PHYS140 Ch31 D1 RQ - Siena Collegemmccolgan/GP140S11/Phys140_Ch31... · PHYS140_Ch31_D1_RQ.ppt...
Transcript of PHYS140 Ch31 D1 RQ - Siena Collegemmccolgan/GP140S11/Phys140_Ch31... · PHYS140_Ch31_D1_RQ.ppt...
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Reading Quiz
What quantity is represented by the symbol J ?
A. Resistivity B. Conductivity C. Current density D. Complex impedance E. Johnston’s constant
2. The electron drift speed in a typical current-carrying wire is
A. extremely slow (≈10–4 m/s). B. moderate (≈ 1 m/s). C. very fast (≈104 m/s). D. Could be any of A, B, or C. E. No numerical values were
provided.
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3. All other things being equal, current will be larger in a wire that has a larger value of
A. conductivity. B. resistivity. C. the coefficient of current. D. net charge. E. potential.
4. The equation I = ∆V/R is called
A. Ampere’s law B. Ohm’s Law C. Faraday’s Law D. Weber’s Law
Reading Quiz
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A. Resistivity B. Conductivity C. Current density D. Complex impedance E. Johnston’s constant
What quantity is represented by the symbol J ?
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A. extremely slow (≈10–4 m/s). B. moderate (≈ 1 m/s). C. very fast (≈104 m/s). D. Could be any of A, B, or C. E. No numerical values were provided.
The electron drift speed in a typical current-carrying wire is
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All other things being equal, current will be larger in a wire that has a larger value of
A. conductivity. B. resistivity. C. the coefficient of current. D. net charge. E. potential.
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The equation I = ∆V/R is called
A. Ampère’s law. B. Ohm’s law. C. Faraday’s law. D. Weber’s law.
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Grade for the “Initial Effort”:
• Satisfactory initial effort : 4pt • Missing explanations or steps:3pt • Major problem parts missing (or didn’t finish): 2pt • Very little coherent effort:1pt No initial effort: 0pt Note:
• it is a grade per problem. • This grade has nothing to do with whether that effort is correct: students can earn a full 4 points on this part and be completely wrong. • No comments are made on the problem.
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• Post the printed solutions on BB or on the website
• Students then use the printed solutions to correct their work using a different color ink.
• Re-collect the work and grade for
• Correction Quality
• Correction Needed
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Correction Quality:
Solution is now completely correct 3 Minor issues were not corrected 2 Major issues were not corrected 1 No correction effort 0
and for Correction needed:
No correction was necessary 3 Minor corrections were needed 2 Important corrections were needed 1 Initial effort needed a complete rewrite 0
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• Pushing on the sea of electrons with an electric field causes the entire sea of electrons to move in one direction like a gas or liquid flowing through a pipe. • This net motion, which takes place at the drift speed vd, is superimposed on top of the random thermal motions of the individual electrons. • The electron current is the number of electrons per second that pass through a cross section of a wire or other conductor. ne is the number density of electrons. • The electron current in a wire of cross-sectional area A is
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If Q is the total amount of charge that has moved past a point in a wire, we define the current I in the wire to be the rate of charge flow:
The SI unit for current is the coulomb per second, which is called the ampere. 1 ampere = 1 A = 1 C/s. The conventional current I and the electron current ie are related by
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The current density J in a wire is the current per square meter of cross section:
The current density has units of Amps/m2.
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For a junction, the law of conservation of current requires that
where the Σ symbol means summation. This basic conservation statement – that the sum of the currents into a junction equals the sum of the currents leaving – is called Kirchhoff’s junction law.
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The conductivity of a material is
Conductivity, like density, characterizes a material as a whole. The current density J is related to the electric field E by:
The resistivity tells us how reluctantly the electrons move in response to an electric field:
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The resistance of a long, thin conductor of length L and cross=sectional area A is
The SI unit of resistance is the ohm. 1 ohm = 1 Ω = 1 V/A. The current through a conductor is determined by the potential difference ΔV along its length:
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• Ohm’s law is limited to those materials whose resistance R remains constant—or very nearly so—during use. • The materials to which Ohm’s law applies are ohmic. • The current through an ohmic material is directly proportional to the potential difference. • Metal and other conductors are ohmic devices.
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• Van de Graff Generator • Capacitor Activity • Workbook p. 31.7 – 31.10