Oersted: how can you be sure it is not an electric field?

[damosuz]

A teacher demonstrates Oerted's experiment in class. He places a compass on top of a straight wire and then shows that when a current flows in the wire, the compass needle moves. Since a compass always moves under the effect of a magnetic field, he says the experiment shows that an electric current produces a magnetic field.

A student asks him how can we be sure it is not an electric field that affects the compass needle. What should the teacher answer?

Thank you.

 

[Bob S]

Place a piece of aluminum foil (or aluminum sheet) between compass and wire (carrying a DC current), or wrap the wire with several layers of aluminum foil.

Bob S

 

[Hyperspace2]

A teacher demonstrates Oerted's experiment in class. He places a compass on top of a straight wire and then shows that when a current flows in the wire, the compass needle moves. Since a compass always moves under the effect of a magnetic field, he says the experiment shows that an electric current produces a magnetic field.

A student asks him how can we be sure it is not an electric field that affects the compass needle. What should the teacher answer?

Thank you.

Electric field will be in the plane perpendicular to the motion of current, and Magnetic field will be perpendicular to the electric field. Right hand rule. Direction of current =E X H, where X denotes Cross product.
Because compass kept over top of wire.
So electric field cannot make deflection ! . Only Magnetic can.

 

[damosuz]

Let's assume that the only thing the student knows about at this stage of the course is how static charges produce electric fields and how electric fields affect matter. The teacher has to show him it cannot be an electric field that makes the needle move.

Bob S, could you tell me more about using the aluminum foil? What would that show?

 

[Vanadium 50]

You guys are making it way too hard.

(1) You measure the electric field (e.g. via the force on a test charge) and show that it is zero.

or, (2) You show that the compass responds to magnetic fields and not electric fields.

 

[Bob S]

Bob S, could you tell me more about using the aluminum foil? What would that show?

Aluminum (or copper) is a non-magnetic electrical conductor, and will block electrical fields, and not DC magnetic fields. In addition, the aluminum foil can be grounded to block any electrostatic electric fields.

If placing the aluminum foil between the wire and the compass has no effect on the deflection of the compass needle, the deflection must be due to something other than electric field from the wire. e.g., magnetic field.

Bob S

 

[damosuz]

You guys are making it way too hard.

(1) You measure the electric field (e.g. via the force on a test charge) and show that it is zero.

or, (2) You show that the compass responds to magnetic fields and not electric fields.

Thanks for your input. However I still see some problems:

(1) If you do that, you will find that there is actually an electric field in the surroundings of the wire (the field due to the static charge on the surface of the wire responsible for the electric field inside the wire that is itself responsible for the DC current). Thus the student could say this electric field is the one that makes the needle of the compass move. He would be wrong, but the teacher will not be able to show him why...

(2) But the teacher would have to put the compass in every possible configuration of electric field, which is impossible! The student could always say the wire may produce a configuration you haven't tried...