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When an electric current passes through a wire, a magnetic field is created around the wire. The lines of force are in circles around the wire. The field is strongest near the wire. The direction of the field is reversed if the direction of the current is reversed. We can use the right hand rule. Thumb points for current, fingers show the magnetic field direction Draw the current lines for: Solenoid: A long coil of wire wound around a tube that may contain an iron core or be filled with air. There is a magnetic field in and around the solenoid. Concentrated at the ends Parallel inside Spread out beyond the ends Reversing current, reverses the field lines Increasing current increases the strength The magnetic field inside a solenoid is used in cathode ray TV tubes & ‘magnetic resonance’ (MR) brain scanner. The magnetic field outside a solenoid is used in scrap yards machines and in electrical relays. Motor Effect: A force is created on a wire when it is in a magnetic field. Electric Motors work due to the motor effect. The wire and the magnet exert equal and opposite forces on each other. The force can be increased by: Increasing the current Using a stronger magnet The direction of the force is reversed if the direction of the current or the magnetic field is reversed. The force depends on the angle between the wire and the magnetic field lines. The force is: Greatest at 90° Zero then they are parallel Fleming’s left hand Rule: ▪ Thumb = Force ▪ Index Finger = Magnetic Field ▪ Middle Finger = Current The magnetic field of the magnet and the wire cancel out on one side and reinforce each other on the opposite side. This results in a force on the wire sideways towards the weakest part of the field. 1. 2. Define ‘motor effect’ What is Flemming’s left hand rule? A coil in a magnetic field can be forced to turn by passing a current through it. When a current goes through the coil, a force acts on each of the two long sides of the coil. As these forces are in opposite directions, they act to make the coil turn. A simple electric motor has a rectangular coil of wire that spins in a magnetic field when a current passes through the coil. In a practical motor, the rotating part of the motor or ‘armature’ consists of several evenly-shaped coils wound on an iron core. The iron core makes the field much stronger so the turning effect is much greater. Electromagnetic Induction: the creation of an e.m.f. when a wire moves across a magnetic field. Induced e.m.f. (makes an emf) How a generator works - a generator contains coils of wire that spin in a magnetic field. Basically the opposite of the motor effect. Instead of using an e.m.f. to move the wire, the wire is moved to make an e.m.f. If the wire is part of a circuit, the induced e.m.f. makes an electric current pass around the circuit. You can test a coil of insulated wire connected to an ammeter to see if a current is induced.