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Electromagnetic induction
- 1. CONTENTS Atomic Structure Electricity and Magnetic Fields Faraday Experiment Electromagnetic Induction and Eddy Currents
- 3. An electric current is a flow of electric charge.
- 4. Electricity is a type of energy fuelled by the transfer of electrons from positive and negative points within a conductor.
- 5. In the copper atom the outside shell has only one electron. Conductive materials have a free electron in their valence shell and this electron can easily move from atom to atom.
- 6. Some common conductors are copper, aluminium, gold, and silver. Some common insulators are glass, air, plastic, rubber, and wood.
- 7. In electric circuits this charge is carried by moving electrons in a wire. A magnetic field is generated anytime an electrical current flows through a conductor.
- 9. Electromagnetic induction is the production of an electromotive force across a conductor exposed to time varying magnetic fields.
Editor's Notes
- Atoms are composed of protons, neutrons, and electrons. The forces of attraction between the electrons and the protons hold an atom together. Electrons are the smallest and lightest of the particles in an atom. Electrons are in constant motion as they circle around the nucleus of that atom. Electrons are said to have a negative charge, which means that they seem to be surrounded by a kind of invisible force field. Protons are much larger and heavier than electrons. Protons have a positive electrical charge.
- Notice that in the copper atom pictured below that the outside shell has only one electron. This represents that the copper atom has one electron that is near the outer portion of the atom. The outer shell of any atom is called the valence shell. When the valence electron in any atom gains sufficient energy from some outside force, it can break away from the parent atom and become what is called a free electron.
- The term electromagnetism is defined as the production of a magnetic field by current flowing in a conductor. We will need to understand electromagnetism in greater detail to understand how it can be used to do work. Electricity can also be used to make measurements and inspect conductive materials. When electricity is flowing in a coil, it produces a magnetic field in and around the coil. This magnetic field of the coil can then be used to generate electrical current in another conductor. These electrical currents are called eddy currents because they travel in circles like eddy currents in a stream. The strength of the eddy currents can be measured and this provides useful information about the material.
- Induction Current is produced in a conductor when it is moved through a magnetic field because the magnetic lines of force are applying a force on the free electrons in the conductor and causing them to move. This process of generating current in a conductor by placing the conductor in a changing magnetic field is called induction. This is called induction because there is no physical connection between the conductor and the magnet. The current is said to be induced in the conductor by the magnetic field.
- Eddy current testing is based on the physics phenomenon of electromagnetic induction. In an eddy current probe, an alternating current flows through a wire coil and generates an oscillating magnetic field. If the probe and its magnetic field are brought close to a conductive material like a metal test piece, a circular flow of electrons known as an eddy current will begin to move through the metal like swirling water in a stream. That eddy current flowing through the metal will in turn generate its own magnetic field, which will interact with the coil and its field through mutual inductance. Changes in metal thickness or defects like near-surface cracking will interrupt or alter the amplitude and pattern of the eddy current and the resulting magnetic field. This in turn affects the movement of electrons in the coil by varying the electrical impedance of the coil. The eddy current instrument plots changes in the impedance amplitude and phase angle, which can be used by a trained operator to identify changes in the test piece.