AP Physics C: Electricity and Magnetism Sample Student

2017

AP Physics C: Electricity

and Magnetism

Sample Student Responses

and Scoring Commentary

Inside:

• Free Response Question 3

• Scoring Guideline

•

Student Samples

•

Scoring Commentary

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AP

®

PHYSICS

2017 S

CORING GUIDELINES

General Notes About 2017 AP Physics Scoring Guidelines

1. The solutions contain the most common method of solving the free-response questions and the

allocation of points for this solution. Some also contain a common alternate solution. Other methods of

solution also receive appropriate credit for correct work.

2. The requirements that have been established for the paragraph length response in Physics 1 and

Physics 2 can be found on AP Central at

https://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

correctly substituted into an otherwise correct solution to part (b), full credit will usually be awarded.

One exception to this may be cases when the numerical answer to a later part should be easily

recognized as wrong, e.g., a speed faster than the speed of light in vacuum.

4. Implicit statements of concepts normally receive credit. For example, if use of the equation expressing a

particular concept is worth one point, and a student’s solution embeds the application of that equation to

the problem 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 those given on the exam equation sheet. For a description of the use of such terms as “derive”

and “calculate” on the exams, and what is expected for each, see “The Free-Response SectionsStudent

Presentation” in the AP Physics; Physics C: Mechanics, Physics C: Electricity and Magnetism Course

Description or “Terms Defined” in the AP Physics 1: Algebra-Based and AP Physics 2: Algebra-Based

Course and Exam Description.

5. The scoring guidelines typically show numerical results using the value

g = 9.8 m s

2

,

but use of

10 m s

2

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

cases answers 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 the question typically receive full credit. Exceptions to these guidelines usually occur when

rounding makes a difference in obtaining a reasonable answer. For example, suppose a solution requires

subtracting two numbers that should have five significant figures and that differ starting with the fourth

digit (e.g., 20.295 and 20.278). Rounding to three digits will lose the accuracy required to determine the

difference in the numbers, and some credit may be lost.

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(a)

i. 2 points

For connections that create a circuit

with the

power

supply,

solenoid,

and resistor

in

series with each other

1 point

For connections that create a circuit with the multimeter in series with the

inductor/resistor

combination

1 point

ii.

1 point

For selecting

the answer consistent with the answer in part

(a)(i) 1 point

iii.

3 points

For selecting point

B

1 point

For any indication that the magnetic field outside the solenoid is less than inside the

solenoid

1 point

For any indication that the closer you are to the center

of the solenoid, the better the

approximation of an ideal solenoid

due to the lack of edge effects

1 point

Example:

The equation for

the magnetic field for

a solenoid is for an ideal solenoid.

This does not work outside the solenoid (this eliminates C

and D). In addition, as

you approach the edge of the solenoid, the ideal approximation is not as accurate

(this

eliminates

A). Thus,

point

B

gives the best approximation of an ideal

solenoid.

(b) 2 points

For any

indication

that Solenoid 1 has more turns per unit

length

1 point

For any indication that

Solenoid 1 is longer and therefore more like an ideal solenoid

1 point

Examples:

The equation for the magnetic field of

a solenoid









is based on an ideal

solenoid in which you can ignore edge effects. Because

Solenoid 1 has more turns

per unit

length, it is a better approximation of

an ideal

solenoid and thus its slope

will give a better value for

the permeability of

free space.

The slope for Solenoid 1 is closer to the accepted value than the slope for Solenoid 2;

therefore, the graph for Solenoid 1 is better.

(c)

i. 3 points

For correctly calculating the slope for the solenoid chosen in part (b) using the best-fit

line and not the data points

 



   

 

1 point

For correctly relating the magnetic permittivity of free space to the slope



1 point

For correct units







For Solenoid 2

 

  



   

 

1 point

ii. 1 point

For using a correct

equation for

% error

including the substitution of the accepted and

the

measured values

   

 

 



  



   



For Solenoid 2

   

 

 



  



   



1 point

(d)

i. 1 point

For a correct explanation involving any extraneous B fields

1 point

Example: A component of Earth’s magnetic field is in the direction of the inductor’s

magnetic field.

ii. 2 points

For stating

that

either

the solenoid or the circuit has near zero resistance

1 point

For stating that as a

result

of near zero resistance

the current becomes very high

1 point

Example:

The inductor has near zero resistance. Therefore,

without

the resistor

in series

with it,

the inductor

will

draw a near infinite current

that

the multimeter cannot

handle.

