6. Magnetism & Electromagnetism

• an a.c generator is a device such as a dynamo that produces alternating current (ac)
• an alternating current is an electric current that flows first one way then the other way in a circuit
• demagnetization is the process of removing the magnetism from a magnet
• the dynamo effect is when electricity is generated when a coil moves near a magnet
• electrostatic charge is a property of an object that causes it to attract or repel other objects with a charge
• magnetic materials include: iron, nickel, cobalt and neodymium
• a compass needle is attracted to the north pole of a magnet because it has a south pole at one end.
• opposite poles attract
• a magnetic field is a region of space where a magnet feels a force
• the direction of a magnetic field can be found by placing a compass near the source of the field
• when two magnets are brought together they experience an attractive or repulsive force depending on whether their like or unlike poles face each other
• the right-hand rule shows how the magnetic field lines are arranged around a wire carrying an electric current
• electric current produces a magnetic field around wires carrying an electric current
• the strength of the magnetic field depends on the amount of current flowing through the wire
• electric current produces a magnetic field, which surrounds the conductor
• alternating current (AC) generators use a split ring commutator to produce a unidirectional output voltage from the alternating input voltage
• in a simple DC generator, when a coil rotates between the poles of a permanent magnet, it cuts through the magnetic field and generates a potential difference across its ends
• in a DC motor, the current flows in one direction only because it has a split ring commutator
• to change the direction of a force you need to reverse the current and reverse the magnetic field
• an electric motor turns switches from positive to negative through brushes, forced up and down
• in a motor a current is passed through the coil, this produced a magnetic field around the coil. the interaction of the two magnetic fields produces a force. the forces on each side of the coil act in opposite directions, this produces a turning effect
• a transformer works by an alternating current in the primary coil producing a changing magnetic field. the soft iron core directs the changing magnetic field in the iron core. this induces an alternating potential difference across the ends of the secondary coil. if the coil is a part of a closed circuit there is an alternating current induced
• an iron core is used as it is easily magnetized and demagnetized and it makes the magnetic field stronger
• an alternating input produces a changing magnetic field which cuts through the secondary coil
• current is only induced in the secondary coil of a transformer when the magnetic field changes
• a changing magnetic field is a magnetic field with changing strength
• if a wire moves near a magnet it induces a potential difference in the conductor
• in a rotating-coil generator, a coil of wire is rotated inside a fixed magnetic field
• to induce a larger potential difference, add more turns in the coil of wire and make a faster change in the magnetic field
• alternators generate AC power because the direction of the force on the moving coil keeps reversing
• in a step-down transformer, the potential difference across the secondary coil is lower than the potential difference across the primary coil
• in a transformer that is 100% efficient, all of the energy transferred to the transformer by the primary coil will be transferred to any components connected to the secondary coil
• energy is lost when transmitting electricity as the current heats up the wires
• step-up transformers are used to increase the voltage from the generator at the power station to the voltage used in the primary transmission grids
• the term used to describe when magnetic materials become magnets in a magnetic field is induced magnetism
• solenoid is a coil of wire that produces a magnetic field around it
• in a step-up transformer the secondary coil has more turns than the primary coil
• factors effecting the speed of a DC motor are increasing the current and increasing the strength of the magnetic field
• the direction of a DC motor can be changed by reversing the direction of the current and reversing the direction of the magnetic fields by reversing the poles of the magnet
• the force supplied on a motor can be increased by increasing the current in the coil, increasing the strength of the magnetic field and adding more turns to the coil