Electric & Magnetic Forces: Facts and Concepts

An electric fan operates on the 'motor principle'. What is that? Name five other devices that also operate on this principle.

'Motor principle' is the way a magnet exerts force on a wire with current to accelerate it. A fan reicieves its current from an outlet and wire, which then passes through a coil within the fan and magnets give the force to keep the fan moving. Other examples would be blenders, electric toothbrushes, electric pencil sharpeners, CD-drives, electric cars, and more. The 'motor principle' is the name given to the ability of a magnet to exert a force upon a current-carrying wire and accelerate it through space. A fan is supplied with current by plugging it into the wall outlet or connecting it to a battery. The current passes through a coil of wire in the fan (armature). Magnets situated with the fan supply the force to turn the fan blades. Other devices could include: blenders, electric toothbrushes or pencil sharpeners, CD-drives, electric cars, etc.

Who was Robert A. Millikan and what were the premise and goal of his "oil-drop experiment"?

An American scientist from the late 1800s to early 1900s. His oil drop experiment involved measuring the time of different charges oil drops to fall or rise depending on the charge around it. He presumed that every electron possessed an identical charge, and found the measurement of the charge. Millikan (1868-1953) was an American scientist who experimentally measured the mass of a single electron. He subjected small charged oil droplets to an electric field force, gravity, and a viscous air drag force simultaneously. The physics equations governing the droplet's motions permitted him to collect numerical data and calculate the electron charge. Among the primary premises of his experiment was that each electron possessed an identical charge and that the charge on any one oil droplet was a integer multiple of the fundamental electron charge.

1. What is Coulomb's Law of Electrostatic interaction? Within what situations is it useful?

Coulomb's Law mathematically states the electric force (of attraction or repulsion) between two charged objects (Q and q) separated by a distance (r): Fe = q(KQ/r2) It is useful when dealing with small, point-like charges or objects with dimensions that are small compared to the value of r.

What are cyclotrons and how are they used to investigate the realm of subatomic physics?

Cyclotrons accelerate charged subatomic particles to high speeds. Electric fields are used to make the particles speed up, and magnetic fields are used to make the particles curve. When the particles collide, the kinetic energy is converted to mass, creating a bunch of new particles which have different properties and can be used to confirm theories or create new ideas. Cyclotrons are used to accelerate charged subatomic particles to high speeds. They use a combination of electric fields (to speed up the particles) and magnetic fields (to curve the particles in a circular path). When these high speed particles are collided with targets or other particles coming from the opposite direction... kinetic energy can be converted into mass... producing a shower of new particles. Many of these new particles can possess different and exotic properties which would obey new physic laws. In this way scientist can expand our understanding of the universe and/or confirm newly developed theories.

3. What are the conditions necessary for an E-field to produce an electric force on a particle? What are the conditions necessary for a B-field to produce a magnetic force on a particle?

For a particle to experience a force from an E-field, the particle must possess charge and be within the E-field. For a particle to experience a force from an B-field, the particle must possess charge and be moving within the B-field with at least a component of its velocity perpendicular to the B-field. E-field = a charged particle moving within the field B-field = a charged particle moving perpendicular to the B-field

6. When a charged particle moves through a magnetic field it can move in a straight, circular or helical path. What would cause each of these trajectories?

If the particle is moving parallel to the field lines, it will move straight. If the particle is moving perpendicular to the field lines, it will move in a circle. If the particle possess a velocity with components both parallel to the field lines and perpendicular to the field lines... it will move in a helical path.

2. How would the electrostatic force between a deuteron and an alpha particle that are ten nanometers apart compare to the electrostatic force between an electron and a positron that are twenty nanometers apart?

