Forces and Fields Flashcards

• Electrons are mobile. they can move easily. While p+ are not mobile. They are "locked up" in the nucleus.
• Insulators do not allow electrons to flow through them, for e.g. plastic, rubber, glass, air. These are the materials that can acquire a static charge. Conductors allow electrons to move through them easilt. Metals are conductors. because of this Metals do not acquire static charges.
• Charge sharing occurs when 2 identical objects are touched together, their total charge is shared equally between the two objects.

Like charges exert repulsive forces on one another, while Unlike charges exert sttractice forces on each other. A negative static charge is an object with excess e-. A positive static charge is an object with more p+ than e-.

• 1. Friction - when two substances (insulators) (e.g. fur and plastic) are rubbed together electrons are transferred from one object to another (fur->plastic).
• 2. Induced charge (charging by induction) - when a charged object is brought near (not touching) a neutral insulator the neutral charge experiences an induced charge as the oppositely charged particles move closer and the similar charged ones repel.
• 3. Conduction - Direct contact with a charged object causes the excess electrons from the negatively charged objects to spread out evenly, causing the object that is touched to gain the same charge as the charged object. This causes both objects to repel aftwerwards.

A coulomb is a unit of measuring static charges (static charges are often measured in micro (x10^-6) or nano (x10^-9) coulombs. One excess e- is equal to the negative elementary charge which is 1.60x10^-19C. One excess p+ is equal to the positive elementary charge and one alpha particle is equal to 3.20x10^-19C.

• It is a device that is fully a conductor with two leaves on the bottom. It responds to static electricity. 
• Coulombs law is used to determine the magnitude of Fe, we use logic to decide direction (either attraction or repulsion). 
• The formula is Fe = kq1q1/r². Where Fe equal the magnitude of the electric force (N) between two point charges. K is Coulomb's constant (8.99x10^9 Nm²/C²). q1 and q2 are the charges (C) on the point charges (will be kept positive). and r is the seperation distance (m) between the 2 point charges.

Electric fields are vectors ( E+ ߯), they point in the direction that a positive "test charge" (proton) would accelerate if released at a point. Note E+ ߯ is electric field (n/c) while E is energy (J). E + ߯ points away from postive point charges and towards negative point charges. E+ ߯ lines can be curved if you have 2 charges.

|E + ߯| can be calculated form the formula |E+ ߯|=kq/r^2. Where |E+ ߯| is the magnitude of the electric field (n/c), k is 8.99x10^9 Nm^2/C^2, Q is the charge (C) of the point charge causing the formula (do not use direction) and r is the distance from the point charge (m). Remember since this is a magnitude direction is found with your positive point charge. If you don't have information about what is causing the field you can use  E+ ߯= (Fe+ ߯)/q. Where E + ߯ is the electric field (with direction), Fe+ ߯ is the electric force (N) acting on a test charge in the field and q= is the magnitude of the charge (C) on the test charge.

Since Ek=Ep you can find Ep with E=Vq and convert to Ek = 0.5mv^2. An electron has the maximum amount of Ep when it is closest to the like charged plate. An electron volt is a equal to the energy a electron has in an electron plate system with a one volt battery it is equal to 1.60x10^-19J.

It was an experiemtn done by experimental physcist Milikan to determine the elementary charge (charge of one e-). To do this he used a spray bottle of oil, that sprayed statically charged oil droplets and began spraying droplets between electric plates with an adjustable power supply. He carefully adjusted the volts until adroplet was suspended (Fg=Fe). He then used the formula mgd/V = q and found the static charges of the oil droplets (he got mass from the volume and density of the oil droplets) He observed these charges were all multiples of 1.60x10^-19 so he suggested that was the charge of one e-.

