Transcript Electricity and Magnetism ISCI 2002

Electricity and Magnetism
ISCI 2002
1. Electricity is a Force – Vector
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Electric charges (attract and repel)
Comb and Ball Example
2. Atoms
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Protons and Electrons
“Atoms are electrically neutral”
Formation of ions
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Rub a rubber rod with fur
3. Insulators
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Does not allow charge to run through it
4. Conductors
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Allows for charges to run through it
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Metals
Van de Graff Generator
How it Works:
• Belt – high RPM
• Picks up charges
• Distributes charges to
spherical dome surface
• Conductors
– Charges concentrate
(curvature of surface is
greatest)
1. Coulomb’s Law
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The force between charges is proportional to the product of
the charges and inversely proportional to the square of the
distance between them.
2. Equation
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Fe = k (q1q2)
r2
k = constant (9.0 x 109)
q = charges
r = distance between
charges
3. Newton’s Universal Law of Gravitation
• Differences
– Charges may be positive or negative (mass always positive)
– Electrical force is greater than gravitational force (k > G)
1. Charged objects creates an electrical field around itself (space
around the object)
2. Electrical Fields exert a force on any object placed into the field.
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Fields may be weak or strong
3. Represented by vector lines (field lines)
4. Direction – field lines point; magnitude – lines closest together
5. If:
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Positive charge – move in direction of lines
Towards negative charged area
Negative charges pushed in opposite direction
Lines point away from positive
charges to negative charges
All field lines begin at the positive
charge
More charge on an object, more
field lines attached to it.
1. If you separate the positive and negative charges from
each other, WORK must be done!
2. This creates Electrical Potential Energy (EPE).
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A charge in a field has energy
External force pushes it the against the field – work
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Creates EPE
3. If Charges are brought together again – Energy is
released
1. Work – Creates Electrical Potential Energy
2. How it is measured:
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Electrical Potential = electric potential / charge
3. Unit – Volt
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1 Volt = 1 joule / charge (coulomb)
1.5 – Volt Battery
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Gives 1.5 joules of energy to convert to 1 coulomb of charge flowing
through a battery
1. Electrical Conductor
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Two electric potentials separated
Charges flow from one to the other
2. How a Chemical Battery Works
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Removes electrons from one terminal
and deposits them on another terminal
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Adds energy to the charge
3. Where does the energy come
from?
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Chemical reaction – breakdown (lead,
zinc)
1. Metals
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Electrons (free) flow through the
wires
Protons do not (bound to nuclei)
2. Non-Metals
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Electrons and Protons both may
flow
Car Battery
1. Conductors
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Diameter
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More narrow – greater resistance
Temperature
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Higher – more resistance
Material
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Metals (free electrons available)
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Great Conductors!
Rubber (no free electrons)
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Poor Conductors or Insulators
2. Measured in Ohm’s (Ω)
3. Types of Current
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AC vs DC
1. Current and Voltage
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Proportional relationship (Current = Voltage)
2. Current and Resistance
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Current = 1/resistance
3. Ohm’s Law
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Current = Voltage / Resistance
I = current
V = volts
R = resistance
1. Path of Electron Flow through wires
2.
Simple
3.
Series
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4.
Current flows through each lamp (resistor)
From positive terminal to negative terminal
Easy to make; Disadvantages?
Parallel
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Electrical devices connected to the same two points of an
electrical circuit
Three lamps
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Electrons travel to lamp one and return
Video Link
What type of Circuit is show at the
Left?
How do you know?
What are the symbols used?
1. Moving charges do work.
2. Rate at which work is done
3. Formula
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Power = current x voltage
4. Unit
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Watt
1. Guess What? Magnetism is a force – vector
2. Where this force comes from
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Metal atoms (Fe)
Domains – aligned atoms
Induced
3. Magnetic Fields
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Space around a magnet – Magnetic Field
Field Lines – North pole to South Pole
Produced by moving electric charges
Video Link 1
Video Link 2
Electric Currents and Magnetic Fields
A magnetic field is produced by the motion of
electric charge.
A magnetic field surrounding a current-carrying
wire makes up a pattern of concentric circles.
1. Magnetic Poles ------ Magnetic Forces
2. Like poles repel; unlike poles attract
3. Break a bar magnet
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Still have a north and south pole
1. Charges are moving – deflected by magnets
2. Particles moving through a wire are deflected
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wire is also deflected
3. Electromagnetic Force – Electric Motors
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Coil of wires turn - magnet
Current in motor – changes direction when coil makes a half-turn
Video Link
4. Meters – magnetic compasses detect electrical current
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Compass in a coil of wire – current loop has effect on needle
Galvanometer (current); Voltmeter (V)