magnetic fields - 2026 ACF Regionals

Packet F: Bonus 17

The topology of these constructs is given by their helicity, which is the sum of their writhe and twist. For 10 points each:

[10e] Name these vector fields whose lines can be visualized using iron filings and are drawn from the north to south pole.

ANSWER: magnetic fields [accept magnetic field lines; or B-field]

[10m] For a particle in a non-uniform magnetic field, they experience a component of this quantity proportional to the gradient of B. For electrons in a uniform electric field, this quantity equals mobility times electric field.

ANSWER: drift velocity [prompt on drift; prompt on velocity; prompt on u]

[10h] Moving particles in a magnetic field also experience a drift velocity term named for and proportional to this quantity. In MHD, magnetic fields with a non-zero value of this quantity experience magnetic tension equal to this quantity times B-squared over permeability of free space.

ANSWER: curvature [accept curvature drift; prompt on kappa; reject “radius of curvature”]

<WashU A, Physics> | Packet F - Chicago C, McGill C, WashU A, Chicago B

Heard	PPB	E %	M %	H %
63	10.48	92%	13%	0%

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Conversion

Team	Opponent	Part 1	Part 2	Total	Parts
Berkeley B	Berkeley D	10	10	20	EM
Birmingham	Imperial B	10	0	10	E
Brock	McMaster B	0	0	0
Bruin	Alabama A	10	0	10	E
CWRU B	Ohio State C	10	0	10	E
Central Florida A	Florida B	10	10	20	EM
Chicago E	Northwestern C	0	0	0
Columbia A	Georgetown	10	0	10	E
Columbia B	NYU C	10	0	10	E
Cornell A	Cornell C	10	10	20	EM
Duke	NC State A	10	0	10	E
Durham	LSE	10	0	10	E
Edinburgh A	Cambridge A	10	0	10	E
Georgia Tech A	Washington B	10	0	10	E
Georgia Tech B	WashU B	10	0	10	E
Georgia Tech C	Kentucky	10	0	10	E
Guelph	Waterloo B	10	0	10	E
Harvard A	Brandeis B	10	10	20	EM
Harvard B	Bowdoin A	10	0	10	E
Illinois B	Illinois A	10	0	10	E
Imperial A	Bristol	10	10	20	EM
Imperial C	City St. George's	10	0	10	E
Indiana A	Purdue B	10	0	10	E
John Jay	Columbia C	10	0	10	E
Johns Hopkins A	Maryland B	10	0	10	E
Louisville B	Alabama B	10	0	10	E
MIT	Brown	10	0	10	E
McGill B	Western C	10	0	10	E
Michigan B	Michigan A	10	0	10	E
Minnesota A	Missouri A	10	0	10	E
Mississipi State	Louisville A	10	0	10	E
Montpellier	Southampton A	10	0	10	E
Murray State A	Texas A	0	0	0
NYU B	Maryland C	10	0	10	E
North Carolina C	South Carolina B	10	0	10	E
Notre Dame B	Chicago D	10	0	10	E
Ohio State A	Michigan C	10	10	20	EM
Ohio State B	CWRU A	10	10	20	EM
Oxford A	Cambridge B	10	0	10	E
Oxford B	Cambridge C	10	0	10	E
Penn State	RIT	0	0	0
Pittsburgh	Michigan State	10	0	10	E
Purdue A	Indiana C	10	0	10	E
Queen's	Simon Fraser	10	0	10	E
Rutgers A	Swarthmore	10	10	20	EM
Rutgers B	NYU A	10	0	10	E
South Carolina A	Wake Forest	10	0	10	E
Stanford A	Berkeley A	10	0	10	E
Stanford B	Berkeley C	10	0	10	E
Toronto B	Carleton University	10	0	10	E
UBC A	Western A	10	0	10	E
UC Irvine	UC Santa Barbara	10	0	10	E
UCL	Edinburgh B	10	0	10	E
UCLA A	Arizona State	10	0	10	E
UCLA B	UCSD A	10	0	10	E
UCSD B	UCSD C	0	0	0
Virginia	NC State B	10	0	10	E
Virginia Tech A	Liberty B	10	0	10	E
Virginia Tech B	James Madison A	10	0	10	E
Warwick A	Manchester	10	0	10	E
Washington A	Claremont	10	0	10	E
Waterloo A	Ottawa	10	0	10	E
William and Mary	North Carolina A	10	0	10	E

Summary

Tournament	Edition	Match	Heard	PPB	E %	M %	H %
California (North)	Main Site	✓	3	13.33	100%	33%	0%
California (South)	Main Site	✓	4	7.50	75%	0%	0%
Canada	Main Site	✓	7	8.57	86%	0%	0%
Florida	Main Site	✓	1	20.00	100%	100%	0%
Great Lakes	Main Site	✓	5	14.00	100%	40%	0%
Lower Mid-Atlantic	Main Site	✓	7	10.00	100%	0%	0%
Midwest	Main Site	✓	5	8.00	80%	0%	0%
Northeast	Main Site	✓	3	13.33	100%	33%	0%
Overflow	Main Site	✓	3	10.00	100%	0%	0%
South Central	Main Site	✓	1	0.00	0%	0%	0%
Southeast	Main Site	✓	5	10.00	100%	0%	0%
UK	Main Site	✓	10	11.00	100%	10%	0%
Upper Mid-Atlantic	Main Site	✓	7	11.43	100%	14%	0%
Upstate NY	Main Site	✓	2	10.00	50%	50%	0%