• Magnets are metals that are made up of magnetic domains (microscopic volumes where electron spin direction is the same). These metals are Fe(s), CO(s), Ni(s) and ferromagnetic elements. Magnets are defined by a N and S pole. Like poles repel and unlikes attract.
• Bar magnets have a non-uniform magnetic field that is strongest at the poles and flows from N to S. When there are unlike poles together a uniform magnetic field is created.
• A compass always points in the direction of the magnetic field (vectorB)

• When electrons move thru a wire they produce a circular magnetic field around the wire. To find the direction you hold your thumb in the direction of e- flow and your fingers dictate the direction of the e- field. When the field is facing into the page we put an x and when it is coming out we put a dot. 
• When electron flow is in the same direction in parallel wires the fields will be opposite and they will attract. If e- flow is oppostie the field direction will be similar between the wires and they will repel. Convetional current is the opposite direction of electron flow (flows from pos to neg. while acc e-'s flow neg to pos)

• IF we wrap a wire around something into a coil and passs electricity through it the electromagneticic forces from each looped wire will complement each other and create one strong magnetic field going through the coil.
• The three factors that affect the strength of the magnetic force are 1. Current strength - a stronger current = stronger force 2. # of loops = the more loops there are the stronger the field 3. Coil diameter - the smaller the diameter the stronger the force.
• To figure out the direction use the second left hand rule, where your fingers are the direction of the e- flow and your thumb points in the direction of the magnetic field inside the coil (this is also the N pole of the magnet). The magnetic field is similar to that of the bar magnet on the outside of the coil.

Since moving charged particles generate a circular magnetic field around them as they move (this is why the wire material has nothing to do with electromagnetism only the e-'s do) they will  experience a force when interacting with/passing through an external magnetic field. To determine the direction of this force you use the third hand rule, where if the particle is negative you use your left hand and if the particle is pos. (alpha or proton) you use your right hand. When using this rule your thumb dictates the particle movement direction, your fingers dictate the external magnetic field direction and the palm of your hand dictates the direction the Fm (magnetic force) is acting.

The formula is Fm=qvB. Where Fm is the magnitude of the magnetic acting on the particle, q is the charge (C) on the particle, v= is the speed in (m/s) of the particle, remember the speed must be perpendicular to the magnetic field - because the less perpendicular it is the less Force is being acted on the particle (parallel = Fm)  and B is the magnetic field strength in teslas (T).

The one in the electric field has a parabolic path and the particle speeds up and the Fe (Fe=qvectorE) direction does not change, while the one in the magnetic field has a constant speed and follows a circular path (VectorFm=VectorFc = qvB=mv^2/r) this means the Fm direction changes in the same way a Fc changes during circular motion.

It is a device with three chambers that identifies unkown particles by charge and/or mass. It works by first accelerating the particle to a speed v in the acceleration chamber, this makes it so that we us the formula √2∆Vq/m (m and q are unknown). Next the particle moves into the velocity selector. It is an overlapping region of "competing" fields (created by electric plates and a magnetic field), creating Fe and Fm in opposite directions. The scientists will adjust the voltage of the electric field weakening or strengthening it until most particles go straight through. This "filters" out different particles as they will move up (slower e-) or down (faster e-). Lastly the particles pass through the seperation chamber which is an external magnetic field which causes them to undergo circular motion, thus you may use the formula qvB=mv^2r (r is inversely porpotional to mass so heavier particles curve less while lighter ones curve more).

The motor effect is when current carrying wires experience a magnetic force when in an external magnetic field just like electrons do. This is because e-'s flow thru wires (therefore we use our 3rd hand rule with our left hand only, to determine direction). TO calculate the force we use the formula Fm=ILB. Fm is the magnetic force on the wire, I is the current (A) moving thru the wire, B is the magnetic field and L is the length of wire (m) that is in the magnetic field. To determine I (A) we use the formula I=q/t since amps are C/s or in other words the rate of flow of e- thru a wire at a given point.

• It is the process of inducing electron flow in a conductor through the use of a moving magnetic field. The three things required to generate e- flow is: 1. A conductor (for e- to move thru) 2. A magnetic field 3. Movement (relative movement between conductor or magnetic field. Generators and motors do opposite E transformations, Generators turn kinetic E into electric while motors turn electrical E into kinetic.
• Lenz's law states that when a magnet moves in relation to a conductor electrons move in a way that will create a magnetic field that opposes the magnets movement. Since this field opposes motion it is hard to make electic E. This is an example of the law of conversation of energy as it shows that to create electric E you must put in kinetic E. To calculate e- direction in coils that are moving in relation to a magnet you use the 2nd LH rule, if it is a conducting rod moving through a magnetic field you use the 3rd LH rule.