Download ELECTRIC FIELD

ELECTRIC FIELD
Electrostatic Force
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For centuries, humans observed “strange” force
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Force is far too strong to be gravity
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Example: Rub amber with fur → it attracts straw
Gravity between amber and straw is too weak to feel
Amber and straw must have something
–
–
–
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Other than mass...
Which causes a noticeable force between them
This “stuff” is called electric charge
Charge is measured in “Coulombs” (C)
Electric Charge
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Electrostatic force can be attractive or repulsive
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Humans named these “positive” and “negative”
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–
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There must be 2 types of charge which interact differently
So the amber has positive charge...
And the straw has negative charge
Opposite charges attract; like charges repel
This was the limit of our understanding for
thousands of years
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More experiments → understanding electricity in 1800's
Coulomb's Law
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After much experimenting:
q1
F=k
d
q2
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F = Force
q1 q 2
d
2
q = charge
d = distance
k = 9 x 109 N m2 / C2
This is an “inverse-square law”
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Just like gravity! (except it can also be repulsive)
Charge and Atoms
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Today we know that matter is made up of atoms
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Charge is always “attached” to a particle
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Positively charged nuclei with negatively charged e–
Positive charge → protons in nucleus
Can never have a charge smaller than 1.6 x 10-19 C
Charge can never be created or destroyed
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It is conserved, just like energy!
The universe is electrically neutral overall...
...as far as we can tell!
Conductors and Insulators
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Most electrons are “bound” to the nucleus
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Strength of this binding varies greatly
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Some electrons are relatively easy to remove from the nucleus
Conductors
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i.e. They orbit around the nucleus
Materials with lots of “free” electrons which move easily
Most metals are good conductors
Insulators
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Materials with few free electrons (rubber, glass, plastic)
Polarization of Charge
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How do “bound electrons” respond to electric fields?
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Shape of the electron cloud is affected
Atom is said to be “polarized”
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Energy is stored inside the atom's electric field
Too much polarization → electron separates from nucleus
Charging Objects
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How can we transfer charge to an object?
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Conduction
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Put objects made of different materials in physical contact
If one attracts e– better than the other → it picks them up
Induction
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Bring a charged object near an uncharged one
Cut uncharged object in half
+
–
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+
+
+
Electric Field
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How can particles exert force without touching?
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“Action-at-a-distance” → just like gravity
Solution: The “electric field”
Charge
creates
exerts force
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exerts force
Electric Field
Charge
creates
Electric charge affects the space around it
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If another charge comes into this “field”, it feels a force
Electric Potential – “Voltage”
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Charge affects space (via the electric field)
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Need to measure the effect at a given point in space
Electric potential (or “voltage”)
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Describes the overall effect of nearby charge
Measured in Volts (V)
+
+
+
A
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Point A has a large
positive voltage
B
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Point B has a large
negative voltage
Storing Electric Energy
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Must separate + and – charges
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Batteries
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Like stretching an imaginary rubber band
Place 2 different substances in contact with acid
+ charges move toward one object...
– charges move to the other
Capacitors
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2 metal plates with a gap between them
Place + charge on one plate and – charge on the other
Good for releasing energy quickly → e.g. camera flash bulb
Releasing Electric Energy – Current
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Allow + and – charges to come back together by
making an electric “circuit”
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Electric current
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Let the imaginary rubber band “snap”
The flow of charge from one location to another
Units: Coulombs / second (C/s) OR Amperes (A)
Currents occur when there is a “voltage drop”
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Difference in voltage between one location and another
Often due to a battery, capacitor, or generator
KE of moving charges can be used → electronic devices
Resistance and Ohm's Law
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Resistor
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Device that converts current to another form of energy
Resistance
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Defined by “Ohm's Law”:
Units: Measured in Ohms (Ω)
Voltage drop
R=
Current
Power in Electric Circuits
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Electric power:
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Energy used
Power =
time
Units: Measured in Watts (W) or horsepower (hp)
Depends on the electric current and the resistance
10 – 500 W
1,000 – 10,000 W
About 1000 W on average