Transcript Electricity and Magnetism unit

Electricity and Magnetism unit
Concept Map
Electrostatics
Magnetism
Electricity
Electromagnetism
Electrostatics and Magnetism are both natural phenomena which were
manipulated by and for technology
Electrostatics - overview
Natural phenomena
Obeys law of opposites where the terms negative and positive
were selected to indicate the two opposites
Materials are divided into Conductors and insulators
Electrostatics - charging
By friction - electron affinity
Induced Charge Separation (occurs before charging)
Grounding - connecting to the earth
contact
Electrostatics - 3 keys
size of the Charge difference
Distance between opposite charges
Conductivity of the medium between the opposite
charges
Electrostatics - pre 1900
Charges either positive and negative - Ben
Franklin
positive charges flow to balance out the negatives
negative charges “aren’t bad”
Electrostatics - post 1900
Protons and electrons are discovered
negative electron is free to move and is the charge
carrier (transfer)
Positve proton is fixed in the nucleus of the atom
Electrostatics - charge difference
Charge difference (symbol Q) is measured in coulombs (Unit C)
Charge difference is a reflection of the
number of excess/deficit electrons in matter - Q=Ne
N - number of excess/deficit electrons
e - charge of 1 electron - -1.6 x 10-19C
charge of 1 proton - +1.6 x 10-19C
Electrostatics to Electricity
Electrostatics is unpredictable
Volta was able to create a small charge difference that was
predictable with a wet cell (primitive battery)in 1793
moving electrons can now do work for some application
(motor, light, heat)
Electricity - Circuits
A loop is created where the electrons can flow from the
power supply (battery - origin of the charge difference) to
one or more loads.
A wire, a highly conductive medium, provides the path for
the electrons
Electricity - Circuits
Battery/Power supply
Q = Ne represents the charge difference
E =VQ represents the work/ potential energy that these
electrons possess
E -Energy(J, Joules)
V - Voltage, Electric Potential, Potential
difference(V, Volts)
Electricity - Clarification
E=VQ should really be ΔE = ΔVQ
ΔE reflects the change in energy as the electrons do
their work
ΔV is called potential difference or voltage, where V is
just called electric potential. In Grade 12, we will clarify
their differences.
Electricity - Circuits
Current (I measured in Amps,A) reflects the moving energy
of the electrons. This energy is lost at the loads
(applications)
Instead of measuring their kinetic energy directly, current
measures the amount of charge that moves past a given area
in a second
I = Q/t
Electricity - Circuits
Resistance is a measure of how much opposition the
electrons experience as they move throughout the
circuit. Sometimes this resistance is desired as it does
work on the load. Sometimes it is not desired as
seen the energy loss within the wire itself
Circuits - Resistance
Resistance at the load (symbol R, unit - Ohm,Ω) R = V/I
Resitivity in the wire
Resistivity - factors
•
the type of metal (gold is the best conductor)
•
the length of the loop
•
the cross sectional path (thickness)
•
the temperature (higher the temperature of the wire the
higher the resistance)
resistance  resistivit y 
R
Length
Area
L
A
Resistivity - Equation
L
R 
A
•
•
R – resistance, L – length, A – cross sectional area
The coefficient “ ρ” depends upon the material. Values can
be found in Table 13.1 of your PRACTICE PROBLEMS
sheet.
Resistance at the load
•
The total resistance in the circuit dictates the voltage and
current based on the limits of the power supply.
•
The greater the resistance, the greater the need for energy
(E=VQ) to pull one electron around the loop. This results in
less electrons completing the loop (I ↓). (and vice versa)
Resistance at the source
•
We first look at the total Resistance in the circuit at
the power supply
•
The Power supply’s overall resistance RT controls the
size of VT and IT
•
Ohm’s Law applies RT = VT/IT.
Resistance at each load
•
The total Resistance in the circuit, mirrored at the
power supply, must be equivalent to the loads
•
Each load will have its own resistance R# controlling
the size of V# and I#
•
Ohm’s Law applies R# = V#/I#
Resistance in the circuit
•
Depends on how the loop is created
•
Rules have been established to distinguish between a
series circuit, parallel circuit and a mixed circuit. tomorrows lesson
Power
•
Power is defined as the rate at which work is done.
•
P = W/t (1 W = 1J/s) or P = VI (1 W = 1V∙A)
•
Note: W =ΔE
Power in the home
•
Homes use energy, not power. Power rates how fast energy is
used. However, because the loads are often listed by their
power requirements – 60 W light bulbs – Electrical
companies will measure ones energy use through the KW∙h
instead of J. Therefore , using an appliance for 1 hour might
require 1 KW∙h or 1 KJ.