Download IB 5.1Electric fields Jan 17 Agenda

Physics 4 – Jan 17, 2017
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Do Now –
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Review of vector addition:
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Add 50 N @ 35 and 60 N @ 110
Objectives/Agenda/Assignment
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Objective:
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5.1 Electrostatics
Assignment:
 p205
#1-14
Agenda:
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About Charge
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Coulomb’s Law
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Electric Field Diagrams
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Current
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Electron drift
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Potential difference
About Charge
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Charge is a fundamental quantity of nature and is a feature of
all matter because matter consists of electrons and protons.
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1) Charge comes in two varieties: positive and negative
(arbitrary names)
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Electrons are negative, protons are positive.
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Neutral matter has approximately equal numbers of electrons and
protons
About Charge
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2) Charge is quantized. The charge of one electron is
the smallest amount of charge you can have. All
charges are some multiple of 1.6 x 10-19 C.
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Charge is measured in Coulombs and given the symbol Q or q.
3) Charge is conserved.
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Charge is neither created nor destroyed
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Touching two objects together instantly equilibrates and evenly
distributes their charges.
Coulomb’s Law
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Two charges will exert a force on one another.
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Opposite charges attract
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Force is proportional to both charges and inversely proportional to the square of
the distance between the two charges.
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Proportionality constant is k = 9.0 x 109 N m2/C2 (To make the units work)
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k = 1/4𝝅𝝐o O = permittivity of vacuum = 8.85 x 10-12 C2/Nm2
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𝑭=𝒌
𝑸𝟏 𝑸𝟐
𝒓𝟐
Same charges repel
A force just like any other. Added just like any other.
Electric Field Diagrams
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Mechanism for how charges act over a
distance: Electric field.
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Direction of electric field is in the same
direction as the force on a small
positive test charge q+
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Strength indicated by arrow length
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Direction: tail of field lines begin at +
and radiate outward.
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Direction: Arrow heads point toward a –
charge and radiate inward.
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Similar to gravitational field
Electric field formulas
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F = qE
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As a result, there is a formula of electric field based on Coulomb’s Law.
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𝑬=𝒌
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Can calculate electric field knowing the force and test charge or
the magnitude of the electric field present at a given distance from
an isolated charge.
𝑸
𝒓𝟐
Current
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Moving charges within a medium creates a current.
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Current is measured as a rate of transfer of charge per time.
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Symbol: I
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1) Positive charge moves in the opposite direction as electrons
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2) A charge can move faster than actual matter can move.
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3) The only thing that physically moves to transfer charge is the
electron. Positive charge is the absence of an electron.
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Q = It
Unit: Ampere, A
1 A = 1 C/s
(SI base unit)
Electron Drift
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So how fast do the electrons actually move? = Electron drift, v
Distance across a given segment of wire = vt
The volume of the cross section is the cross sectional area A times the
length or Avt
If there are n electrons in a given volume (#electrons/m3), the number
of electrons in the volume is nAvt
Q is created by n electrons, each with q charge, so Q = nqAvt
Current is the charge that passes in a given time I = Q/t
I = nqAvt/t
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So I
= nqAv
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v = electron drift speed and A is the cross sectional area of the wire
I = current,
n = number of e/m3
Electron Drift calculation
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How fast are electrons moving in a copper wire with a diameter of
1.5 mm when a current of 0.350 A is applied?
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Density of copper is 8.96 g/cm3. (Assume about 10 electrons per
copper atom are available for metallic bonding.)
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Compare this to the speed of charge on the wire?
Electric Potential Energy
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The electric force is a conservative force so there is a potential energy that
corresponds to the opposite of the work done by the conservative force.
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The state for 0 electrical potential energy is if a test charge is infinitely far away from
an isolated positive charge.
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As the charge moves closer to the charge, it is more and more repelled and the
electric force is doing negative work. The charge therefore is experiencing a positive
potential energy change.
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Positive charges are at a high potential energy
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The work done by the electric force is equal to r times the force or W= −𝒌
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The potential energy is the U = 𝒌
𝑸𝒒
𝒓
𝑸𝒒
𝒓
Electric potential
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The potential energy per unit charge is known as the electric
𝑸
potential or V = 𝒌
and is measured in volts.
𝒓
W = QV
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1 V = 1 J/C
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Electric potential is to electrical potential energy as height is to
gravitational potential energy. The test charge is akin to the mass.
Electric potential
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Positive charges create a relative high potential state.
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Negative charges create a relative low potential state.
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Moving from a lower to a high potential is a positive voltage.
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Just like height, the word potential can refer to both a fixed state
and a difference in states.
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Wherever there is an electric field there will be a potential
difference.
The electron Volt (eV)
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Because electrons are so small, the associated energies are very small,
much smaller than a J.
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1 eV is the energy of one electron charge over 1 volt
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1 eV = (1.6 x 10-19 C) (1 J/C)
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1 eV = 1.6 x 10-19 J
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If you get a very small amount of energy, convert to eV. Or you may be
given information in eV.
Exit slip and homework
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Exit Slip – What is the electrostatic force (magnitude and direction)
between a positive charge of 1.3 x 10-3 C and a negative charge
of 6.2 x 10-3 C when they are separated by a distance of 15.0 cm?
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What’s due? (homework for a homework check next class)
 P205
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# 1-14
What’s next? (What to read to prepare for the next class)
 Read
5.2 p 207-226