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# Electrical and Electronic Systems: Electromagnetic Systems

• Drive in many mechanical systems provided by electric motors, which are electromagnetic devices.
• Magnetic fields are established around any current carrying conductor.
• By convention it is assumed that a magnetic field circulates clockwise around a conductor carrying current into the page and anticlockwise around a conductor where the current flows out of the page. This is the right hand screw rule.
• Solenoid - if a current carrying conductor is would into a cylindrical solenoid, magnetic field around each conductor merges to produce a field pattern associated with bar magnets. Right hand screw rule is used to find direction of field through the centre of the solenoid.
• Toroid - magnetic field associated with both single conductor and solenoid are non-uniform and difficult to calculate. Toroid = closed system, ring shaped core of magnetic or non-magnetic material wrapped with a conducting coil insulated from the core. Current flows in coil, uniform magnetic field is established and circulates around the ring core.
• Flux density = Flux/Cross-sectional Area
• Magnetic Force = Current x Number of turns in coil
• Flux = Magnemotive Force/Reluctance
• Resistance = Resistivity x Length / Cross-Sectional Area
• Reluctance = Length/(Permeability of Core x Cross-Sectional Area)
• Magnetic Field Strength = Magnemotive Force/Length
• Flux Density = Permeability of Core x Magnetic Field Strength
• In circuits with non-magnetic material, Flux Density and Magnetic Field Strength have a directly proportional relationship.
• Electromagnetic induction - wind two coils on a toroid, connect one through a switch to a battery, and the other wired to a galvanometer, which is used to measure small voltages. When switch is closed, galvanometer needle deflects and then returns to zero. When the switch is opened, the needle moves in the opposite direction before going back to zero.
• From his experiments Faraday was able to deduce that the magnitude of the emf, e, induced in the coil - as measured by the galvanometer - was proportional to the rate of change of magnetic flux linkages with the coil.
• emf = Number of turns x change in instantaneous flux / change in time
• emf = inductance x change in current/ change in time
• The emf induced in a coil is equal to its inductance multiplied by the rate of change of the current flowing through it.
• Mechanical Work done to move the conductor = force on the conductor x distance moved by the conductor
• Mechanical work done = electrical energy supplied
• Force = Flux Density x Length  x Inductance