Presentation on theme: "Magnetism and Electromagnetism. The basics of magnetism Named for Magnesia, an island in the Aegean Sea >2000 years ago Lodestones or magnetite, Fe 2."— Presentation transcript:
The basics of magnetism Named for Magnesia, an island in the Aegean Sea >2000 years ago Lodestones or magnetite, Fe 2 O 3 Chinese used magnets as compasses in the 12 th century
The basics of magnetism 1750 in England, John Mitchell shows that magnetic poles obey the inverse-square law 1820 Oersted discovered that magnetism is related to electricity… Ampere later proposed that electric currents are the source of all magnetism
Magnetic Forces and Poles Like the electrostatic force it can repel or attract “regions” of magnets called magnetic poles give rise to magnetic forces All magnets have both a north and south pole Like poles repel, opposite poles attract No magnetic monopoles exist in nature
Magnetic Fields The space around a magnet is “altered”… the alteration is called a magnetic field Lines of force… always in closed loops
Magnetic materials (cont’d) Magnetic force is due to the motion of electric charges Magnetism is produced by two motions of electrons… their spin and their orbiting motion about the nucleus The space around a moving electric charge is “altered”… the alteration is called a magnetic field
Magnetic materials (cont’d) In many substances, magnetism produced cancels out In some materials… such cancellation does not occur… called Ferromagnetic materials such as iron, nickel, cobalt, and alloys e.g. alnico… an alloy of aluminum – nickel - cobalt
Electromagnetism 1820, Oersted finds that a current carrying wire will cause a compass needle to deflect Implication: the current carrying wire itself behaves like a magnet (since it can exert a force on another magnet… like a permanent magnet would) Let’s use field theory to understand what’s happening
Magnetic Field for a coil of wire called a solenoid
Electromagnetism (cont’d) 1820, Oersted finds that a current carrying wire will cause a compass needle to deflect… so there was a discovery waiting to be made… 1832, Michael Faraday found that a changing magnetic field could produce an electric current… electromagnetic induction Principle underlying electric generators
Contrasting Electric motors and generators Electric motors – convert electrical energy into mechanical energy… the motion of electrons into the motion of something else Electric generators – convert mechanical energy into electrical energy… the motion of something into the motion of electrons
+ - + + + + The world is filled with electrical charges: + + + + + - - - - - - - - -
What is this electrical potential called? Static Electricity -- - - - - - + + + + +
The build up of an electric charge on the surface of an object. The charge builds up but does not flow. Static electricity is potential energy. It does not move. It is stored.
Static Discharge… Occurs when there is a loss of static electricity due to three possible things: Friction - rubbing Conduction – direct contact Induction – through an electrical field (not direct contact)
Electricity that moves… Current: The flow of electrons from one place to another. Measured in amperes (amps) Kinetic energy
How can we control currents? With circuits. Circuit: is a path for the flow of electrons. We use wires.
There are 2 types of currents: Direct Current (DC) – Where electrons flow in the same direction in a wire.
There are 2 types of currents: Alternating Current (AC) – electrons flow in different directions in a wire
There are 2 types of circuits: Series Circuit: the components are lined up along one path. If the circuit is broken, all components turn off.
What is Resistance? The opposition to the flow of an electric current, producing heat. The greater the resistance, the less current gets through. Good conductors have low resistance. Measured in ohms.
What Influences Resistance? Material of wire – aluminum and copper have low resistance Thickness – the thicker the wire the lower the resistance Length – shorter wire has lower resistance Temperature – lower temperature has lower resistance
What is Voltage? The measure of energy given to the charge flowing in a circuit. The greater the voltage, the greater the force or “pressure” that drives the charge through the circuit.
Difference b/t Volts and Amps Example – you could say that… Amps measure how much water comes out of a hose. Volts measure how hard the water comes out of a hose.
Ohm’s Law Resistance = Voltage / Current Ohms = Volts / Amps
Practice with Ohm’s Law OhmsVoltsAmps 410025 1515010 23015 9455 6488
What is an electromagnet? Electromagnet – a magnet made from a current bearing coil of wire wrapped around an iron or steel core.
What is a generator? Generator – a machine that changes mechanical energy to electrical energy Usually use moving magnets to create currents in coils of wire.
What is a motor? Motor – a device that changes electrical energy to mechanical energy that can do work.