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Transcript
Electromagnetism. We found out earlier that electric charges produce electric force fields So what is a magnetic field? Magnetic Field So how are magnetic fields produced? This experiment shows that the moving electrons in the wire also produce a…… Magnetic Field So electric charges always produce an electric field. If the electric charges are moving, they also produce a magnetic field Wire with electrons flowing OUT Produces circular magnetic field GRIP RULE Fingers curl in direction of thing going round a circle Thumb points in the direction of thing going straight Left Hand Grip Rule for Wires Electron flow Magnetic field What direction do the electrons flow? Left Hand Grip Rule for Coils Magnetic field Electron flow GRIP RULE Fingers curl in direction of thing going round a circle Thumb points in the direction of thing going straight What direction is the field in the coil? Which end is North?? Electrons Moving in Fields 200 Volts link to cathode ray tube Magnetic Force on Electrons electron flow SLAP Magnetic force on electron (out of page) Magnetic field electron flow magnetic field FORCE Find the direction of the Magnetic Force x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x Find the direction of the Magnetic Force x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x Find the direction of the Magnetic Force x x x x ELECTRON x COMING OUT x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x Trajectory of a Moving Electron Electrons cutting across a magnetic field bend in a circle LINK TO CHARGE IN A MAGNETIC FIELD The size of the magnetic force on a charge depends on: The strength of the magnetic field. The speed of the charge across the field The amount of charge F Bvq Find the size and direction of the Magnetic Force 3.0x106 ms-1 B = 0.040 T Find the size and direction of the Magnetic Force 3.0x106 ms-1 B = 0.040 T Find the size and direction of the Magnetic Force B = 0.040 T 3.0x106 ms-1 link to F=Bvq Magnet on a TV Aurora, Invercargill Aurora, Antarctica LHC Cern circumference = 27 km depth= 100 m Proton Tube Particle Detector What keeps the protons going in a circle? If the protons go clockwise, what direction is the magnetic field?? Your Particle Accelerator CRT. Electrons can be deflected using electric OR magnetic fields Colour Monitor link to electrons in a TV link to TV Ex. Proton mass = 1.67 x 10-27 kg. Proton charge = 1.6 x 10-19 C Circumference = 27 km calculate the magnetic field strength required when the protons are travelling at 0.99c Extension: there is a flaw in your calculation, what is it???? Measuring the mass of an electron with a “magic eye tube” No magnetic field small magnetic field large magnetic field By measuring the radius of the curve, we can calculate the magnetic field strength. mv Bvq r 2 Bqr m v Force on a Current Carrying Wire Electron flow current flow 1: electrons in roller are moving... 2: causing electrons to be pushed… 3: causing roller to be pushed…… This is called the motor effect electron motion (this is the moving wire) SLAP Magnetic force on electron (this is the current) Magnetic field The size of the electromagnetic force depends on: 1. 2. 3. Strength of magnetic field Current in wire Length of wire F BIl Applications Thousands of devices use this force to do something useful, e.g. The electric motor in…… The electric motor in…… The speakers in your phone or stereo ELECTRIC CURRENT MOTION ELECTRIC MOTOR Magnet + Wire current flow link to Fendt Electric Motor Parts of a Motor World’s Simplest Motor + e- flow Mag Field Loudspeaker The Current into a Speaker is Alternating S N AC from amplifier N S S PUSH N Electron flow AC from amplifier S S N N PULL AC from amplifier S N Electromagnetic Induction We have seen that…… ELECTRIC CURRENT MOTION ELECTRIC MOTOR Magnet + Wire Do you think that…… If you move a wire through a magnetic field you can produce electric current? MOTION ELECTRIC MOTOR Magnet + Wire ELECTRIC CURRENT Here’s a hint Tyger Tyger, burning bright, In the forests of the night; What immortal hand or eye, Could frame thy fearful symmetry Remember this? Electron motion Force x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x How about moving a wire full of electrons across a magnetic field? Move the wire Force on electrons x x x x x x x x Electron xmotion x x x x x x x x x x x x x x x x x x x x x x x + x x x x x x x x _ There is a voltage induced across the wire x x x x x x x x x x x x x x x x x x x onx Force x x x electrons x x x x x x x x x + x x x x x x x x _ Electron motion This is called……… Electromagnetic Induction It is used to generate almost all the electricity you use. It is also used in things like …… Voltage Induced in Moving Wire Y X 1: electrons in roller are moving 2: causing them to be pushed 3:The bottom end (X) becomes negative (Y becomes positive) 4:There is an induced voltage 1. 2. 3. The size of the induced voltage depends on: Strength of magnetic field Speed or wire across the magnetic field Length of wire V Bvl link to Fendt Generator Motor Effect and Generator Effect Electron flow Force on roller 1: electrons in roller are moving 2: causing them to be pushed 3: electrons in roller are now flowing 4: causing them (and roller) to be pushed diagram shows conventional current Microphone S N AC to amplifier S N Sound waves vibrate coil This causes alternating current We have seen that…… ELECTRIC CURRENT MOTION ELECTRIC MOTOR Magnet + Wire MOTION Generator Magnet + Wire ELECTRIC CURRENT ALTERNATING CURRENT SOUND SPEAKER Magnet + Wire ALTERNATING CURRENT SOUND MICROPHONE Magnet + Wire Extension James Clerk Maxwell showed that…… a changing electric field created a changing magnetic field. and a changing magnetic field created a changing electric field. he worked out these fields would travel through space at a speed of ……… 3.0 x 108 ms-1