Magnetism. What causes magnetism? Magnetic Force

Transcription

1 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 4 Magnetic Force What causes magnetism? 5 6 1

2 Magnetism Magnetism 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. The 1 st Right Hand Rule 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 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

3 Solenoids: Electromagnets Magnetic Force on Current carrying wire Appropriate materials can be mad into magnets and this magnetic filed can be turned on and off, e.g. videotape Charges and magnetic Fields 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 15 Magnetic Force on Current- Carrying Wires 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 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 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

4 Can this property be harnessed? The Speaker 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

5 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. Faraday s law 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 Energy In EM Field A changing magnetic field produces an electric field

6 Generators and AC Current Transformers Transformer: A device used to increase or decrease an alternating potential difference

7 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. 37 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? 38 Power Transmission Electron in magnetic Filed Charged particle In magnetic and electric fields 41 7