Chapter 10Chapter 10MagnetsMagnets

All magnets have the following All magnets have the following common properties:common properties:

Magnets always have two opposite “Magnets always have two opposite “polespoles,” ,” called called northnorth and and southsouth..

If divided, each part of a magnet has both If divided, each part of a magnet has both north and south poles; we never see an north and south poles; we never see an unpaired north or south pole.unpaired north or south pole.

When near each other, magnets exert When near each other, magnets exert magnetic forces on each other. magnetic forces on each other.

The forces between magnets depend on the The forces between magnets depend on the alignment of the poles; two unlike poles will alignment of the poles; two unlike poles will attract each other and two like poles will repel attract each other and two like poles will repel each other.each other.

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

History of History of MagnetismMagnetism

The magnetic force depended on the The magnetic force depended on the directiondirection and and orientationorientation of the two magnets and also on the of the two magnets and also on the distancedistance between them. The model of a magnetic between them. The model of a magnetic field was developed to describe how a magnet field was developed to describe how a magnet exerts magnetic force.exerts magnetic force.

First, every magnet creates an energy First, every magnet creates an energy field, called the field, called the magnetic fieldmagnetic field, in the , in the space around it.space around it.

Second, the Second, the fieldfield exerts forces on any exerts forces on any other magnet that is within its range. other magnet that is within its range.

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

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

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

As expected the more current would make As expected the more current would make an electromagnet stronger. an electromagnet stronger.

There are two ways to increase the current.There are two ways to increase the current. apply more voltageapply more voltage by adding a second battery. by adding a second battery. add more turnsadd more turns of wire around the nail. of wire around the nail.

The second method works because the magnetism in The second method works because the magnetism in the electromagnet comes from the the electromagnet comes from the totaltotal amount of amount of current flowing current flowing around around the nail. If there is 1 amp of the nail. If there is 1 amp of current in the wire, each loop of wire adds 1 amp to current in the wire, each loop of wire adds 1 amp to the total amount that flows around the nail. the total amount that flows around the nail. Ten Ten loops of 1 amp each make 10 total amps flowing loops of 1 amp each make 10 total amps flowing aroundaround. By adding more turns, the same current is . By adding more turns, the same current is used over & over to get stronger magnetism. used over & over to get stronger magnetism.

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

strong enough magnet.strong enough magnet.

The The magnetic forcemagnetic force exerted by an exerted by an electromagnet depends on three factors:electromagnet depends on three factors:

The The amount of electric currentamount of electric current in the in the wire.wire.

The The amount of iron or steelamount of iron or steel in the in the electromagnet’s core.electromagnet’s core.

The The number of turnsnumber of turns in the coil. in the coil.

Electric motorsElectric motors convert electrical convert electrical energy into mechanical energy.energy into mechanical energy.

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

All electric motors must have three All electric motors must have three things to work:things to work:

1.1. A rotating element (A rotating element (rotorrotor) with magnets.) with magnets.

2.2. A A stationary magnetstationary magnet that surrounds the rotor. that surrounds the rotor.

3.3. A A commutatorcommutator that switches the that switches the electromagnets from north to south at the right electromagnets from north to south at the right place to keep the rotor spinning.place to keep the rotor spinning.

The electromagnet The electromagnet mustmust switch from north to switch from north to south as each rotor magnet passes by to keep south as each rotor magnet passes by to keep the rotor turning. The switch that makes this the rotor turning. The switch that makes this happen is called a happen is called a commutatorcommutator..

Both Both electrical forceelectrical force and and magnetic forcemagnetic force exist between exist between electric chargeselectric charges. Scientists . Scientists now believe both forces are two aspects of now believe both forces are two aspects of one force, the one force, the electromagnetic forceelectromagnetic force..

A current through a wire creates a magnet. The A current through a wire creates a magnet. The reverse is also true: If a magnet is moved reverse is also true: If a magnet is moved through a coil of wire, then electric current is through a coil of wire, then electric current is created. created.

This process is called This process is called electromagnetic inductionelectromagnetic induction because a moving magnet because a moving magnet inducesinduces electric electric current to flow. current to flow.

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

the electricity flow.the electricity flow.

Electromagnetic inductionElectromagnetic induction enables us to enables us to transform mechanical energy (moving magnets) transform mechanical energy (moving magnets) into electrical energy. Any machine that causes into electrical energy. Any machine that causes magnets to move past wire coils generates electric magnets to move past wire coils generates electric currents. These machines include giant electric currents. These machines include giant electric power plants and computer disk drives.power plants and computer disk drives.

Power plantsPower plants use use electromagnetic electromagnetic inductioninduction to create electricity. A to create electricity. A generatorgenerator is is a combination of mechanical and electrical a combination of mechanical and electrical systems that converts kinetic energy into systems that converts kinetic energy into electrical energy. electrical energy.

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