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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.
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
A. Resistivity B. Conductivity C. Current density D. Complex impedance E. Johnston’s constant
What quantity is represented by the symbol J ?
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
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.
The equation I = ∆V/R is called
A. Ampère’s law. B. Ohm’s law. C. Faraday’s law. D. Weber’s law.
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.
• 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
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
• 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
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
The current density J in a wire is the current per square meter of cross section:
The current density has units of Amps/m2.
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.
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:
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:
• 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.
• Van de Graff Generator • Capacitor Activity • Workbook p. 31.7 – 31.10
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