We know spinning electrons make a magnetic field… …so do FLOWING electrons! Hans Christian Oersted 1820 accidently discovered this link between magnetism and electricity. DEMO – compass and current-carrying wire

Magnetic Field and Right Hand Rule

A loop of wire results in a stronger field inside loop… Right hand rule still holds

A coiled wire strengthens magnetic field even more Field strengthened along axis. Looks just like magnetic field around bar magnet! “Solenoid”

Electromagnets If you wrap a wire around a non-magnetized piece of iron, then send current through the wire, you can create a magnet that turns on and off! (The iron becomes a magnet when the electric current is on.)

Electromagnets

What if we send current through a wire that is already in a magnetic field? The magnetic fields interact There will be a FORCE on the wire PERPENDICULAR to the magnetic field

Extension of Right Hand Rule Thumb points in direction of current (or positively charged particles) Fingers point in direction of magnetic field (N to S) Palm is direction of force

Left Hand for Negative! For negatively charged particles, like electrons, use your LEFT HAND.

Force on Charged Particles Any flowing charges in a magnetic field will have a force on them Oscilloscope has a beam of electrons that hit screen – see as dot Can deflect beam by using magnets This is the idea behind Cathode Ray tubes on old TV screens

Particle Acclerators Extremely large-scale machines that accelerate elementary particles (like electrons or protons) to extremely high energies (close to the speed of light) Strong magnets direct and accelerate these charged particles to a target

Particle Accelerators The target (which can be atoms or other elementary particles), is usually located inside a magnetic field in which charged particles are bent. The bending radius and mass of particle can be used to calculate the particle’s momentum

Magnetic Force Equations The equation for the Force on a charged particle is given by: Fmag = qBv Where q = charge (in Coulombs) B = magnetic field (in Teslas) v = velocity (in m/s) then Fmag will be in Newtons

Magnetic Force Equations The equation for the Force on a current-carrying wire in a magnetic field: Fmag = B I l Where B = magnetic field (in Teslas)) I = current (in Amps) l = length of wire in field (m)

Loudspeakers Sound signal converted to electrical signal by microphone Electrical signal is amplified and sent to speaker

How a speaker works Varying electric current causes varying magnetic force on coil Alternating force on coil  vibrations on cone Air vibrates  sound

Motor Effect We’ve seen that when a current passes through a wire that cuts across a magnetic field, a force occurs on the wire. This is called the MOTOR EFFECT – when an electric current produces motion

Simple Electric Motors

Natural question that followed… We know a current carrying wire can induce a magnetic field… Can you induce a current by using a magnetic field?

Generator Effect When the motion of a wire in a magnetic field induces an electric current, this is called the GENERATOR EFFECT DEMO – magnet, wire and ammeter

Electromagnetic Induction Inducing a current in a circuit by changing the magnetic field that passes through the circuit No battery or power source needed!

Electromagnetic Induction

Generators

Recap: Motors Motors are machines that convert electrical energy into mechanical energy Current is supplied to loop by voltage source Magnetic Force on current loop causes it to rotate

Recap: Generators Generators are devices that convert mechanical energy into electrical energy Turbines (connected to wire loop) are turned in some way in magnetic field By moving water in hydroelectric plant By rising steam (water heated up) in coal, natural gas, nuclear plant

Bar Magnet Into Coil of Wire Induced current produces it’s own magnetic field in OPPOSITE direction of the increasing magnetic field. We don’t get something for nothing by increasing the number of loops. It gets more difficult to push the magnet into coil with more loops.

Lenz’s Law Magnet falling through tube induces a current in the tube. This current creates its own magnetic field, and thus a force that opposes its falling Lenz’s Law: “An induced current is always in such a direction as to oppose the motion or change causing it”

Explaining Lenz’s Law

Another way of inducing voltage When switch on primary is closed, current flows through primary  A brief surge of current flow through secondary When switch is opened, another brief surge, but in opposite direction A change in magnetic field of primary induces voltage in secondary

Improving on the design.. Placing an iron core inside coils intensifies magnetic field Using AC current eliminates need to turn on and off

Improving even more… Even more efficient with an iron loop Changing the number of turns in the coil allows you to change the voltage on the secondary

Transformer A device for increasing or decreasing voltage through electromagnetic induction Works by inducing a changing magnetic field in one coil which induces an AC current in a nearby secondary coil Voltages can be STEPPED UP or STEPPED DOWN with a transformer

Transformer Equations Primary Voltage = Secondary Voltage # of Primary turns # of Secondary turns You DON’T get something for nothing! ELECTRIC POWER IN = ELECTRIC POWER OUT (voltage x current)primary = (voltage x current)sec.

Np = Vp Ns Vs

Transmitting Electrical Power Most electrical power is transmitted in AC not DC because it’s relatively easy to step up and step down voltages with transformers.

Transmitting Electrical Power Over great distances, power is transmitted at high voltages and low currents (more efficient, less heat lost) 120,000 V from power plants to cities 2400 V in cities 120 V to house

Another extension of electromagnetism If I shake a stick back and forth in water, I get waves on water surface If I shake a charged rod back and forth, we get something called ELECTROMAGNETIC WAVES

Electromagnetic Waves A moving electric charge is known as electric current Surrounding this moving charge is a changing magnetic field This changing magnetic field creates a changing electric field These regenerate each other No medium is required

Electromagnetic Waves James Clerk Maxwell first saw this connection between electrical and magnetic fields He developed a series of equations that showed that electricity, magnetism, and even light were manifestations of the same phenomenon: The electromagnetic field

Electromagnetic Waves All EM waves consist of oscillating electric and magnetic fields, which radiate outwards at the speed of light There exist a wide range of frequencies and wavelengths of these EM waves We call this the ELECTROMAGNETIC SPECTRUM Visible light is simply EM waves in the range of frequencies our eyes can sense!

Electromagnetic Waves Radio waves Microwaves Infrared Waves Visible Light Ultraviolet Waves X-Rays Gamma Rays All of these waves travel at c = 3.0 x 108 m/s 300,000,000 m/s