Monthly Archives: July 2018

Magnetic B-field of a Solenoid

The purpose of the solenoid simulation is to let the user explore the magnetic field inside and  surrounding a solenoid.  A complete worksheet is available to guide students in this exploration.  Note the following: 

  • B-field strength is proportional to the current in the solenoid (I amps), and the number of turns per meter (NTPM) in the solenoid coil.  The exact formula for a long solenoid is    B = μ 0  I NTPM ,  where μ 0 = 4 π × 10 -7 .
  • The B-field strength is proportional to the spatial density of the B-lines: that is, a lot of lines close together means a strong B-field
  • The B-field is strong and very uniform inside the solenoid, and very weak outside.  
  • The direction of the solenoid field is determined by the “Right-Hand-Rule” and the solenoid produces North and South poles similar to a magnet.  

In order access the dual current-loop website, use the following information:

Student Worksheet Downloadable Word Doc

We used the following word document as a lab procedure in the PHY112 class.  However, it can be used with more advanced or engineering-level classes, because the concepts are significant.  Document:

Solenoid-Bfield

Points for Discussion
  • Explain why the |B| at the end of a long solenoid is very close to half the B-field magnitude inside the solenoid?
  • The solenoid produces a uniform magnetic field which is very strong inside the solenoid.  How is this analogous to the capacitor and the electric field?
  • There is much similarity between B-fields produced by a bar magnet and a solenoid.  Is there an underlying similarity between magnets and solenoids that would explain this similarity?
  • Do adjacent turns in the solenoid coil attract one another or repel?

Magnetic Field Surrounding 2 Circulating Current Loops

The purpose of the current loop simulation is to let the user explore the magnetic fields surrounding one or two current loops.  A complete worksheet is available to guide students in this exploration.  Note the following: 

  • The number of B-lines is proportional to the current in the loops (I amps).   
  • The B-field strength is proportional to the spatial density of the B-lines: that is, a lot of lines close together means a strong B-field
  • B-lines are complete: that is, they close on themselves and do not terminate on any “magnetic charges”. “Magnetic charges” do not exist
  • The B-field is stronger near a wire carrying current.  

In order access the dual current-loop website, use the following information:

Student Worksheet Downloadable Word Doc

We used the following word document as a lab procedure in the PHY112 class.  However, it can be used with more advanced or engineering-level classes, because the concepts are significant.

ParallelCoils-Bfield

Points for Discussion
  • The magnetic field is a force field: The force on a current is perpendicular both to current vector and the B-field line. The magnitude of force is proportional to the current acted upon and to the spatial density of the B-field lines.
  • All magnetic field lines are complete loops. They appear to terminate on the poles of a permanent magnet. But actually, they do not stop, but penetrate into the magnet and continue right through it.
  • If the spacing between two loops carrying parallel currents is 1/2 the loop diameter, this is called a “Helmholtz Pair”. This configuration gives a rather uniform B-field in between the loops.
  • When the two loops are near to each other, and the currents are circulating in the same direction, the magnetic fields in between them tend to add.