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Magnetic Forces

The Superconducting Magnets The superconducting magnets To study the properties of elementary particles such as protons, neutrons,etc., which needs to be accelerated in very high magnetic fields, to achieve such large magnetic fields, super conducting materials such as niobium–tin and niobium-titanium are used as coil winding's. These are termed as superconducting magnets, with solenoid geometries as shown in the figure. Most of the high energy accelerators now use superconducting magnets, to accelerate the particles. These magnets are also used in the construction of MRI (Magnetic Resonance Imaging) apparatus for medical imaging. Lets learn more about such applications of magnets and its forces in this topic.

Learning objectives

After completing the topic, the student will be able to:

  • Understand why a moving charge gets deflected in a strong magnetic field and determine the force experienced by the charge moving in the magnetic field.
  • Discuss the importance of Fleming’s left hand rule in determining the direction of force on the conductor carrying current in a uniform magnetic field.
  • Discuss and relate magnetic field and flux through a surface and investigate how to measure the flux through any given surface using Gauss’ law.
  • Investigate and explore the motion of a charged particle in a magnetic field, with relevance to practical observations like trajectory of electron beam in a helmholtz coil.
  • Discover the working of a mass spectrometer and its application to determine the mass of a charged particle.
  • Discuss the hall effect and its application to determine the force on the charged particle in a uniform magnetic field.
Magnetic field of a bar magnet Magnetic field of a bar magnet Iron filings in the magnetic field around a bar magnet. Flux lines which make up the magnetic field around the magnet form closed loops flowing from its north pole to its south pole. Each iron filing becomes magnetized and aligns itself accordingly along the nearest line of flux. Although the magnetic field is actually continuous, interactions between the iron filings cause them to accumulate in thin arcing lines
Magnetic field

We discussed about the concept of electric field surrounding an electric charge. In the same way, we can imagine a magnetic field surrounding a magnet. The force one magnet exerts on another can then be described as the interaction between one magnet and the magnetic field of the other. We represented electric interactions in two steps:

  • A distribution of electric charge at rest creates an electric field E in the surrounding space.
  • The electric field exerts a force F = q E on any other charge q that is present in the field.

We can describe the magnetic interactions in a similar way:

  • A moving charge or a current creates a magnetic field (in addition to the electric field) in the surrounding space
  • The magnetic field exerts a force F on any other moving charge or current that is present in the field.

Magnetic field is analogous to the electric field. Like the electric field, the magnetic field is a vector field caused by the charges. Magnetic field is generally designated with the vector B. The SI unit of magnetic field is Tesla. Another unit of magnetic field is gauss (G) and 1 G = 10−4 T.

Magnetic force of a moving charge Magnetic force is attraction or repulsion that arises between electrically charged particles because of their motion. Electric force exists among stationary electric charges; both electric and magnetic forces exist among moving electric charges.
Magnetic forces

A charged particle at rest will not interact with a static magnetic field. But if the charged particle is moving in a magnetic field, the magnetic character of a charge in motion becomes evident. It experiences a deflecting force. The force is greatest when the particle moves in a direction perpendicular to the magnetic field lines. At other angles, the force is less and becomes zero when the particles move parallel to the field lines. In any case, the direction of the force is always perpendicular to the magnetic field lines and to the velocity of the charged particle. So a moving charge is deflected when it crosses through a magnetic field, but when it travels parallel to the field no deflection occurs. The force that causes this sideways deflection is very different from the forces that occur in other interactions, such as the gravitational forces between masses, the electric forces between charges, and the magnetic forces between magnetic poles.

The force that acts on a moving electron does not act along the line that joins the sources of interaction, but instead acts perpendicularly to both the magnetic field and the electron's path. This fact is employed to guide electrons onto the inner surface of a TV tube and provide a picture. More interestingly, charged particles from outer space are deflected by the earth's magnetic field. The intensity of harmful cosmic rays striking the earth's surface would otherwise be greater.

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