earth. (unnatural movement) When released, the potential energy is converted into kinetic energy, and the object moves in the direction of the force field naturally.

Electric potential energy A charged object has PE due to its location within an electric field. Work is required to move a charged object against the electric field. When released, that PE will be converted into KE naturally in the direction of the field.

Guiding questions Answer questions #1 - #5 now please.

Electric Potential In electricity, rather than deal with total PE, it is more convenient to deal with electric potential energy per charge since there could be many charges in the field (think a circuit) We call this electric potential for short – Measured in Volts, and symbolized with a “V”

Electric potential Rearranging the previous equation, we get… W = qV When we get to circuits, this will be useful because if we are using a 9V battery, the amount of work we get out of a charge moving through the circuit will be 9V*charge.

More about Volts 1 volt = 1 joule of energy per coulomb of charge Example, if a conductor has a potential of 1000 volts, it would take 1000 joules of energy per coulomb to bring a small charge from far away and add it to the charge of the conductor.

Guiding questions Answer #6 now please

Current The movement of electrons through a conductor. The rate at which charge flows. DC (Direct Current) – All charges move in one direction in the circuit. AC (Alternating Current) – charges move one way and then the other, changing direction from moment to moment. I = q t units: Coulombs Second = Ampere = Amp = A

CIRCUIT CIRCUIT: A path where electrons flow and their energy is used. RESISTANCE RESISTANCE: Opposition to the flow of electrons in a circuit.

THE DAM ANALOGY Dam = Battery, Outlet, Power Supply Water Depth = V = Voltage or Potential Valve = Resistance or Current Control Water Wheel = Energy User/Converter (Light Bulb, Motor…) Pipe = Wire Current GROUND or Lowest Potential

How can the resistance change? What are the variables that effect the resistance of the flow of the water in the Dam Analogy? Resistance is much like friction. The more “friction” against a current, the more resistance.

First variable What would happen if we widen the path for the water? The resistance would be less. Therefore: R is in proportion to 1/A

2 nd variable What would happen if we shorten the path of the valve? There would be less to flow through, therefore the resistance would be less. Therefore, R is proportional to L

3 rd variable What would happen if we thicken the walls of the pathway for the water? There would be more resistance. Therefore, R is proportional to density of the material.

4 th variable What would happen if we heat up the pathway of the water? The resistance would increase As resistance goes up, temperature goes up.

Summary Resistance of an object depends on 1)Cross-sectional area of the resistor 2)Length of the resistor 3)Density of the resistor 4)Temperature of the resistor

Guiding questions Answer #7 now please

What does the dam analogy tell us about the relationship between I and R? As R increases, I …… DECREASES! or

How could the current be increased in a circuit whose resistance is held constant? Increase the “push” or VOLTAGE

Georg Simon Ohm Ohm’s Law: OR More Typically Written As: Can Be Written As: UNITS FOR RESISTANCE This tells you the number of volts necessary to push 1 amp of current through the device.

Power Deriving power of a circuit P = Work/time How is work related to potential of a circuit?

Guiding questions Answer #8 now please.