1. Chapter 10 Magnets

2. All magnets have the following common properties:  Magnets always have two opposite “poles,” called north and south.  If divided, each part of a magnet has both north and south poles; we never see an unpaired north or south pole.  When near each other, magnets exert magnetic forces on each other.  The forces between magnets depend on the alignment of the poles; two unlike poles will attract each other and two like poles will repel each other.

4. A compass needle is a magnet that is free to spin until it lines up in the north-south direction. The origin of the terms “north pole” and “south pole” of a magnet come from the direction that a magnetized compass needle points. The end of the magnet that pointed north was called the north pole of the magnet and the end that pointed south was called the south pole.

6. History of Magnetism

7. The magnetic force depended on the direction and orientation of the two magnets and also on the distance between them. The model of a magnetic field was developed to describe how a magnet exerts magnetic force.  First, every magnet creates an energy field, called the magnetic field, in the space around it.  Second, the field exerts forces on any other magnet that is within its range.

8. Electromagnets = A magnet that is created by electric current. This type of magnet is called an electromagnet.

9. The right hand rule: When your fingers curl in the direction of current, your thumb points toward the magnet’s north pole.

10. Wrap the wire in many turns around the nail and connect a battery. When current flows in the wire, the nail becomes a magnet. Use the right hand rule to figure out which end of the nail is the north pole and which is the south pole. To reverse north and south, reverse the connection to the battery, making the current flow the opposite way.

11. As expected the more current would make an electromagnet stronger. There are two ways to increase the current.  apply more voltage by adding a second battery.  add more turns of wire around the nail. The second method works because the magnetism in the electromagnet comes from the total amount of current flowing around the nail. If there is 1 amp of current in the wire, each loop of wire adds 1 amp to the total amount that flows around the nail. Ten loops of 1 amp each make 10 total amps flowing around. By adding more turns, the same current is used over & over to get stronger magnetism.

12. Of course, nothing comes for free. By adding more turns the resistance of the coil is also increased. Increasing the resistance makes the current a little lower & generates more heat. A good electromagnet is a balance between too much resistance and having enough turns to get a strong enough magnet.

13. The magnetic force exerted by an electromagnet depends on three factors:  The amount of electric current in the wire.  The amount of iron or steel in the electromagnet’s core.  The number of turns in the coil.

14. Electric motors convert electrical energy into mechanical energy. Permanent magnets and electromagnets can work together to make electric motors and generators. The secret is in the ability of an electromagnet to reverse from north to south. By changing the direction of electric current, the electromagnet changes from attract to repel, and spins the motor.

15. All electric motors must have three things to work: 1. A rotating element (rotor) with magnets. 2. A stationary magnet that surrounds the rotor. 3. A commutator that switches the electromagnets from north to south at the right place to keep the rotor spinning. The electromagnet must switch from north to south as each rotor magnet passes by to keep the rotor turning. The switch that makes this happen is called a commutator.

16. Both electrical force and magnetic force exist between electric charges. Scientists now believe both forces are two aspects of one force, the electromagnetic force. A current through a wire creates a magnet. The reverse is also true: If a magnet is moved through a coil of wire, then electric current is created. This process is called electromagnetic induction because a moving magnet induces electric current to flow.

17. When a magnet moves into a coil of wire, it induces electric current to flow in the coil. The current stops if the magnet stops moving. If the magnet is pulled back out again, the current flows in the opposite direction. A changing magnetic field is what makes the electricity flow.

18. Electromagnetic induction enables us to transform mechanical energy (moving magnets) into electrical energy. Any machine that causes magnets to move past wire coils generates electric currents. These machines include giant electric power plants and computer disk drives. Power plants use electromagnetic induction to create electricity. A generator is a combination of mechanical and electrical systems that converts kinetic energy into electrical energy.

20. A power plant generator contains a turbine that turns magnets inside loops of wire, generating electricity. In the top sketch the north pole on the disk induces a south pole in the electromagnet, causing current to flow one way. When the disk rotates, the magnetism in the coil is reversed, and the electric current generated also reverses.