A Powerful Attraction or A Class of Phenomena caused by Moving Electric Charges

Cause of Magnetism The root cause of magnetism is the movement of electrons. In permanent magnets, unpaired electrons within the orbitals of atoms spin in the same direction (synchronized spinning). In temporary magnets, the flow of an electric current (usually through a wire) produces a magnetic field.

Magnetic Poles Magnets are polarized—the force of the magnetic field is directional. The direction of force from a magnetic field is the often referred to as a pole. The north pole is defined as the end that will point to the geographic north pole of the earth. On a permanent magnet, the poles cannot be separated. Breaking a magnet will create two magnets, each with both north and south poles.

Magnetic Domains Individual iron atom forces cause them to form clusters called magnetic domains. In unmagnetized iron, these domains point in random directions. In magnetized iron, the domains are aligned. A magnet causes the domains in unmagnetized iron to become aligned and attracted.

Materials in Permanent Magnets Iron is a common magnetic material, as are nickel and cobalt. Soft magnetic materials (for example iron) are easily magnetized but tend to lose their magnetism easily. Hard magnetic materials (for example nickel) tend to retain their magnetism. Many permanent magnets are made of an iron-based alloy (ALNICO) with added aluminum, nickel and cobalt. Some rare earth elements make strong permanent magnets. Examples are neodymium and gadolinium.

Magnetic Field Lines A magnetic field is a region in which a magnetic force can be detected. Magnetic field lines show the direction and strength of a magnetic field. The lines are drawn point away from the north pole and towards the south pole. Magnetic field strength (B) is measured in Teslas.

The Earth’s Magnetic Field The earth is a huge magnet. The poles of the earth’s magnetic field do not exactly line up with the geographic poles. It is not known what causes the earth’s magnetic field. It may be related to the moving charges within the molten core of the earth. It may be due to convection currents and the earth’s rotation.

Magnetic Field around a Straight Wire Electric current in a wire produces a cylindrical magnetic field. Changing the direction of the current changes the direction of the field. Right Hand Rule for a Straight Wire – point thumb in direction of the current, and the fingers will curve in the direction of the magnetic field.

Magnetic Field of a Loop The overall direction for a magnet formed from coiled wire can be determined using the Right-hand Rule. – Wrap the fingers around the coil form in the direction of the current flow – the thumb will point in a direction indicating the end which becomes the N-pole

Charged Particles in a Magnetic Field A charge moving through a magnetic field will experience a force proportional to the charge and velocity of the particle in addition to the strength of the magnetic field. The strength of the force can be calculated with the formula: F magnetic = Bqv F magnetic = (magnetic field)(charge)(velocity)

Charged Particles in a Magnetic Field The direction of the magnetic force on a moving charge is always perpendicular to both the magnetic field and the velocity of the charge. A different right-hand rule can be used to find the direction of the magnetic force. A charge moving through a magnetic field follows a circular path.

Example Problem Particle in a Magnetic Field A proton moving east experiences a force of 8.8  10 –19 N upward due to the Earth’s magnetic field.At this location, the field has a magnitude of 5.5  10 –5 T to the north. Find the speed of the particle.

Forces on Objects in Magnetic Fields Any current carrying wire in an external magnetic field undergoes a magnetic force. This force is perpendicular to the magnetic field. This produces a force that can be calculated using the formula : F magnetic = BIL. F is force (N), B is magnetic field strength (T), I is current (A), and L is the length of the wire (m).

Force on a Current Carrying Wire

The Magnetic Force on Two Current-Carrying Wires Two parallel current-carrying wires exert a force on one another that are equal in magnitude and opposite in direction. If the currents are in the same direction, the two wires attract one another. If the currents are in opposite direction, the wires repel one another. Loudspeakers use magnetic force to produce sound.

Force Between Parallel Wires

Example Problem Force on a Current-Carrying Conductor A wire 36 m long carries a current of 22 A from east to west. If the magnetic force on the wire due to Earth’s magnetic field is downward (toward Earth) and has a magnitude of 4.0  10 –2 N, find the magnitude and direction of the magnetic field at this location.