If you decrease Q by 2, and increase r by 2, it is times 1/8 A deuteron has one proton and one neutron bound together: q = 1.6 x10-19 C An alpha particle has two protons and two neutrons bound together: Q = 3.2 x10-19 C Fe = q(KQ/r2) = (1.6 x10-19)(8.99 x109)(3.2 x10-19)/(10 x10-9)2 = 4.6 x10¬-12 N Since the electron has the same amount of charge as the deuteron... it will not affect the force. Since the positron has the same half the charge as the alpha particle... it will cut the force in half. Since the separation is doubled... it will quarter the force. So the electron and positron will be attracted with: (4.6 x10¬-12 )( ½ )( ¼ ) 0.57 x10-12 N Fe = q(KQ/r2) = (1.6 x10-19)(8.99 x109)(1.6 x10-19)/(20 x10-9)2 = 0.57 x10¬-12 N

What does a mass spectrometer do and how does it do it?

It separates ions by mass (think spout of different ions of masses) . Material is heated, vaporized, and ionized. Then, the ions are put into the magnetic field where their paths, particularly their radii's of curvature will depend upon their masses. A mass spectrometer is used to separate ions by mass. A sample of material may be heated, vaporized and ionized. Then the ions may be injected into a magnetic field where their trajectories' radii of curvature will depend upon the mass of the particle (r = mv/qB).

How do iPod earbuds (acoustic speakers) work?

Magnetic force exerted upon a small coil of wire carrying a current within the speaker forces it to vibrate back and forth, the faster it vibrates the louder the sound is produced. A speaker is forced to vibrate back and forth... to produce sound waves. The force used is a magnetic force exerted upon a small current-carrying coil of wire within the speaker. A nearby magnet within the speaker supplies the necessary magnetic field. When the current is increased, the force is stronger and the sound grows louder.

Suppose I were to build a 'clock' based upon the mutual repulsion of electron pairs. The two electrons start at the center of the clock and accelerate out to the rim in exactly one second. After this a second pair of electrons repeats the process... and then a third and fourth and so on. The clock therefore 'ticks' at one second intervals. What would happen to this clock if I put it on a train and had it rush by me at a high rate of speed

Still in relation to me: Electrons will accelerate outwardly at a rate of one second. Moving sideways in relation to me: A magnetic field will be produced by the electrons (because they're moving charge, aka current) so they will move apart slower. Therefore the clock will be moving slower, since it'll take more than one second for the electrons to move apart. When the clock sits at rest relative to me... the electrons move apart based upon the repulsive force of their mutual electric charge. It will tick at the standard rate. However, when the clock moves sideways relative to me... the electrons will produce magnetic fields which will provide an additional attractive force... the electrons won't accelerate apart as rapidly. It will tick at a slower rate. Einstein's Theory of Relativity shows that clocks of all designs will tend to slow under motion. In fact, it's time itself that is passing at a slower rate!

4. An electron runs alongside a current-carrying wire. What is the direction of the magnetic force on this electron?

The wire's B-field is oriented into the page At the location of the electron (RHR#1). The electron will experience a force away from the wire due to its motion (RHR#2). Note: RHR#2 implies the force is toward the wire, but the electron is negative.

5. What would be the direction of the electro-magnetic forces on these three moving particles? p+ × × × B E e- B E ⓪ Fm × × × Fe Fe H+ × × × Fm

Within electric fields: positive particles are forced "with" the field lines. Within electric fields: positive particles are forced "against" the field lines. The force on charged particles within magnetic fields is dictated by RHR#2

If I were to suspend a slinky vertically and then pass a high current through it... would it do anything interesting?

Yes, the slinky would contract. A coil with a current would have parallel wires with currents flowing in parallel directions, so they would contracts (the in and out would be moving in opposite ways, so they'd contract to 'cancel eachother out'). Passing current along the slinky will cause current to be flowing in parallel directions among adjacent coils. When two nearby wires carry current in parallel directions, they will magnetically attract (each wire will produce a B-field which will act upon the other wire.) Thus, the Slinky will contract.

7. What determines the radius of an ion's circular path within a magnetic field?

a = F/m The radius of the path is directly proportional to the mass and speed. v2/r = qvB/m The radius of the path is inversely proportional to the charge and field strength. v/r = qB/m r = mv/qB