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12.4 Solenoids p. 559-562 https://www.youtube.com/watch?v =EsJXZLwSCdA Ampere’s Experiment Investigated further aspects of electricity and magnetism Took two parallel wires to see if they would attract or repel one another when opposing currents were sent through them Two magnetic fields interact with one another to cause a force; force can be attractive or repulsive depending on the direction of the two fields If two field lines are in same directionrepulsion force applied If two field line are in opposite directionattractive force applied Solenoid A wire that has been bent into a loop has a stronger magnetic field than a straight wire conductor. We can represent the magnetic field lines created by a looped wire in two ways. Figure (a) shows the field lines all pointing in the same direction (into the page) on the inside of the loop, and out of the page outside the loop. We can also represent the magnetic field lines in a loop of wire in three-dimensional form as in Figure (b). An × represents a field line going into the page. A • represents a field line coming out of the page. If several loops of wire are used, the intensity of the magnetic field through the loop increases. A wire that has been looped many times to increase the intensity of the magnetic field when a current is applied is called a solenoid. A current passing through a solenoid produces a straight magnetic field that is similar to the magnetic field of a bar magnet. Discussion: Solenoids Section 12.4 Looking at the diagrams, how is a solenoid like a bar magnet? How are they unlike each other? Answers for Discussion Questions: Section 12.4 A solenoid and a bar magnet both have similar magnetic fields. However, the field of a bar magnet is always present, whereas a solenoid’s magnetic field can be turned on or off as part of an electric circuit. Usually an iron core is used in a solenoid since the magnetic fields pass through iron better than through air because the domains in iron align with the field, which strengthens the field. A solenoid is often referred to as an electromagnet because the magnetic field is generated by a current that flows through the wire. The magnet can be turned on or off by turning the current on or off. As the number of loops increases, the intensity of the magnetic field inside the loop also increases. Conventional current Use the RHR to determine the direction of the magnetic field in a solenoid for conventional current. If you grasp the solenoid with your right hand and wrap your fingers around the solenoid in the direction of conventional current, then your thumb indicates the direction of the magnetic field lines inside the core toward the north pole of the core magnet. Right hand rule for a solenoid Electron Flow Use the LHR to determine the direction of the magnetic field in a solenoid if the moving charge is negative. If you grasp the solenoid with your left hand and wrap your fingers around the solenoid coil in the direction of electron flow, then your thumb indicates the direction of the magnetic field lines inside the core. p. 561 Applications of Solenoids Subwoofer p. 561 An electric bell When switch is closed current is directed into the solenoid. The solenoids produce a magnetic field that is amplified by the soft iron cores. The soft iron armature is attracted to the core and the bell rings once. Now armature pulls away from the contact so the circuit is interrupted. Armature is on a spring and makes contact again…until switch opened Electromagnets An electromagnet uses a current-carrying solenoid to generate a magnetic field that is easy to switch on and off. The strength of an electromagnet can be increased by: • increasing the current through the wire. • increasing the number of loops in the solenoid • decreasing the diameter of the loops in the solenoid • changing the core of the solenoid Section 12.4 Solenoids Match each term to the appropriate description. Name • produces a magnetic field whose strengths can change depending on the value for the current, the number of turns in the wire, and the spacing of wire, etc. Description • amplifies a • produces a • solenoid magnetic field all magnetic field produced by the time an electric • soft iron core current • bar magnet Section 12.4 Solenoids Match each term to the appropriate description. Name Description • soft iron core • amplifies a magnetic field produced by an electric current • solenoid • produces a magnetic field whose strengths can change depending on the value for the current, the number of turns in the wire, and the spacing of wire, etc. • bar magnet • produces a magnetic field all the time Section 12.4 Solenoids in Action 1. Which features do the devices below have in common? spring battery contact point soft-iron bell armature switch light bulb soft-iron core In common: 2. Label the diagram using the features listed in Question 1. Section 12.4 Solenoids in Action 1. Which features do the devices below have in common? spring battery contact point soft-iron bell armature switch light bulb soft-iron core soft-iron spring contact point switch In armature common: 2. Label the diagram using the features listed in Question 1. switch spring battery spring soft-iron armature light bulb contact point soft-iron core bell switch soft-iron core soft-iron armature contact point soft-iron core soft-iron core Homework • 12.4 P. 562 questions #1,4,7