1 2 Magnetism Top view of iron filings sprinkled around a magnet The filings trace out a pattern of magnetic field lines in the space surrounding the magnet Interestingly the magnetic field lines continue inside the magnet not revealed by the filings and form closed loops 3 Magnetic Force 4 What causes magnetism 5 6 1 Magnetism Magnetism The 1st Right Hand Rule II Electric currents produce magnetism Hans Christian Oersted in 1820 accidentally discovered that a compass needle was affected by a nearby electric current The electric current produced a magnetic field Magnetic fields are caused by moving electric charges I When to use To find the direction of the magnetic field around a current carrying wire How to apply Point your thumb in the direction of the positive charge flow conventional current Your fingers wrap to show the direction of the magnetic field 7 8 Earth is a Magnet A suspended magnet or compass points northward because the Earth itself is a huge magnet The compass aligns with the magnetic field of the Earth 9 10 What Causes Magnetism Electric Current Causes Magnetism In 1820 a Danish HS teacher Hans Orsted made amazing discovery Wire carrying current deflects a magnet Basis of telegraph radio TV 11 12 2 Magnetic Force on Current carrying wire Solenoids Electromagnets Appropriate materials can be mad into magnets and this magnetic filed can be turned on and off e g videotape 13 Charges and magnetic Fields 14 Magnetic Force on CurrentCarrying Wires A charged particle at rest will not interact with a static magnetic field But if the charged particle is moving in a magnetic field the magnetic character of a charge in motion becomes evident It experiences a deflecting force Simple logic tells you that if a charged particle moving through a magnetic field experiences a deflecting force then a current of charged particles moving through a magnetic field experiences a deflecting force also 15 When the current flows in the wire that is placed in the magnetic field as shown the wire is forced upward If the wire is made to form a loop as shown the loop will tend to 16 When the current flows in the wire that is placed in the magnetic field as shown the wire is forced upward If the wire is made to form a loop as shown the loop will tend to 1 rotate clockwise 2 rotate counterclockwise 3 remain at rest 1 rotate clockwise 2 rotate counterclockwise 3 remain at rest 17 18 3 Can this property be harnessed The Speaker 19 20 Meter 21 Electromagnetic Induction If a wire carrying current causes a magnetic field is the inverse thru i e does a magnetic field cause current 22 Lenz s Law If a wire carrying current causes a magnetic field is the inverse thru i e does a magnetic field cause current An induced current flows in such a direction so that the magnetic field it creates opposes the change in magnetic flux that causes the current 23 24 4 Faraday s Law 25 26 27 28 Faraday s law The induced voltage in a coil is proportional to the product of the number of loops and the rate at which the magnetic field changes within those loops Energy In EM Field Summary of EM Induction Move a magnet near a electrical conductor and current flows in the conductor Move conductor near a stationary magnet and current flows in conductor Make an Electromagnet by passing current thru it move it near conductor and current flows in 2nd conductor loop Place 2 conductors in proximity change the current in one conductor current flows in other conductor A changing magnetic field produces an electric field 29 30 5 Generators and AC Current 31 32 Transformer A device used to increase or decrease an alternating potential difference Transformers 33 34 35 36 6 We see that if the secondary has more voltage than the primary it will have less current than is in the primary The ease with which voltages can be stepped up or down with a transformer is the principal reason that most electric power is ac rather than dc Check yourself 1 If 100 V of ac are put across a 100 turn transformer primary what will be the voltage output if the secondary has 200 turns 37 Power Transmission 38 Electron in magnetic Filed 39 40 Charged particle In magnetic and electric fields 41 7