Magnetism and Electromagnetism

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 Field of a Bar Magnet

Earth has a magnetic field

Magnetic materials

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

Magnetic Field for Solenoid with an Iron Core

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

Electricity & Magnetism Static, Currents, Circuits Magnetic Fields & Electro Magnets Motors & Generators

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.

Series Circuit

There are 2 types of circuits: Parallel Circuit – there are several branching paths to the components. If the circuit is broken at any one branch, only the components on that branch will turn off.

Parallel Circuit

Conductors vs. Insulators Conductors – material through which electric current flows easily. Insulators – materials through which electric current cannot move.

Examples Conductors: MetalWater Insulators: StyrofoamRubberPlasticPaper

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

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.