Electricity and Magnetism

Electricity and Magnetism • Properties of Magnets • Magnetic Properties of Materials • The Magnetic Field of the Earth

Objectives • Predict the direction of the force on a moving charge or current carrying wire in a magnetic field by using the right-hand rule. • Explain the relationship between electric current and magnetism. • Describe and construct a simple electromagnet. • Explain the concept of commutation as it relates to an electric motor. • Explain how the concept of magnetic flux applies to generating electric current using Faraday’s law of induction. • Describe three ways to increase the current from an electric generator.

Vocabulary Terms gauss right-hand rule coil solenoid magnetic field tesla Faraday’s law induction induced current magnetic flux commutator generator electromagnet polarity

Electric Current and Magnetism Key Question: Can electric current create a magnet? *Students read Section 23.1 AFTER Investigation 23.1

Electric Current and Magnetism • In 1819, Hans Christian Oersted, a Danish physicist and chemist, and a professor, placed a compass needle near a wire through which he could make electric current flow. • When the switch was closed, the compass needle moved just as if the wire were a magnet.

Electric Current and Magnetism • Two wires carrying electric current exert force on each other, just like two magnets. • The forces can be attractive or repulsive depending on the direction of current in both wires.

Question • Opposite currents through parallel wires cause the wires to move • Away from each other • Towards each other • nowhere

Electric Current and Magnetism • The magnetic field around a single wire is too small to be of much use. • There are two techniques to make strong magnetic fields from current flowing in wires: • Many wires are bundled together, allowing the same current to create many times the magnetic field of a single wire. • Bundled wires are made into coils which concentrate the magnetic field in their center.

Electric Current and Magnetism • The most common form of electromagnetic device is a coil with many turns called a solenoid. • A coil takes advantage of these two techniques (bundling wires and making bundled wires into coils) for increasing field strength.

Question • A solenoid resembles which type of magnet • A BAR Magnet • A disk magnet • A cylindrical magnet

The true nature of magnetism • The magnetic field of a coil is identical to the field of a disk-shaped permanent magnet.

Electric Current and Magnetism • The electrons moving around the nucleus carry electric charge. • Moving charge makes electric current so the electrons around the nucleus create currents within an atom. • These currents create the magnetic fields that determine the magnetic properties of atoms.

Question: Concept check • How are electricity and magnetism related • Magnetism and electricity are two different concepts • Magnetism is caused by a movement of electrons • Magnetism is caused by a movement of atomic nuclei

Magnetic force on a moving charge • The magnetic force on a wire is really due to force acting on moving charges in the wire. • A charge moving in a magnetic field feels a force perpendicular to both the magnetic field and to the direction of motion of the charge.

Question • A magnetic field is pointed towards the left if the velocity of the particle is pointed out of the page. What direction is the force • Up • Down • Left • Right

Magnetic force on a moving charge • A magnetic field that has a strength of 1 tesla (1 T) creates a force of 1 newton (1 N) on a charge of 1 coulomb (1 C) moving at 1 meter per second. • This relationship is how the unit of magnetic field is defined.

Magnetic force on a moving charge • A charge moving perpendicular to a magnetic field moves in a circular orbit. • A charge moving at an angle to a magnetic field moves in a spiral.

B = 2x10-7 I r Magnetic field near a wire The field of a straight wire is proportional to the current in the wire and inversely proportional to the radius from the wire. Current (amps) Magnetic field (T) Radius (m)

Question • What is the strength of a magnetic field In a wire that has a diameter of 0.5 meters and is connected to a 9V battery and a 3 ohm resistor?

B = 2p x10-7 NI r Magnetic fields in a coil The magnetic field at the center of a coil comes from the whole circumference of the coil. No. of turns of wire Magnetic field (T) Current (amps) Radius of coil (m)

Question • What is the Magnetic force in a coil of wire that has 10 turns and has a radius of 0.5 meters if the current is 2 amps?

Calculate magnetic field • A current of 2 amps flows in a coil made from 400 turns of very thin wire. • The radius of the coil is 1 cm. • Calculate the strength of magnetic field (in tesla) at the center of the coil.

Electromagnets and the Electric Motor Key Question: How does a motor work? *Students read Section 23.2 AFTER Investigation 23.2

Electromagnets and the Electric Motor • Electromagnets are magnets that are created when electric current flows in a coil of wire. • A simple electromagnet is a coil of wire wrapped around a rod of iron or steel. • Because iron is magnetic, it concentrates and amplifies the magnetic field created by the current in the coil.

Electromagnets and the Electric Motor • The right-hand rule: • When your fingers curl in the direction of current, your thumb points toward the magnet’s north pole.

The principle of the electric motor • An electric motor uses electromagnets to convert electrical energy into mechanical energy. • The disk is called the rotorbecause it can rotate. • The disk will keep spinning as long as the external magnet is reversed every time the next magnet in the disk passes by. • One or more stationary magnets reverse their poles to push and pull on a rotating assembly of magnets.

The principle of the electric motor

Commutation • The process of reversing the current in the electromagnet is called commutation and the switch that makes it happen is called a commutator.

Electric Motors • Electric motors are very common. • All types of electric motors have three key components: • A rotating element (rotor) with magnets. • A stationary magnet that surrounds the rotor. • A commutator that switches the electromagnets from north to south at the right place to keep the rotor spinning.

Electric Motors • If you take apart an electric motor that runs on batteries, the same three mechanisms are there; the difference is in the arrangement of the electromagnets and permanent magnets.

Electric motors • The rotating part of the motor, including the electromagnets, is called the armature. • This diagram shows a small battery-powered electric motor and what it looks like inside with one end of the motor case removed.

Electric motors • The permanent magnets are on the outside, and they stay fixed in place. • The wires from each of the three coils are attached to three metal plates (commutator) at the end of the armature. commutator

Electric Motors • As the rotor spins, the three plates come into contact with the positive and negative brushes. • Electric current flows through the brushes into the coils.

Key Question: How does a generator produce electricity? Induction and the Electric Generator *Students read Section 23.3 AFTER Investigation 23.3

Induction and the Electric Generator • If you move a magnet near a coil of wire, a current will be produced. • This process is called electromagnetic induction, because a moving magnet induceselectric current to flow. • Moving electric charge creates magnetism and conversely, changing magnetic fields also can cause electric charge to move.

Induction • Current is only produced if the magnet is moving because a changingmagnetic field is what creates current. • If the magnetic field does not change, such as when the magnet is stationary, the current is zero.

Induction • If the magnetic field is increasing, the induced current is in one direction. • If the field is decreasing, the induced current is in the opposite direction.

Magnetic flux • A moving magnet induces current in a coil only ifthe magnetic field of the magnet passes through the coil.

Faraday's Law • Faraday’s law says the current in a coil is proportional to the rate at which the magnetic field passing through the coil (the flux) changes.

Faraday's Law

Generators • A generator is a device that uses induction to convert mechanical energy into electrical energy.

Transformers • Transformers are extremely useful because they efficiently change voltage and current, while providing the same total power. • The transformer uses electromagnetic induction, similar to a generator.

Transformers • A relationship between voltages and turns for a transformer results because the two coils have a different number of turns.

Application: Trains that Float by Magnetic Levitation

Electricity and Magnetism