Electric Fields
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Electric Fields
16.7 The Electric Field
The electric field is the force on a small charge, divided by the charge:
(16-3)
16.7 The Electric Field
For a point charge:
(16-4a)
(16-4b)
16.7 The Electric Field
Force on a point charge in an electric field:
(16-5)
Superposition principle for electric fields:
16.7 The Electric Field
Problem solving in electrostatics: electric forces and electric fields
1. Draw a diagram; show all charges, with signs, and electric fields and forces with directions
2. Calculate forces using Coulomb’s law
3. Add forces vectorially to get result
16.8 Field Lines
The electric field can be represented by field lines. These lines start on a positive charge and end on a negative charge.
16.8 Field Lines
The number of field lines starting (ending) on a positive (negative) charge is proportional to the magnitude of the charge.
The electric field is stronger where the field lines are closer together.
16.8 Field Lines
Electric dipole: two equal charges, opposite in sign:
16.8 Field Lines
The electric field between two closely spaced, oppositely charged parallel plates is constant.
16.8 Field Lines
Summary of field lines:
1. Field lines indicate the direction of the field; the field is tangent to the line.
2. The magnitude of the field is proportional to the density of the lines.
3. Field lines start on positive charges and end on negative charges; the number is proportional to the magnitude of the charge.
Electric Potential
Electric Potential and Gravitational Potential have several similarities Mass/Charge Height/Distance
Electric Potential is given as the amount of work that can be done to move the charge
Unit of electric potential is Volts (V)
If the electrical circuit in your Ipod were analogous to a water circuit at a water park, then the battery would be comparable to _____. a. the people which slide from the elevated positions to the
ground b. the obstacles which stand in the path of the moving water
c. the pump which moves water from the ground to the elevated positions
d. the pipes through which water flows e. the distance which water flows through the circuit
If a battery provides a high voltage, it can ____.a. do a lot of work over the course of its
lifetime b. do a lot of work on each charge it
encountersc. push a lot of charge through a circuitd. last a long time
Electric Circuit
RequiresClosed path
The path can only be created by conductors
Energy Source
Electric Current
Current – the rate at which charge passes a point in a circuit
I = Q/t Unit is Coulomb per second
Ampere (A) or amps Current requires a closed path.
Positive to negative
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1. If an electric circuit could be compared to a water circuit at a water park, then the ... ... battery would be analogous to the ____. ... positive
terminal of the battery would be analogous to the ____. ... current would be analogous to the ____. ... charge would be analogous to the ____. ... electric potential difference would be analogous to the
____. Choices:
A. water pressure B. gallons of water flowing down slide per minute C. water D. bottom of the slide E. water pump F. top of the slide
Resistance
All things resist the flow of charge Resistance – a measurement of resistance to
current Measured in Ohms (Ω) When charge encounters resistance, work is
done In wires, resistance is based on material, width,
and length
Material Resistivity (ohm•meter)
Silver 1.59 x 10-8
Copper1.7 x 10-8
Gold 2.4 x 10-8
Aluminum 2.8 x 10-8
Tungsten 5.6 x 10-8
Iron 10 x 10-8
Platinum 11 x 10-8
Lead 22 x 10-8
Nichrome 150 x 10-8
Carbon3.5 x 105
Polystyrene 107 - 1011
Polyethylene 108 - 109
Glass 1010 - 1014
Hard Rubber 1013
Power
Power – the rate at which electric energy is used by a circuit
Unit of Power is Watts (W) or kilowatts (kW)
Frequently used as kW*h Power = IV
Ohms law
There exists a relationship between Current, resistance, and voltageV=IR
This equation is often rearranged as I=V/R
http://www.physicsclassroom.com/Class/circuits/u9l3c.cfm
Two wires - A and B - with circular cross-sections have identical lengths and are made of the same material. Yet, wire A has four times the resistance of wire B. How many times greater is the diameter of wire B than wire A?
Which of the following will cause the current through an electrical circuit to decrease? Choose all that apply.a. decrease the voltage b. decrease the resistancec. increase the voltaged. increase the resistance
You have likely been warned to avoid contact with electrical appliances or even electrical outlets when your hands are wet. Such contact is more dangerous when your hands are wet (vs. dry) because wet hands cause ____. a. the voltage of the circuit to be higher b. the voltage of the circuit to be lower c. your resistance to be higher d. your resistance to be lower e. the current through you to be lower
If the resistance of a circuit is tripled, then the current through the circuit would be ____.a. one-third as much b. three times as muchc. unchangedd. ... nonsense! There would be no way to
make such a prediction.
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