22.3 Magnetic Fields and Magnetic Field Lines

22.3 Magnetic Fields and Magnetic Field Lines

• The image of a spinning electron is not in line with modern physics.
• They give a useful way of understanding phenomena.

• Einstein is said to have been fascinated by a compass when he was a child.
• His ability to think deeply and clearly about action at a distance enabled him to create his revolutionary theory of relativity.

• A magnetic field line is a direction that a small compass points at.
• The strength of the field is determined by the density of the lines.

• A magnetic field will not be disturbed by small compasses.
• A small compass that is placed in these fields will align itself with the field line at its location, with the north pole pointing in the direction of B.
• There are symbols used for field into and out of the paper.

• The fields shown here could be mapped with small compasses.
• The symbols used for the field pointing inward and outward are similar to the symbols used for the tip of an arrow.

• A field is a way of mapping forces surrounding an object that can act on another object at a distance.
• The object is represented by the field.
• Magnetic forces, electrical forces, and electric forces are mapped.

• There are a number of hard-and-fast rules.
• Magnetic field lines are not a physical entity in and of themselves and are used to represent the field.

• At any point in space, the direction of the magnetic field is related to the field line.
• The direction of the field line will be pointed out by a small compass.

• The field's strength is determined by how close the lines are.
• The areal density is proportional to the number of lines per unit area.

• Magnetic field lines can't cross at any point in space.

• Closed loops are formed by continuous magnetic field lines.
• They go from the north to the south.

• The north and south poles cannot be separated.
• Electric field lines begin and end on positive and negative charges.
• Magnetic field lines would begin and end on magnetic monopoles.

• The answer is related to the fact that magnetism is caused by the flow of charge.
• Magnetic fields exert force on other magnets that have moving charges.

• The magnetic force on a moving charge is one of the most fundamental.

• The magnetic force is more complex than the simple Coulomb force because of the number of factors that affect it.

• The force on a charged particle moving in a magnetic field is how we define magnetic field strength.
• The tesla relates to other SI units.

• The strongest magnets have fields of 2 T or more.
• The Earth's magnetic field is very small.

• To determine the direction of the magnetic force on a positive moving charge, you have to point the thumb of the right hand at the direction of the fingers on the palm.
• One way to remember is that there is one velocity and the thumb represents it.
• The fingers represent the field lines.
• You would push the force with your palm.
• The force on a negative charge is the opposite of the force on a positive charge.

• Magnetic fields exert force on moving objects.
• One of the most basic forces is this force.
• The direction of the magnetic force on a moving charge is related to the plane formed by it and follows the right hand rule-1.
• The magnitude of the force is determined by the angle between and and.

• There is no force on static charges.
• There is a force on moving charges.

• Magnetic fields are created when charges move.
• The electric and magnetic fields affect each other when there is relative motion.

• A positively charged object moving due west in a region where the Earth's magnetic field is due north experiences a force that is straight down as shown.
• A negative charge moving in the same direction would feel a force.

• The magnetic field strength and direction is given to us.
• The equation can be used to find the force.

• The angle between the velocity and the direction of the field is what we see.

• This force is not significant on any object.
• The effects of the Earth's magnetic field are very important.

• Magnetic force can cause a particle to move.
• Some of the Cosmic rays that approach the Earth are energetic charged particles.
• The Earth's magnetic field can force them into spirals.
• Magnetic force keeps the particles in a circular path.
• The basis of a number of phenomena can be found in the curved paths of charged particles.

• The artist's depiction of a bubble chamber has high-energy charged particles moving through the liquid hydrogen.
• The curved paths of the particles are caused by a strong magnetic field.
• The mass, charge, and energy of the particle can be found by using the radius of the path.

• Magnetic force doesn't work on the charged particle because it's always perpendicular to velocity.
• The particle's speed and energy remain the same.
• The direction of motion is not affected.
• This is typical of circular motion.
• The centripetal force is supplied by the magnetic force.

• There are small circles with x's--like the tails of arrows--represented by a negatively charged particle moving in the plane of the page.
• The magnetic force is the same as the velocity, but it is not magnitude.

• The radius of the path of a charged particle with mass and charge is moving at a speed that is close to a magnetic field of strength.
• The component of the velocity that is perpendicular to the field is called the velocity component.
• Since the magnetic force is zero for motion parallel to the field, the component of the velocity parallel to the field is unaffected.
• This produces a spiral motion.

• To demonstrate this, use a magnetic field of strength equal to the speed of sound to calculate the radius of the path of an electron.

• The side view shows what happens when a computer monitor or TV screen has a magnet in it.
• The component of their velocity is parallel to the field lines as they move toward the screen.
• The image on the screen is distorted.

• Since all other quantities in the equation are known, we can find the radius of curvature directly from the equation.

• A large effect is indicated by the small radius.
• The electrons in the TV picture tube are made to move in very tight circles.

• The figure shows how electrons don't follow the magnetic field lines.
• The charges spiral along the field lines because the component of velocity parallel to the lines is unaffected.
• A kind of magnetic mirror can be formed if field strength increases in the direction of motion.

• When a charged particle moves along a magnetic field line into a region where the field becomes stronger, the particle experiences a force that reduces the component of velocity parallel to the field.

• The properties of charged particles in magnetic fields are related to other things.
• As seen above, charged particles approaching magnetic field lines may get trapped in spirals rather than crossing them.
• Cosmic rays follow the Earth's magnetic field lines and enter the atmosphere near the magnetic poles, causing the southern or northern lights to illuminate.
• The particles that approach the middle latitudes must cross magnetic field lines.
• Cosmic rays give a higher radiation dose at the poles than at the equator.

• Cosmic rays from the Sun and deep outer space do not cross the Earth's magnetic field lines.

• The Van Allen radiation belts are formed by charged particles trapped in the Earth's magnetic field.

• In the few minutes it took lunar missions to cross the Van Allen radiation belts, astronauts received radiation doses more than twice the allowed annual exposure for radiation workers.
• Jupiter has strong magnetic fields and is one of the planets with similar belts.

• The Earth's magnetic field traps energetic charged particles in the Van Allen radiation belts.

• One belt is about 300 km above the Earth's surface, the other 16,000 km.
• The charged particles in these belts migrate along magnetic field lines and are reflected away from the poles by the stronger fields there.
• The Sun and sources in outer space replenish the charged particles that enter the atmosphere.

• Magnetic fields can be used to contain charged particles.
• The giant particle accelerators have been used to explore the substructure of matter.
• Magnetic fields can be used to control the direction of the charged particles, as well as to focus them into beams and overcome the repulsion of like charges in these beams.

• The most powerful particle accelerator in the world until 2008 is located in Illinois and uses magnetic fields to direct its beam.
• This and other accelerators have been in use for a long time and have allowed us to understand some of the laws.
• One of the most promising devices is the tokamak, which uses magnetic fields to trap and direct the charged particles.
• The microwaves are sent into the oven.