Foundations of Physics CPO Science Foundations of Physics Unit 7, Chapter 23

Unit 7: Electricity and Magnetism Chapter 23 Electricity and Magnetism 23.1 Properties of Magnets 23.2 Magnetic Properties of Materials 23.3 The Magnetic Field of the Earth

Chapter 23 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.

Chapter 23 Vocabulary Terms gauss right-hand rule coil solenoid magnetic field tesla Faraday’s law induction induced current magnetic flux commutator generator electromagnet polarity

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

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.

An apparatus can be built that shows the magnetic field around a straight wire. The compass needles all form a circle when the current is switched on in the wire.

23.1 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.

The direction of the force can be deduced from the right-hand rule. If you bend the fingers of your right hand as shown, your thumb, index, and middle finger indicate the directions of the force, current and magnetic field.

23.1 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.

When wires are bundled, the total magnetic field is the sum of the fields created by the current in each individual wire. By wrapping the same wire around into a coil, current can be “reused” as many times as there are turns in the coil

23.1 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.

Coils are used in electromagnets, speakers, electric motors, electric guitars, and almost every kind of electric appliance that has moving parts.

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

23.1 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.

23.1 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.

23.1 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.

23.1 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.

23.1 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) B = 2x10-7 I r Magnetic field (T) Radius (m)

23.1 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) B = 2p x10-7 NI r Current (amps) Radius of coil (m)

23.1 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. 1) You are asked for the magnetic field in tesla. 2) You are given the current, radius, and number of turns. 3) Use the formula for the field of a coil: B = 2π x 10-7 NI ÷ R 4) Solve: B =(2π x 10-7)(400)(2A) ÷(.01m)= 0.05 T

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

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.

23.2 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.

23.2 The principle of the electric motor An electric motor uses electromagnets to convert electrical energy into mechanical energy. The disk is called the rotor because 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.

23.2 The principle of the electric motor To keep the disk spinning, the external magnet must be reversed as soon as magnet (B) passes by. Once the magnet has been reversed, magnet (B) will now be repelled and magnet (C) will be attracted. As a result of the push-pull, the disk continues to rotate counterclockwise.

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

23.2 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.

23.2 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.

23.2 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.

23.2 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

23.2 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. As the motor turns, the plates rotate past the brushes, switching the electromagnets from north to south by reversing the positive and negative connections to the coils. The turning electromagnets are attracted and repelled by the permanent magnets and the motor turn

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

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 induces electric current to flow. Moving electric charge creates magnetism and conversely, changing magnetic fields also can cause electric charge to move.

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

23.3 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.

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

23.3 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. Consider a coil of wire rotating between two magnets

23.3 Faraday's Law When the coil is in position (A), the magnetic flux points from left to right. As thecoil rotates (B), the number of field lines that go through the coil decreases. As a result, the flux starts to decrease and current flows in a negative direction. At position (C), the largest negative current flows because the rate of change in flux is greatest. The graph of flux versus time has the steepest slope at position (C), and that is why the current is largest. At position (C), no magnetic field lines are passing through the coil at all and therefore the flux through it is zero. As the coil continues to rotate (D), flux is still decreasing by getting more negative. Current flows in the same direction, but decreases proportionally to the decreasing rate of change (the slope of flux versus time levels out). At position (E), the flux through the coil reaches its most negative value. The slope of the flux versus time graph is zero and the current is zero. As the coil rotates through (F), the flux starts increasing and current flows in the opposite direction.

23.3 Generators A generator is a device that uses induction to convert mechanical energy into electrical energy. Because the magnet near the coil alternates from north to south as the disk spins, the direction of the current reverses every time a magnet passes the coil. This creates an alternating current.

23.3 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.

23.3 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