DASI-P - Ranking Magnetic Field Selected Response

What important physics idea is this question targeting?

Magnets are not like masses or electrical charges: the strength isn't how far you are from the center of the magnet but has to do with how dense the field lines are. So this targets if students understand how to determine that strength. (C)
This question targets whether or not students understand that magnetic field lines are closed loops and that the field has the same strength anywhere on that field line. (I)
This will determine if students can use images to determine the shape and strength of a magnetic field. (V)

Student A's response:

Which answer best describes what student A's response indicates about their physics understanding?

The student's response suggest that they think of field lines, which originate on north poles and end on south poles, as conveying a kind of energy or power that weakens as it travels. (C)
This student is able to draw the field lines correctly but does not interpret them correctly. With two field lines at A and D, and one each at B and C, the field should be strongest at A and B, but weaker and the same at C and D. (I)
The student is using strong reasoning of the field lines to make inferences about the field strength but could use more evidence to back up their ideas. (V)

Which of the following options would best support Student A in being able to correctly respond to this question?

I would have them use a simulator (like PhET) to measure the strength of the field at various points and use that data to decide if the agree or disagree with a set of statemetns that say things like "a magnetic field line points from a high magnetic field to a low magnetic field," "the field is always strongest closest to the north pole of a magnet" and " the density of field lines tells us the strength of the field." (C)
I would have them sprinkle iron filings over a bar magnet to help visualize the full magnetic field around the magnet, and then draw a more accurate reprseentation of the field. We would then discuss how the field looks the same towards the two ends of the magnet and different on the edges. (I)
I would give a lecture on magnetism with a note-catcher to ensure that they get the correct definitions for a field and for magnetism and then use that to re-work this problem. (V)

Student B's response:

Which answer best describes what student B's response indicates about their physics understanding?

They are treating the magnetic field as originating from one point in the middle of the magnet, and not all along the magnet.
The student has trouble drawing field lines appropriately for a bar magnet.
The student has not provided reasoning for their answer.

Which of the following options would best support Student B in being able to correctly respond to this question?

I would affirm for them that the distance from the magnet matters, but it isn't as simple as finding the distance to the center of the magnet. We could the model the magnet as made up of smaller magnets to make sense of how the field can be weak close to the middle of the magnet. (C)
I would begin by having them use a compass to measure the magnetic field at those four points around the magnet. (V)
I would remind them that field lines point out of north poles and into south poles; since the middle of the magnet is far from a pole the field lines must be weakest there. (I)