Magnetic Fields
advertisement
Lesson 11: Magnetic Fields! ! Part 1: Introduction to magnetism! ! A magnet is made up of a north pole and a south pole! ! ! ! ! ! You can’t have an isolated north or south pole —> they always come together! ! ! ! ! ! ! ! ! ! Magnetic Forces! ! F is attractive for unlike poles! ! ! ! ! ! F is repulsive for like poles! ! ! ! ! ! ! m m Magnetic forces are action at a distance forces (like electric and gravitational forces) so they can be explained by magnetic fields. ! ! A magnetic pole in a magnetic field will always experience a force (attractive or repulsive depending on the type of pole)! ! Direction of magnetic field:! ! ! ! ! ! ! ! ! Examples of magnetic field lines:! !! !! ! !! !! !! !! ! ! Field Gravitational Electric Magnetic Source !! !! !! ! Earth’s Magnetic Field! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! Symbol Unit Direction Part 2: Induced Magnetic Fields in wires! ! In 1820, Hans Christian Orsted discovered the link between electricity and magnetism during a lecture. ! ! He set up a wire carrying electric current and noticed that a compass needle moved when the current was turned off and on.! ! Orsted’s discovery:! ! ! ! ! ! ! ! ! ! ! ! Hand rules: Used to find the direction of vectors in magnetism! ! Left hand !! !! ! ! Electron flow vs. conventional current:! ! ! ! ! ! Right Hand First Hand Rule: Use to determine the direction of the magnetic field around a wire carrying current. ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! Example: ! electron flow ! ! ! ! ! ! Example: ! ! electron flow ! ! ! ! ! ! ! ! ! ! ! conventional current ! ! ! ! electron flow New vector directions: Into the page Out of the page ! Example:! ! ! ! ! electron flow Example: (current is electron flow)! ! ! ! ! ! ! ! ! ! ! ! ! Part 3: Induced Magnetic Fields Around Solenoids! ! If a current carrying wire is looped, the magnetic field will spiral around it.! ! ! The result is that the magnetic field is one direction inside the loop and the other direction outside. ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! This is magnified if you add more loops. A solenoid is a type of electromagnetic that is made from a series of looped wires:! ! ! Second Hand Rule: Gives the direction of a magnetic field of a solenoid! Example: Will these solenoids attract or repel?! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! ! Example: Draw the magnetic field inside and outside the solenoid! ! ! ! ! Practice Problems:! ! 1.! In the diagram below, a compass is placed under the conductor and the conductor is carrying conventional current from A to B. In which direction will the compass needle point?! ! ! ! ! ! ! ! ! 2.! A answer here B In the diagram below, a compass is placed over a conductor and the conductor is carrying electrons from B to A. In which direction will the compass needle point?! ! ! ! ! ! ! ! ! 3.! A B In the diagram below, the compass is pointing toward the right side of the page. In which direction is current flowing in the conductor, A to B or B to A?! ! ! ! ! ! ! ! ! 4.! A _____ to _____ B In the diagram below, current is flowing up out of the page. In what direction will a compass needle point at point B?! ! ! ! ! ! ! ! 5.! ! 6.! ! B• • A magnetic compass is allowed to come to rest in a north-south orientation. A wire is placed over it, and connected so that the electron flow is from north to south. Will the compass needle deflect clockwise or counterclockwise as seen from above?! A wire is connected in a North-South direction and a compass is placed over top of the wire and allowed to come to rest in the North-South direction. Electrons are allowed to flow through the wire from North to South. Will the compass needle deflect clockwise or counterclockwise as seen from above?! 7.! ! 8.! ! 9.! ! A magnetic compass is allowed to come to rest in a North-South direction. A wire is placed over it and connected so that the electrons flow from South to North. Will the compass needle deflect clockwise or counterclockwise as seen from above?! A wire is connected in a North-South direction and a compass is placed over top of the wire and allowed to come to rest in the North-South direction. Electrons are allowed to flow through the wire from South to North. Will the compass needle deflect clockwise or counterclockwise as seen from above?! If you fire a negatively charged bullet horizontally and watch it recede away from you, what is the direction of the magnetic field that surrounds it? (clockwise or counterclockwise)! 10. If you fire a positively charged bullet horizontally and watch it recede, what is the direction of the magnetic field that surrounds it?! ! ! 11. ! ! .! ! ! ! ! ! ! A Ping-Pong ball of mass 3.0 x 10-4 kg is hanging from a light thread 1.0 m long, between 2 parallel plates 10 cm apart (as shown to the right). When the potential difference across the plates is 420 V, the ball comes to equilibrium 1.0 cm to one side of its original position.! a)! What is the electric field intensity between the plates? (4.2 x 103 N/C)! b)! What is the tension in the thread? (2.9 x 10-3 N)! c)! What is the magnitude of the electric force deflecting the ball? (2.9 x 10-5 N)! d)! What is the charge on the ball? (7.0 x 10-9 C)! 1.0 m 1.0 cm 10 cm 12. Two wires that are parallel to each other with current flowing in opposite directions, do they attract? Or repel?! ! ! 13.! In the diagram below, electrons are flowing from A to B. Which end of the solenoid becomes the north pole?! ! ! ! ! ! ! ! ! ! ! ! A B 14.! In the diagram to the right, current is flowing from A to B. Which end of the solenoid becomes the south pole?! ! ! ! ! ! ! ! 15.! In the diagram to the right, the top of the solenoid is the induced north pole. Which way is the current flowing, A to B or B to A? ! ! ! ! ! ! ! ! ! ! ! 16.! A coil of wire is placed on a table with its axis in the vertical direction. A flow of electrons is sent through the wire in the counterclockwise direction as seen from above. In which direction (up or down) is the North end of the magnetic field inside the coil generated?! ! 17.! Draw the magnetic field lines for the following configurations. Indicate whether the objects experience an attractive or repulsive magnetic force. Do the drawings on a separate piece of paper. (Do not cram them onto this page.)! ! ! 18.! (a) A large flat plastic disk is negatively charged. if the disk is spinning clockwise as seen from above, in what direction (up or down) will the North end of the generated magnetic field point?! ! (b) Answer the same question for a positively charged disk.! ! 19.! Is the North Magnetic Pole area on Earth a North Pole or a South Pole? Explain using a diagram of the Earth’s magnetic field.! ! 20.! ! ! ! ! ! Identify the sources of electric fields, magnetic fields, and gravitational fields.!
