AP Physics C: Electricity and Magnetism · 2018-09-06 · AP Physics; Physics C: Mechanics, Physics C: Electricity and Magnetism Course Description. or “Terms Defined” in the
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
2018
AP Physics C: Electricity and MagnetismSample Student Responses and Scoring Commentary
General Notes About 2018 AP Physics Scoring Guidelines
1. The solutions contain the most common method of solving the free-response questions and the allocation ofpoints for this solution. Some also contain a common alternate solution. Other methods of solution alsoreceive appropriate credit for correct work.
2. The requirements that have been established for the paragraph-length response in Physics 1 and Physics 2 canbe found on AP Central athttps://secure-media.collegeboard.org/digitalServices/pdf/ap/paragraph-length-response.pdf.
3. Generally, double penalty for errors is avoided. For example, if an incorrect answer to part (a) is correctlysubstituted into an otherwise correct solution to part (b), full credit will usually be awarded. One exception tothis may be cases when the numerical answer to a later part should be easily recognized as wrong, e.g., aspeed faster than the speed of light in vacuum.
4. Implicit statements of concepts normally receive credit. For example, if use of the equation expressing aparticular concept is worth 1 point, and a student’s solution embeds the application of that equation to theproblem in other work, the point is still awarded. However, when students are asked to derive an expression,it is normally expected that they will begin by writing one or more fundamental equations, such as thosegiven on the exam equation sheet. For a description of the use of such terms as “derive” and “calculate” onthe exams, and what is expected for each, see “The Free-Response Sections Student Presentation” in theAP Physics; Physics C: Mechanics, Physics C: Electricity and Magnetism Course Description or “TermsDefined” in the AP Physics 1: Algebra-Based Course and Exam Description and the AP Physics 2: Algebra-Based Course and Exam Description.
5. The scoring guidelines typically show numerical results using the value 29.8 m sg , but the use of 210 m s is of course also acceptable. Solutions usually show numerical answers using both values when they
are significantly different.
6. Strict rules regarding significant digits are usually not applied to numerical answers. However, in some casesanswers containing too many digits may be penalized. In general, two to four significant digits are acceptable.Numerical answers that differ from the published answer due to differences in rounding throughout thequestion typically receive full credit. Exceptions to these guidelines usually occur when rounding makes adifference in obtaining a reasonable answer. For example, suppose a solution requires subtracting twonumbers that should have five significant figures and that differ starting with the fourth digit (e.g., 20.295 and20.278). Rounding to three digits will lose the accuracy required to determine the difference in the numbers,and some credit may be lost.
Overview The responses to this question were expected to demonstrate the following:
• The ability to apply Ampere’s law. • An understanding of the superposition of magnetic fields. • An understanding of magnetic forces and their effects on current-carrying wires placed in a
nonuniform magnetic field. • The ability to apply Faraday’s law. • The ability to apply Lenz’s law.
Sample: E Q3 A Score: 13
Parts (b), (c), (e), (f), and (g) earned full credit. Part (a) has a correct answer but does not show how Ampere’s law is used, so 1 point was earned. Part (b) has the correct relationship between the two magnetic fields and has a correct vector sum, which earned 2 points. Part (c) has a correct substitution into an expression that correctly relates the angle to the fields, which earned 2 points. Part (d) has a correct explanation that the net force is equal to zero but has no mention of torques, so 1 point was earned. Part (e) has the correct selection, which earned 1 point. Part (f) shows the correct derivative of the flux, a correct use of Ohm’s law, and a correct answer, which earned 3 points. Part (g) has a correct selection and correct explanations of flux and Lenz’s law, which earned 3 points.
Sample: E Q3 B Score: 7
Parts (a) correctly uses Ampere’s law and has a correct answer, which earned full credit of 2 points. Part (b) correctly uses vector addition of the magnitudes of the fields but has an incorrect value for B2, so 1 point was earned. Part (c) does not show a correct relation between the angle and the magnetic fields, so no points were earned. Part (d) has no discussion of the forces or the torques, which earned no points. Part (e) has an incorrect selection, which earned no points. Part (f) shows the derivative of the flux and uses Ohm’s law but has an incorrect answer, so 2 points were earned. Part (g) has a correct selection and a correct explanation of the flux and Lenz’s law, which earned 2 points.
Sample: E Q3 C Score: 5
Parts (a) and (b) earned full credit of 4 points. In part (c) no substitution or expression relating the angle to the fields are indicated, so no points were earned. Part (d) has an insufficient justification, so no points were earned. Part (e) has a correct selection and earned 1 point. Part (f) does not show the derivative of the flux or the use Ohm’s law, so no points were earned. Part (g) has no mention of flux or Lenz’s law, so no points were earned.