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.

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”

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energypotentialelectricV

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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

CurrentCurrentThe movement of electrons

through a conductor. The rate at which charge flows.

time

chargecurrent

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: CoulombsSecond

= Ampere = Amp = A

CIRCUITCIRCUIT:

A path where electrons flow and

their energy is used.RESISTANCERESISTANCE:

Opposition to the flow of electrons in a

circuit.

THE DAM ANALOGYDam = 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

2nd 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

3rd 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.

4th 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 on1) Cross-sectional area of the resistor2) Length of the resistor3) Density of the resistor4) Temperature of the resistor

Guiding questions

• Answer #7 now please

What does the dam analogy What does the dam analogy tell us about the relationship tell us about the relationship

between I and R?between I and R?As R increases, I ……As R increases, I ……

DECREASES!DECREASES!oror

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

VOLTAGEVOLTAGE

Georg Simon Ohm

Ohm’s Law:

R

V I

Resistance

Voltage Current OR

IR V I

V R

More Typically Written As:Can Be Written As:

UNITS FOR UNITS FOR RESISTANCERESISTANCE

Amp

VoltsOhm

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.