IB Physics: Magnetism and Electromagnetic Induction. “Magnetic Field Lines Always Point Away from the _____ and Toward the _____.” North South LNK2LRN

Magnetite: From Magnesia (Greece). Formula: Fe3O4. Description: Dark grey, slightly shiny. Magnetite is naturally magnetic. It is also called Lodestone. In Middle Ages, pilots were called lodesmen. The lodestar is the Polar star, the leading star by which mariners are guided. The name probably comes from Magnesia, but there is a fable of Magnes, a Greek shepherd, who discovered magnetite when the nails in his shoes stuck to the ground!

Magnets in Ancient Times Magnetism has been known since ancient times because it occurs naturally in loadstone, a rock rich in magnetite, a form of iron oxide. It was believed by some that magnetic fields permeated humans and their manipulation could affect health. Some Chinese cities are laid out along the direction of the Earth’s magnetic field. The first compasses were made in China in ~1000 AD.

Sir William Gilbert (1544-1603) Magnets have two poles, which he called north and south. Like poles repel and opposite poles attract. Iron can be magnetized. Gilbert’s book, De Magnete, was enormously popular and influenced Kepler and Galileo. The Earth is a giant magnet.

Like poles repel, and unlike poles attract. The Magnetic Field The ‘Gilbert Model’ Like poles repel, and unlike poles attract. Cut a magnet in half and you will have two magnets. A single pole (monopole) has never been isolated.

Magnetic Field of a Bar Magnet. Field lines always point away from the North and toward the South.

Filing demonstration of magnetic field lines.

Until 1820, the only magnetism known was that of iron magnets and of "lodestones", natural magnets of iron-rich ore. This was changed by a professor of Physics at University of Copenhagen, Hans Christian Oersted (1777-1851).                         

The Magnetic Field 1820 - Electromagnetism, Current In 1820, a physicist Hans Christian Oersted, learned that a current flowing through a wire would move a compass needle placed beside it. This showed that an electric current produced a magnetic field. LNK2LRN

Oersted’s Compass Deflections LNK2LRN

Michael Faraday (1791-1867) was a British scientist who contributed to the field of electromagnetics. 1820 –Faraday observed Oersted’s compass needle move and wrote, “Use magnetism to produce electricity.” 1831 - Faraday built two devices to produce what he called electromagnetic rotation: the electric motor, t hat used continuous circular motion from the circular magnetic force around a wire. 1832 - The electric generator used a magnet to generate electricity.

Earth’s Magnetic Field

Magnetic Field of Earth LNK2LRN

The Right Hand Rule for Wires. B = μoI / 2πr μo= 4πx10-7 Tm/A Single Loop B = μoI / 2r

Examples: 1. A long, straight wire carries current from West to East. What is the direction of the magnetic field directly Above the wire. (b) Below the wire.   2. If the current in the wire from #1 is 2.5 A, find the magnitude of the magnetic field at a distance of 1.5 cm from the wire. 3. A circular loop of copper wire with current 8.4 A determines a magnetic field. The area of the circle is 2.0 m2. Find the magnitude of the magnetic field.

Magnetic Field Generated by a Coil B = μonI/L μo= 4πx10-7 Tm/A B magnetic field strength N/(Ampere meter) I current in wire (Amperes) n number of turns of wire L length of coil (meters)

A Solenoid N B = μoNI, N=n/L LNK2LRN

The Toroidal Solenoid B = μoNI/(2πr) LNK2LRN

Force on a Charged Particle moving in a Magnetic Field. The magnitude of the magnetic force is F = q v B sin θ, where q is the charge v, its velocity B, magnetic field θ, angle between v and B Force is centripetal and RHR Fc = m v2 / R , M, particle mass R, radius of the circular path Force on a Charged Particle moving in a Magnetic Field.

Force on a Current-carrying Wire in a Magnetic Field. F = B I L sin θ B is the external magnetic field measured in N/Am. I is the current measured in amps. L is the length of the current segment inside of the magnetic field, B θ, angle between L and B F, direction by RHR

1820 - Andre Marie Ampere showed that two parallel wires carrying current could attract or repel. F/L=(μoI1I2)/(2πa) ATTRACT – current going in SAME direction. REPEL – current in OPPOSITE direction.