General Notes About 2007 AP Physics Scoring Guidelines

AP

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PHYSICS C: ELECTRICITY AND MAGNETISM 2007 SCORING GUIDELINES

General Notes About 2007 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. 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.

3. 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 contains the application of that equation to the problem but the student does not write the basic equation, the point is still awarded. However, when students are asked to derive and expression it is normally expected that they will begin by writing one or more fundamental equations, such as those given on the AP Physics 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 Course Description .

9.8 m s

2

=

g , but use of

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

10 m s is of course also acceptable. Solutions usually show numerical answers using both values

2

when they are significantly different.

5. 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.

© 2007 The College Board. All rights reserved.

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AP ^® PHYSICS C: ELECTRICITY AND MAGNETISM 2007 SCORING GUIDELINES

Question 2

15 points total Distribution

of points (a)

(i) 2 points

0

Q

enc

d ⑀

◊ =

Ú ^{E A Q}

^enc

r < a

enc

, where is the charge enclosed by the Gaussian surface Use a concentric sphere of radius as the Gaussian surface.

For correctly calculating Q 1 point

( )

^{3 33}

3

4

4 3

3

Q Qr

r a

a

r p

p

Ê ˆ

Á ˜

= = Á ˜ =

Ë ¯

Q

enc

V

E is normal to the surface everywhere, so applying Gauss’s law,

( ) ^{4 Qr}

³³

E r

p

²

a

0

1 Ê ˆ

= ⑀ Á Ë ˜ ¯

For the correct answer (This point was not awarded if no supporting work was shown.) 1 point

0 3

1 4 E Qr

p a

= ⑀ ^{or E} = ^kQr a

³

2 a < < r a

enc

Q

c

= Q (ii) 2 points

Use a concentric sphere of radius as the Gaussian surface.

For correctly identifying Q 1 point

en

E is normal to the surface everywhere, so applying Gauss’s law,

( )

²

^Q

₀

E ⁴ p r = ⑀

For the correct answer (This point was not awarded if no supporting work was shown.) 1 point

0 2

1 4 E Q

p r

= ⑀ ^{or E = kQ} _r

²

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6

AP ^® PHYSICS C: ELECTRICITY AND MAGNETISM 2007 SCORING GUIDELINES

Question 2 (continued)

Distribution

of points

2 a < < r 3 a

Q

enc _tal

(iii) 4 points

Use a concentric sphere of radius as the Gaussian surface.

1 point For recognizing that is the sum of two charges (or that is the sum of two

components)

E

to 0 0

Q

enc

= - Q r V , where r is the charge density of the outer sphere and V is the volume

₀

of the outer sphere enclosed by the Gaussian surface

0

( ) ( ) ( )

0

4

3

4

3

3 2

3 a 3 a

r

p p

= =

- 4

³

3 19

Q Q

p a

( )

³

( )

3 3 3

0

4 4 4

2 8

3 3 3

V = p r - p a = p r - a

For correctly calculating Q

enc

or just the charge enclosed by the shell 1 point

( ) ( (

^{3 3}

) )

^{33 33}

3

8

4 8 27

3 19 19 19

19

Q Qr Q Q r

r a Q

a a

p

Ê ˆ

Ê ˆ

Á ˜ - = - + = Á - ˜

Á 4 ˜ Ë ¯

3 Q

enc

Q

p a

= -

Ë ¯

Gauss’s law,

0

Q

enc

◊ d =

E A

Ú ⑀ , is applied with E normal to the surface everywhere.

For recognizing that _Ú ^{E ◊ d A = E} ( ) ^{4 p r}

²

enc

1 point For correctly substituting Q into Gauss’s law to find E 1 point

( )

³3

0

4 1 27

19

Q r

a E p r

2

= ⑀ Ê Á Ë Ê Á Ë - ˆˆ ˜˜ ¯¯

3

2 3

0

76 27

Q r

E p r a

Ê ˆ

= ⑀ Á Ë - ˜ ¯

3a < r Q

enc

0 - Q =

( )

or equivalent

(iv) 2 points

Use a concentric sphere of radius as the Gaussian surface.

