Magnets Learning Objectives By the end of this section, you will be able to: Describe the difference between the north and south poles of a magnet. Describe how magnetic poles interact with each other. Universal Characteristics of Magnets and Magnetic Poles It is a universal characteristic of all magnets that like poles repel and unlike poles attract.
Note the similarity with electrostatics: unlike charges attract and like charges repel. That is why the north pole of your compass is attracted toward the geographic north pole of the Earth—because the magnetic pole that is near the geographic North Pole is actually a south magnetic pole!
Figure 3. Unlike poles attract, whereas like poles repel. See if you can show this for two refrigerator magnets. Will the magnets stick if you turn them over? Adam Gray , Manchester, UK Electric and magnetic fields are both components of an electromagnetic field. Trending Latest Video Free. Paralysed mice walk again after gel is injected into spinal cord How Minecraft is helping children with autism make new friends New mineral davemaoite discovered inside a diamond from Earth's mantle The surprising upsides of the prions behind horrifying brain diseases COP People from climate-ravaged regions say we need action now.
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The magnetic field, however, interacts with another magnet, e. The arrow of the lines of magnetic flux indicate what direction a compass will point if it is placed there. If you place a campus in the equator of the bar magnet, the compass will point straight to the south, and align itself in parallel to the bar magnet. How are magnetic poles and electrical charges similar? Dec 25, Their field patterns look similar provided the electric charges form a dipole.
In the case of a negative charge, the direction of the field is reversed. The electric field is directed tangent to the field lines. Of course, we imagine the field lines are more densely packed the larger the charges are. One can see clearly that the curl of the electric force is zero.
Electric Field Generated by Point Charges : The electric field surrounding three different point charges: a A positive charge; b a negative charge of equal magnitude; c a larger negative charge. If multiple charges are involved, field lines are generated on positive charges, and terminate on negative ones. The curl of a magnetic field generated by a conventional magnet is therefore always non zero. Charged particles will spiral around these field lines, as long as the particles have some non-zero component of velocity directed perpendicular to the field lines.
A magnetic field may also be generated by a current with the field lines envisioned as concentric circles around the current-carrying wire. The magnetic force at any point in this case can be determined with the right hand rule, and will be perpendicular to both the current and the magnetic field. If an object experiences no net force, then its velocity is constant: the object is either at rest if its velocity is zero , or it moves in a straight line with constant speed if its velocity is nonzero.
There are many cases where a particle may experience no net force. The particle could exist in a vacuum far away from any massive bodies that exert gravitational forces and electromagnetic fields. Or there could be two or more forces on the particle that are balanced such that the net force is zero.
This is the case for, say, a particle suspended in an electric field with the electric force exactly counterbalancing gravity. If the acceleration is zero, any velocity the particle has will be maintained indefinitely or until such time as the net force is no longer zero.
Because velocity is a vector, the direction remains unchanged along with the speed, so the particle continues in a single direction, such as with a straight line.
Recall that the magnetic force is:. Zero Force When Velocity is Parallel to Magnetic Field : In the case above the magnetic force is zero because the velocity is parallel to the magnetic field lines. In this case a charged particle can continue with straight-line motion even in a strong magnetic field. If is between 0 and 90 degrees, then the component of v parallel to B remains unchanged.
Since the magnetic force is always perpendicular to the velocity of a charged particle, the particle will undergo circular motion. Magnetic forces can cause charged particles to move in circular or spiral paths. Particle accelerators keep protons following circular paths with magnetic force.
The bubble chamber photograph in the figure below shows charged particles moving in such curved paths. The curved paths of charged particles in magnetic fields are the basis of a number of phenomena and can even be used analytically, such as in a mass spectrometer. There is a strong magnetic field perpendicular to the page that causes the curved paths of the particles.
The radius of the path can be used to find the mass, charge, and energy of the particle. So, does the magnetic force cause circular motion? Magnetic force is always perpendicular to velocity, so that it does no work on the charged particle.
The direction of motion is affected, but not the speed. This is typical of uniform circular motion. The simplest case occurs when a charged particle moves perpendicular to a uniform B-field, such as shown in. If this takes place in a vacuum, the magnetic field is the dominant factor determining the motion. Here, the magnetic force Lorentz force supplies the centripetal force. The magnetic force is perpendicular to the velocity, and so velocity changes in direction but not magnitude.
Uniform circular motion results. Here, r , called the gyroradius or cyclotron radius, is the radius of curvature of the path of a charged particle with mass m and charge q , moving at a speed v perpendicular to a magnetic field of strength B. In other words, it is the radius of the circular motion of a charged particle in the presence of a uniform magnetic field.
If the velocity is not perpendicular to the magnetic field, then v is the component of the velocity perpendicular to the field. The component of the velocity parallel to the field is unaffected, since the magnetic force is zero for motion parallel to the field. A particle experiencing circular motion due to a uniform magnetic field is termed to be in a cyclotron resonance. The term comes from the name of a cyclic particle accelerator called a cyclotron, showed in. The cyclotron frequency or, equivalently, gyrofrequency is the number of cycles a particle completes around its circular circuit every second and can be found by solving for v above and substituting in the circulation frequency so that.
Cyclotron : A French cyclotron, produced in Zurich, Switzerland in
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