Magnetic Field Pattern

Field Pattern around One Wire

Again, to find the magnetic field pattern around a wire, we can sprinkle iron filings around the wire or use a plotting compass as discussed in Magnetism with the setup as shown below.
The following magnetic field pattern will be observed for different direction of current flow through the wire. The direction of magnetic field is determined by Right Hand Grip Rule.
Note that the stronger the current flowing, the stronger the magnetic field strength produced.

Field Pattern between Two Wires

Flowing current in each wire produces its own magnetic field lines and interact with those produced by another wire, which resulting in a combined magnetic field lines.

Concepts for combined magnetic field:
  • Magnetic field lines must not cross each other.
  • Magnetic field strength decreases with increasing distance from the wire.
  • Magnetic field lines at same direction => strengthen the magnetic effect
  • Magnetic field lines at opposite direction => weaken the magnetic effect
Wires with like current

Like current means the current in the wires flow in the same direction.
① Field lines at opposite direction and equidistant from wires
=> Complete cancellation

② Field lines at opposite direction but not equidistant from wires
=> Weaken 
=> Field lines are further from the wire

③ Field lines at same direction
=> Strengthen 
=> Field lines are closer to the wire
Likewise for unlike current where current in the wires flow in the opposite direction.

Field Pattern around a Flat Coil

It is same as the field pattern for unlike current.
magnetic Field Pattern around a Flat Coil
Courtesy of PenguinPhysics

Field Pattern around a Solenoid

Solenoid is several hundred turns on a coil.

By combining each field pattern formed by a flat coil, we will get a field pattern around a solenoid as shown below. 
magnetic Field Pattern around a Solenoid
Courtesy of OnlinePhys
Its pattern is quite similar to that of a bar magnet. Thus, we can say that it acts like an electromagnet with two poles. The polarity of solenoid can be determined by Right Hand Grip Rule. However, unlike magnet, there are no field lines coming out from the side of solenoid.

To increase magnetic field strength in a solenoid, we can
  • Increase the current flowing through it;
  • Increase the number of turns on the coil per unit length;
  • Insert a soft iron core into the solenoid.

Note
  • The magnetic field is stronger inside the solenoid.
  • The magnetic field lines are parallel inside the solenoid, which means the magnetic field strength is uniform inside the solenoid.
  • Like Magnet, the direction of magnetic field is from South pole to North pole within the solenoid while the direction of magnetic field is from North pole to South pole outside the solenoid,
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