For correctly calculating 1 point

Q

enc

= + Q

Applying Gauss’s law

2

0

4 Q

^enc

0

E p r = ⑀ =

For the correct answer (This point was not awarded if no supporting work was shown.) 1 point E = 0

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AP ^® PHYSICS C: ELECTRICITY AND MAGNETISM 2007 SCORING GUIDELINES

Question 2 (continued)

Distribution

of points

= - Ú

•

^{E i r 0}

(b) 3 points

For correctly identifying that V = 0 1 point

For a correct, complete explanation 2 points

Examples:

V

3a

d , and since E = 0 and V

_•

= , it follows that V = 0 OR Work to bring a charge from • to the outer surface is 0 since E = 0, so

(

3_a

) (

3_a

0 ) 0

W = q V - V

_•

= q V - = . Thus V

_3a

= 0 .

Note: 1 point only was awarded for a correct but incomplete explanation such as ∑ or E = 0.

0 q =

X

(c) 2 points

Let be the potential inside the outer sphere due to charge on the outer sphere. Use superposition of the potentials resulting from the charges on inner and outer spheres.

V

0

For correctly identifying V 1 point

0

V

X

V 4

0

Q p a

= + ⑀

( )

For correctly identifying V

_Y

1 point

0

4

0

2

Y

V V Q p a

= + ⑀

( ) ( )

0 0

0 0 0

1 1

4 4 2 4 2

X Y

Q Q Q

V V

a a a a

p p p

Ê ˆ

Ê ˆ

- = Á Ë + ⑀ ˜ Á ¯ Ë - + ⑀ ˜ ¯ = ⑀ -

V V

8

0

X Y

V V Q p a

- = ⑀

Alternate solution Alternate points

E dV

= - dr V E dr D = Ú

For setting up a correct integral with proper limits and sign (limits could be switched from those shown below if the integral had a positive sign)

1 point

For correct substitution of E in to the integral 1 point

( )

2 0 2 0

2 0

1 1 1

4 4 2

4

a a

X Y

a a

Q Q Q

dr r a a

r p p

p

Ê ˆ

- = - Ú _⑀ = - _⑀ Á Ë - ˜ ¯ = _⑀ -

V V

8

0

X Y

V V Q p a

- = ⑀

© 2007 The College Board. All rights reserved.

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8

©2007 The College Board. All rights reserved.

Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents).

©2007 The College Board. All rights reserved.

Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents).

©2007 The College Board. All rights reserved.

Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents).

©2007 The College Board. All rights reserved.

Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents).

©2007 The College Board. All rights reserved.

Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents).

©2007 The College Board. All rights reserved.

Visit apcentral.collegeboard.com (for AP professionals) and www.collegeboard.com/apstudents (for students and parents).

AP ^® PHYSICS C: ELECTRICITY AND MAGNETISM 2007 SCORING COMMENTARY

Question 2 Overview

The purpose of this question was to evaluate students’ ability to use Gauss’s law to find the electric field that resulted from a specified spherically symmetric charge distribution, as well as their understanding of the relationship between an electric field and electric potential.

Sample: E2A Score: 14

This very succinctly written response lost only 1 point for an incomplete explanation in part (b). In part (a)(ii) the explicit statement of the sphere acting as a point charge was sufficient for full credit. In part (a)(iv) the indication that the enclosed charge is zero implies applying Gauss’s law. Part (c) illustrates the bare minimum to get full credit for the superposition method.

Sample: E2B Score: 9

Parts (a)(i), (a)(ii), and (a)(iv) received full credit, but part (a)(iii) shows no work and earned no credit. Part (b) received 2 points, but the explanation is incomplete, referring only to sum of the charges. Part (c) uses the alternate method of integrating over the field and correctly substitutes for E, but the sign is incorrect for the limits of integration shown, so only 1 point was awarded.

Sample: E2C Score: 6

Part (a)(i) received no credit. Parts (a)(ii) and (a)(iv) each received 2 points full credit. No credit was given for part (a)(iii) for showing no work beyond writing down Gauss’s law. Part (b) was given only 2 points because the explanation is incomplete. Part (c) was left blank and hence earned no credit.

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