Magnetism and Electricity Chapter 19

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

Pole Force Law, or Law of Poles Like magnetic poles repel, and unlike poles attract.

Magnets, Magnetic Poles, and Magnetic Field Direction Two magnetic poles of opposite kind form a magnetic dipole. All known magnets are dipoles (or higher poles); 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.

B, Magnetic Field, is a Vector Quantity, Described by both Magnitude and Direction 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 Force A magnetic field can exert a force on a moving charged particle.

Calculating Magnetic Field Strength The magnitude of the force is proportional to the charge and to the speed of the charged particle moving through the magnetic field: SI unit of magnetic field: the tesla, T

Evaluating Metric Units Gauss is commonly used by geologists to describe Earth’s magnetic field

Calculating Magnetic Force 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. EXAMPLE: 19.2, p. 628

The Right-Hand Rule Gives the Direction of the Force RHR is for a + charge LHR is for a – charge Acceleration is in the direction of the force v B F F F v v v B B B

Sketching Magnetic Fields If the orange "diamond" particle shown above is negatively charged, in which direction is the magnetic force acting on it while it is at the position shown? towards the right side of the page (+x) B. towards the left side of the page (-x) C. towards the top of the page (+y) D. towards the bottom of the page (-y) none of the above X represents a magnetic field pointing into the plane of the page represents a magnetic field pointing out of the page

True or False? As the magnetic force acts on the particles shown in the diagram above, it does work on each one. This would be evidenced by a change in the kinetic energy of each particle.

Bellwork 04/27/09 What is the magnitude of force on a particle of charge +3.0 μC that moves through a magnetic field of 1.5 T at 7.8 x 104 m/s at 30o from the parallel? **F=qvBsin** What is the direction of the force on a positive charge that travels through a magnetic field as shown to the right? S N +

Bellwork Solutions 1. F = (3.0x10-6 C)(7.8x104 m/s)(1.5T)(sin30o) F= 0.18N 2. The index finger points in the direction of motion, the other fingers north to south in the direction of the magnetic field, leaving the thumb, showing the direction of the force pointing out of the page, or upwards.