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AP

®

PHYSICS C: ELECTRICITY AND MAGNETISM



2017 SCORING COMMENTARY

Question 3

Overview

The responses to this question were expected to demonstrate the following:

• How to use a multimeter to measure current in a circuit.

• An understanding of the magnetic properties of an ideal solenoid, as well as the properties of a real

solenoid that best enable it to approximate the ideal.

• An understanding of how coil density and aspect ratio are invoked when using The Law of Biot &

Savart to

determine the value of magnetic field of an ideal solenoid.

• An understanding that real solenoids experience nonzero magnetic fields outside of the coils, as

well as edge

effects at the ends.

• An understanding that trendlines should be used rather than the data points from which they are

formed in determining physical

values from data.

• An understanding of how to compare measured and expected values using percent difference.

• An understanding that key physical values can be determined from graphs and how to extract

those values.

• An understanding that the Earth’s magnetic field can affect measurements of magnetic field

strength; no

experiment is in complete isolation from its surroundings. An understanding that real

solenoids have a nonzero resistance that is very small.

Sample: E&M Q3 A

Score: 11

Parts (a) and (b) earned full credit. In part (a)(i) all of the circuit components are connected in series, including

the

multimeter. Part (a)(ii) has the correct selection consistent with the connections in part (a)(i). Part (a)(iii)

has the correct selection and discusses both the strength outside the solenoid and edge effects. Part (b) has a

correct selection and discusses both coil density and length. Part (c)(i) correctly calculates the slope from the

line and not data points and relates the slope to the permeability, but does not include the units, so 2 points

were earned. Part (c)(ii) has an incorrect value for the measured value, so no credit was earned. Part (d)(i)

does not mention extraneous fields, so no credit was earned. Part (d)(ii) discusses excessively high current,

but does not discuss the resistance of the solenoid/circuit, so 1 point was earned.

Sample: E&M Q3 B

Score: 10

Parts (a)

(i) and (a)(ii) earned full credit. Part (a)(iii) has a correct selection and discusses edge effects but does

not mention the field strength, so 2 points were earned. Part (b) discusses the loops but not the length, so 1

point was earned. Part (c)(i) correctly relates the slope to the permeability constant and has correct units, but

it does not include





10

5

in the values from the vertical scale and, thus, the answer is incorrect and 2 points

were earned. In part (c)(ii) the answer is unreasonably high, but it is consistent with the incorrect answer in

part (c)(i), so full credit was still earned. Part (d)( i) does not mention extraneous fields, so no credit was

earned. Part (d)(ii) discusses excessively high current, but does not sufficiently discuss the resistance of the

solenoid/circuit, so 1 point was earned.

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AP

®

PHYSICS C: ELECTRICITY AND MAGNETISM

2017 SCORING COMMENTARY

Question 3 (continued)

Sample: E&M Q3 C

Score: 5

Part (a)(i

) earned full credit. Part (a)(ii) is inconsistent with part (a)(i), so no credit was earned. Part (a)(iii) has a

correct selection and attempts a justification, but the justification is incorrect, so 1 point was earned. Part (b)

discusses the loops but not the length, so 1 point was earned. Part (c)(i) correctly relates the slope to the

permeability constant but has incorrect units and does not include

¥

-

10

5

in the values from the vertical

scale; therefore, the answer is incorrect, so 1 point was earned. Part (c)(ii) does not calculate percent error, so

no credit was earned. Part (d)(i) does not mention extraneous fields, so no credit was earned. Part (d)(ii) does

not discuss excessively high current or the resistance of the solenoid/circuit, so no credit was earned.

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