electromagnetism

• a moving charge creates a magnetic field
• when a current flows through a wire, a magnetic field is created around the wire. the field is made of concentric circles perpendicular to the wire, with the wire in the centre
• changing direction of current changes direction of magnetic field
• the strength of the magnetic field produced changes with the current and the distance from the wire. the larger the current through the wire, or the closer to the wire you are, the stronger the field is
• you can increase the strength of the magnetic field that the wire produces by wrapping the wire into a coil called solenoid
• solenoid allows the field lines around each loop wire to line up with each other, resulting in lots of field lines pointing in the same direction that are very close to each other, making the field stronger
• the magnetic field inside a solenoid is strong and has the same strength and direction at every point
• you can increase field strength of the solenoid more by putting a block of iron in the centre of the coil. this iron core becomes an induced magnet whenever the current is flowing
• if you stop the current, the magnetic field disappears. a solenoid with an iron core is called an electromagnet
• you can turn electromagnets on and off. used in some cranes to attract and pick up things made of magnetic materials. switch off to drop it. can be used in other circuits as switches. when the switch is closed, it turns on the electromagnet, which attracts the iron contact on the rocker. the rocker pivots and closes the contacts and turning on the motor
• change polarity (direction)

  Flip the switch to turn off the power to an electromagnet, which is a coil of wires through which a current flows. The flow of electricity through the wires induces a magnetic field in the coil -- one pole is at the top of the coil and the pole is at the bottom.
• 3 ways of increasing strength of magnetic field produced by a solenoid

  -increase size of current increases strength of magnetic field

  -increase number of turns in coil

  -placing piece of iron in solenoid, called an iron core