Electricity & Magnetism
Static Electricity
The Bohr Model - Protons and neutrons exist in the nucleus of the atom and
electrons "orbit" outside of the nucleus. It is the outermost electrons that can
transfer from one atom to another. If electrons transfer away from an atom, the
atom becomes positively charged. If electrons transfer to an atom, the atom
becomes negatively charged. Either way, electrons are the currency of electricity.
The protons stay put.
A charge is a condition that an object assumes as a result of an excess or depletion
of electrons. It may be positive , negative or neutral. Charge (q) is measured in
Coulombs (C). One Coulomb is equivalent to the charge of 6.25 x 1018 electrons.
Analogy
A coulomb is to electrons as a dozen is to eggs
- 1 C = charge on 6.25 x 1018 electrons
q = # electrons / 6.25 x 1018
The most common method to demonstrate a charge is by rubbing two materials
together. Electrons are transferred from one material to another leaving them
oppositely charged. Some materials "give away" electrons more readily than others
and some accept electrons more readily. See the electrostatic series.
Example
What is the charge on one electron?
So another way of looking at the formula is:
A: -1.6 x 10-19 C.
q=Ne
Practice
p. 422 # 2-5
First law of charges:
o like charges repel
o opposite charges attract
Coulomb’s Law
F = Kq1q2 / d2 where K = 9.0 x 109 Nm2 / C2
Problems
p. 420 # 1-5
p. 422 # 1, 6 - 14
Electric Fields
Every charged object creates an electric field of force in the space surrounding it. Any charged
object placed in this field experiences a force of attraction or repulsion.
Fields are represented with force vectors drawn from a point charge pointing in the direction that
a positive test charge would move if placed in this field.
+
-
The closer the field lines, the stronger the “electric Field Intensity”
E = Fe / q
where q is +1 Coulomb - a unit positive test charge
Substituting Coulomb’s law into this expression yields:
E = K Q / d2
More complicated fields form when more than one point charge interfere (see p.429) In any
event, the electric field intensity at any point is equal to the vector sum of the electic fields of all
point charges
Sample
p. 427
Practice
p. 427 # 1 - 3
p. 444 # 1 - 6
Current Electricity
Demo Any simple circuit
A simple circuit usually consists of at least these four things:
i)
A source of electrical energy - a battery or generator
ii)
A device that converts electrical energy to some other form of energy - a load or resistor
iii)
A path along which the energy travels from the source to the load - wires
iv)
A method of controlling the rate at which energy is “consumed” - switch, potentiometer
or variable resistor
Voltage
The terminals of a battery have different charges This difference gives rise to a
force attracting charge from one terminal to the other. If there is a conducting
path from one terminal to the other then charge will “flow” from one terminal to
the other. The difference in charge at the “poles” is sometimes referred to as a
potential difference. Another word for this potential difference is voltage (V).
Potential Difference == E.M.F. == Voltage (V)
Units volts(v) or Joules /Coulomb
The equivalent unit for volt is Joules per Coulomb. This means a 1 volt potential
difference can transfer 1 Joule of energy for every 1 Coulomb of charge that
passes through the light bulb.
Work and Electricity
Recall that work is defined as the transfer of energy from one form to another.
When charge flows through a circuit, electrical energy is transferred to other
forms of energy and so work is done. The amount of work done (W) on a given
amount of charge (Q) to move it through an electric field is equal to the change
in electric potential of this charge (i.e. the voltage drop).
𝑉=
∆𝐸
𝑄
=
𝑊
𝑄
Practice - Hand out questions
Current
The amount of charge, that passes any given point in the circuit, in a given
amount of time, can be quantified. This rate is known as current (I). Current is
the rate of flow of charge though a given point in a circuit.
Current (I) = Charge(q) / time (t)
Units == Amps (A) == Coulombs(C) / Second(s)
the equivalent unit for amp is C/s. Provide some examples i.e. if 6.25 x 1018
electrons pass a given point in a circuit in 1 second the current is 1 Amp. 2 sec?
5? etc
Analogy
Water flowing in a river.
Practice
p. 451 # 1 – 4 and Hand Out
Resistance
Any hurdle or obstacle that obstructs the flow of charge as it travels through the
circuit is referred to as a resistor. Resistors do just that. They resist the flow of
charge through a circuit. To overcome this resistance (R) or load, work is done
(i.e. energy is transferred). A simple example is a light bulb.
Unit == Ohm ()
Ohm’s Law
Two things effect the rate of flow of charge(i.e. the current) through a circuit:
i)
The force driving the charge (i.e. the voltage)
IV
ii)
Any obstacles or hurdles that might get in the way (Resistance)
I  1/R
This leads to Ohm’s Law
Example
V=IxR
A 12 volt battery runs a 600 Ohm radio. What is the current?
A: 20 milliamps
Try p. 454 # 5 - 10
Power
Recall
𝑃=
Δ𝐸
∴ 𝑃 = ∆𝐸 ÷ ∆𝑡
Δ𝑡
𝑏𝑢𝑡 ∆𝐸 = 𝑉𝑄 𝑎𝑛𝑑 ∆𝑡 =
𝑄
𝐼
𝐼
∴ 𝑃 = 𝑉𝑄 ∗ 𝑄 = VI
𝑉2
𝑠𝑜 𝑃 = 𝑉𝐼 = 𝐼 𝑅 =
𝑅
2
Try p. 459 # 14 – 17
Series/Parallel Circuits
Demo
lampboard rheostat
hook it up both ways and students note the differences
What is Series?
What is Parallel?
KVL –
KCL –
Resistors in Series and parallel
Magnetism
Demo Magnetic Field lines (which are imagnary, they just help us visualize)
Always form closed loops and go from N to S(outside the material)
Notes The number of magnetic field lines passing through a surface is called magnetic flux.
The flux per unit area represents the strength of the magnetic field at that point.
Where the field lines are closest together, the magnetic field is strongest.
Field lines have direction. The arrow always points from north to south (outside
of the bar magnet).
1st RHR - with the thumb pointing in the direction of the current, the fingers
point in the direction of the field lines that are generated.
2 - RHR - with electromagnets, the fingers wrap around the wire in the direction
that the positive current flows and the thumb points to the north end of the
electromagnet.
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