Chapter 14 Magnetism 11/28/2018

Uses of Magnets Magnets are used to make electric motors and generators. Without these electric motors and generators we would not have telephones, lights, electric heat, television, or computers. Magnets are used in alarm systems and small electrical appliances. 11/28/2018

Magnets & Magnetic Fields 14.1 Lodestone: naturally occurring magnet rocks. Composed of iron called Magnetite. Named after the region of discovery in Greece - Magnesia. 11/28/2018

Materials such as lodestone that are always magnetic are referred to as Permanent magnets. Lodestone can be used to induce other materials to become permanent magnets. These materials will lose their magnetism. Iron can lose magnetism by heating or hammering. 11/28/2018

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Magnets are classified as hard or soft Hard magnets Materials that are difficult to magnetize, yet they hold magnetism well. Cobalt, Nickel Soft Magnets Material is easily magnetized, and loses magnetism easily. Iron Things such as heat and pounding with a hammer can cause a magnet to weaken. 11/28/2018

The magnetic force can be passed through objects. Chain of nails or paper clips. When the magnet is removed the force no longer acts between the nails or paper clips. 11/28/2018

Coulomb’s Law of Magnetic Force Similar to electrical field forces which depend on distance and charge. 11/28/2018

Magnets have a pair of poles Magnetic poles: the location of the strongest magnetic force. Magnets have a pair of poles North and South (at opposite ends.) Impossible to separate a north and south pole. Opposites attract/ Like poles repel 11/28/2018

Magnetic Fields Two like poles exert a force on each other that acts at a distance. This force is due to the interaction of magnetic fields. Magnetic force is a field force. 11/28/2018

All magnets exert a magnetic field. The strength of the field depends and the material and strength of the magnet. The field lines for magnets are similar to those for electrical charges. Similar to electrical field forces which depend on distance and charge. 11/28/2018

Conpass & Magnetic Fields A Compass is a magnet that floats on cork in water or is balanced on a point. One end will point north and the other south. 11/28/2018

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Earth is a giant bar magnet. Like poles repel each other. Opposite poles attract. Earths poles have reversed many times. 11/28/2018

The geographic south of earth is now the magnetic north. The magnetic south is the geographic north. This is done to confuse students. 11/28/2018

Magnetism from electric currents 14.2 An electric current through a wire will always produce a magnetic field. Due to this field iron filings form a pattern around the wire. When a compass is brought near a wire carrying a current the needle direction changes. The needle points in a direction tangent to the circles of the iron filings. 11/28/2018

Now a compass placed above the wire points in the direction of the net magnetic field above the wire, while a compass placed below the wire points in a different direction, which is the direction of the net magnetic field below the wire 11/28/2018

No Current Current 11/28/2018

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Use right hand rule to determine direction of magnetic field produced Use right hand rule to determine direction of magnetic field produced. Thumb points in the direction of the conventional(+) current. 11/28/2018

Magnetic field of a coiled wire resembles that of a bar magnet The magnetic field around a wire is weak. Increase by increasing the current through the wire (fire hazard) or wrapping wire into a coil. 11/28/2018

Solenoid Wire wrapped into a coil is a solenoid. The strength of a magnetic field of a solenoid depends on the number of loops and the amount of current being carried. 11/28/2018

Electromagnets A coil of wire with current running through it. The more turns of wire the more current and the stronger the magnet. Reversing the current will reverse the polarity of the electromagnet. 11/28/2018

Magnetism is caused by moving charges Magnetism of bar magnets is due to movement of charged particles. The movement is electron spin, which creates a magnetic field around each electron. 11/28/2018

When electron spins cancel each other out the material is not magnetic. Materials that are magnetic have a magnetic field around each atom. Within a material magnetic fields of atoms align themselves with nearby atoms creating smaller regions called domains 11/28/2018

Domains in demagnetized materials are not aligned. Magnetism can be induced by bringing a strong magnet in proximity with the material. The domains realign to produce a magnet. 11/28/2018

Using a magnet to induce magnetism 11/28/2018

Electromagnetic Devices Galvanometer Devices used to measure current or voltage. Ammeter or voltmeter 11/28/2018

Electric Motors Electric motors turn electrical energy into mechanical energy The coil of wire continues to spin in a motor; when the coil is attached to a shaft it can do useful work. 11/28/2018

Commutator A device used to reverse current direction in the coil every half turn. This causes magnetic field of the coil to be reversed every half turn. The coil is repelled by both N and S poles of magnetic field and thus keeps turning in one direction. 11/28/2018

Stereo Speakers Speakers consist of a permanent magnet and a coil of wire is attached to a paper cone. Current through the coil causes a magnetic field which interacts with the permanent magnet. Changing current causes the paper cone to move creating sound waves (longitudinal wave). 11/28/2018

Faraday’s Law Electromagnetic Induction Electric Currents & Magnetism 14.3 Electromagnetic Induction Electromagnetic induction is considered a law of Physics, known as Faraday’s Law Faraday’s Law An electric current can be produced in a circuit by a changing magnetic field. 11/28/2018

Energy is produced in this process. Moving a magnet in and out of a circuit will create a current in the wire. Energy is produced in this process. Does not violate conservation of energy. Energy is put into the system by moving the magnet. 11/28/2018

Charged particles will experience a force when moving through a magnetic field. The force experienced is zero when a wire moves parallel to the magnetic field lines. The force is at a maximum when the particle moves perpendicular to the magnetic field lines. 11/28/2018

Generators AC current is produced A generator uses electromagnetic induction to convert ME to electrical energy. Generators are similar to motors. Only they convert ME to electrical energy. In each ½ rotation of the loop the current reverses direction. AC current is produced 11/28/2018

Generators The magnitude and direction of the current depend on the orientation of the loop in the field. When the loop is perpendicular to the field the current is zero. As the loop turns the current increases until it reaches its maximum. This is when the loop is parallel to the field. (see page 477) Electrical Generator - AC or DC 11/28/2018

Electricity & Magnetism Visible light travels in EM waves. EM waves are made up of oscillating electric and magnetic fields. Each field regenerates the other. 11/28/2018

Transformers A device that can change an AC voltage to a different AC voltage. Transformers may increase the voltage (step-up) or decrease the voltage (step-down). This allows for a more efficient movement of the electricity from the power plant to the site of use. 11/28/2018

10.0 volts Step up Transformer The Primary coil has fewer turns of wire than the secondary coil. 10.0 volts Primary receives 5.0V and has10 turns. Secondary has 20 turns. What is the resulting voltage? 11/28/2018

2.5V Step Down Transformer Primary coil has more turns than the secondary. Primary receives 10.0V and has 20 turns; secondary has 5 turns. How many volts leave the transformer? 2.5V 11/28/2018