Download ppt - Magnetism. Magnets, Magnetic Poles, and Magnetic Field Direction Magnets have two distinct types of poles; we refer to them as north and south

Magnetism

Magnets, Magnetic Poles, and Magnetic Field Direction

Magnets have two distinct types of poles; we refer to them as north and south.


Like magnetic poles repel, and unlike poles attract.


Two magnetic poles of opposite kind form a magnetic dipole. All known magnets are dipoles; magnetic monopoles could exist but have never been observed.

A magnet creates a magnetic field:The direction of a magnetic field (B) at any location is the direction that the north pole of a compass would point if placed at that location.


North magnetic poles are attracted by south magnetic poles, so the magnetic field points from north poles to south poles.

The magnetic field may be represented by magnetic field lines.The closer together (that is, the denser) the B field lines, the stronger the magnetic field. At any location, the direction of the magnetic field is tangent to the field line, or equivalently, the way the north end of a compass points.

Magnetic Field Strength and Magnetic Force

A magnetic field can exert a force on a moving charged particle.


The magnitude of the force is proportional to the charge and to the speed:

SI unit of magnetic field: the tesla, T

B = Magnetic Field (T)F = Force (N)Q = charge (C)V = velocity (m/s)

Or F = qvB


In general, if the particle is moving at an angle to the field,

The force is perpendicular to both the velocity and to the field. So if the particle is not moving perpendicular to the field, this formula must be used. If the particle is moving parallel to the field, then there is no force on it.


For a clearer picture, check page 623 in your text book.


• A) No magnetic force acts on a charge moving with a velocity v that is parallel to a magnetic field B.

• B) The charge experiences a maximum force F when the charge moves perpendicular to the field.

• C) If the charge travels at an angle θ with respect to B, only the velocity component perpendicular to B gives rise to a magnetic force F, which is smaller than the one in B). This component is v sin θ


A right-hand rule gives the direction of the force.

THE RIGHT HAND RULE #1

Extend the right hand so the fingers point along the direction of the magnetic field B and the thumb points along the velocity v of the charge. The palm of the hand then faces in the direction of the magnetic force F that acts on a positive charge.


• If the moving charge is negative instead of positive, the direction of the magnetic force is opposite to that predicted by the Right Rule #1.

Always assume the charge is positive when applying the rule, and then simply change the direction if it is actually a negative charge.

Applications: Charged Particles in Magnetic Fields

A cathode-ray tube, such as a television or computer monitor, uses a magnet to direct a beam of electrons to different spots on a fluorescent screen, creating an image.

Magnetic Forces on Current-Carrying Wires

The magnetic force on a current-carrying wire is a consequence of the forces on the charges. The force on an infinitely long wire would be infinite; the force on a length L of wire is:

θ is the angle between I and B. **Note L = v/t


• If the current is parallel to or directly opposite the magnetic field, then the force on the wire is zero.

• If the current is completely perpendicular to the to the magnetic field, then the equation becomes

F = ILB

Where I is the current (A)

L is the length of the wire (m)

B is the magnetic field (T)

F is the force on the wire (N)


The direction of the force is given by a right-hand rule:When the fingers of the right hand are pointed in the direction of the conventional current I and then curled toward the vector B, the extended thumb points in the direction of the magnetic force on the wire.


• You can also use the ‘same’ right hand rule but replace the velocity with current

• When the fingers of the right hand are extended in the direction of the magnetic field and the thumb pointed in the direction of the conventional current I carried by the wire, the palm of the right hand points in the direction of the magnetic force on the wire


• If we have two parallel straight wires that carry current in the same direction, the force between them will be attractive

• If the wires have current that is in the opposite direction of each other, the force between them will be repelling

Extra: Geomagnetism: The Earth’s Magnetic Field

The Earth’s magnetic field is similar to that of a bar magnet, although its origin must be in the currents of molten rock at its core.

Its magnitude is approximately 10–5 to 10–4 T.


The magnetic poles are not in exactly the same place as the geographic poles; when navigating with a compass, you need to know the angle between them, called the declination, at your position.


Charged particles can become trapped around magnetic field lines. Such trapping of solar wind particles has resulted in bands of charged particles around the Earth called Van Allen belts.