# How can I detect an electric field?

I can easily detect the presence of magnetic field over a surface by using compass or by placing some iron filings. But how could we similarly detect electric field over a place?

I can easily detect the presence of magnetic field over a surface by using compass or by placing some iron filings. But how could we similarly detect electric field over a place?

If all you want is a visual demonstration of the electric field then you can use an electric compass needle which is the equivalent of a magnetic compass needle

as described here.

or you could use semolina (instead of iron filings) immersed in castor oil

as described here

• This answer got me thinking: can you detect an homogenous electric field with the "electric needle" (I don't think so) and can you detect an homogenous magnetic field with a compass (again, I don't think so, but not as sure as in the first case) Oct 21, 2018 at 9:15
• @Jasper What do you mean by a homogeneous electric field? Oct 21, 2018 at 9:16
• The thing between infinite charged conducting plates. Oct 21, 2018 at 9:30
• @Jasper The electric compass needle works because charges are induced on the pointer with the greatest density of charges being at the two most pointed ends. You then have an the equivalent of an electric dipole which orientates itself parallel to the field as in that position there is no net torque on it. So the needle will orientate itself parallel to the electric field lines even if the electric field is uniform. As a demonstration it is particularly good when used with a Van de Graaff generator with another earthed dome close to it. The shape of the field is easily demonstrated. Oct 21, 2018 at 11:31
• Yes, now I see. You will get torque, but the center of mass of the needle would'nt start moving. Oct 21, 2018 at 21:06

If u want to check if there is an EF in a space, take two small rods and an ammeter / voltmeter. Now connect the two rods to both the ends of ammeter / voltmeter and if the needle of the machine deflects as soon as u put it in some space, u can conclude there is an EF in that region of space