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All Physics 18 Tips
| Question | Answer |
|---|---|
| Electrons | Part of an atom, carries a negative charge, found orbiting around the nucleus |
| Protons | Part of an atom, carries a positive charge, found inside nucleus |
| Neutrons | Part of an atom, carries NO charge, found inside the nucleus |
| SI Unit for Charge | Coulomb (C) |
| Electron, Proton, Neutron Charges & Masses | Each of these have different masses that can be found on the formula sheet. Electrons and protons have the same charge amount, but proton's is positive while electron's is negative. Neutron has a charge of 0. |
| Process of Charging Objects | two objects rubbed together, electrons from one material will jump towards the other. The one that loses electrons will turn positive and the one that gains is negative. |
| When you rub fur and ebonite together... | The fur loses negative charge and becomes positively charged. The ebonite gains negative charge and becomes negatively charged. |
| Conservation of Electric Charge | During the rubbing process, electrical charge is not created or destroyed. The quantity of excess on each object is exactly the same. |
| Triboelectric Series (most positive) | Rabbit fur, glass, hair - meaning if you rub a more negative object against it, the glass will turn positive. |
| Triboelectric Series (most negative) | Teflon, PVC pipe, styrofoam, rubber - if you rub a more positive object against it, the Teflon will turn negative. Objects can get charged one way or another depending on what they're rubbed against. |
| Van de Graaf Generator | The generator removes electrons from its metal dome, making it positively charged. When a grounded object comes close, electrons jump through the air back to the dome, creating sparks. |
| Conductor | A material where electrons are weakly bound to the nucleus and free to move. Examples of good conductors include metals like silver, copper, gold, and mercury. |
| Insulator | Material in which electrons are tightly held by the nucleus and are not free to move within the material. Examples of good insulators include substances like glass, rubber, plastic, wood. |
| Semiconductors | A few materials like silicon, germanium, and carbon are called semiconductors. At ordinary temperatures, there are a few free electrons and the material is a poor conductor of electricity. As the temp rises, electrons break free and move through nucleus. |
| Electroscopes | An electroscope detects electric charge because its metal knob transfers charge to the connected foil leaves. When both leaves get the same type of charges, they repel each other and spread apart. |
| Charging by Conduct | Negatively charged rod makes contact with a metal sphere, which is a good conductor, and transfers electrons to it. The sphere distributes its charge uniformly all over its sphere; insulating stand prevents flow down to earth |
| Charging by Induction | Negatively charged rod is brought near - NO CONTACT - to an uncharged electrical conductor. The negative charges in the conductor travel to the far end of the conductor, repelled. |
| Charging by Induction (Part Two): | If the conductor is now connected to the ground, the negative charge would flow to the ground. If the ground (finger) is removed and then the rod is removed, a permanent positive charge would be left on the conductor |
| Tip about Movement | notice that only electrons move while the protons do NOT. They remain tethered to the nucleus |
| Tip about Grounding Wire | You have to remove the grounding wire first, then the rod. If you do it the opposite way, then the charges that have escaped will instantly come back to the sphere and turn the sphere neutral. |
| Like Charges Demo | If you touch potentially charged paper to the rod of the electrosphere, it will shred itself because like charges repel. Similarly, if you touch your hand to the top of the electrosphere and hold paper in the other, the paper will still shred. |
| Rod on a Swivel | Rub the rod against the fur, making the rod negatively charged. Then the rod is placed on a swivel and if you bring a similarly charged rod close, the swivel will begin to spin because of repulsion. |
| If you place a neutral conducting soda on the swivel... | The negatively charged rod pushes the free electrons away, leaving the closer side of the can to become positively charged. The can is attracted to the rod because unlike charges attract each other. |
| Coulomb's Law | States that two point charges exert a force (F) on one another that is directly proportional to the product of the magnitudes of the charges (q) and inversely proportional to the square of the distance (r) between their centers. ON FORMULA SHEET |
| Electric Field | Electric fields exist in space surrounding a charged particle or object. Electric fields are represented by lines of force that start on a positive charge and end on a negative charge. When EF comes from a neg charge, they point towards it, pos point away |
| Rule about Electric Field Lines | represented by lines of force that start on a positive charge and end on a negative charge. The lines never cross each other because that would be a violation to the concept of vectors (a single vector cannot have two directions) |
| Tip about Electric Line Strength | whenever the lines are closer to each other, the field is strong. Whenever the lines are spaced out, the fields are weaker. |
| How to solve a question asking where the third charge won't experience net force | Use the Fe = qE and know that E = 0 for no force. Electric fields point away from pos charges and towards neg. You need to find a space where the vectors are pointing opposite and the magnitude of E1 = E2. |
| Solving for where the third object won't experience net force continued | Set E1 = E2, then substitute for kQ1/rq^2 = kQ2/rq^2. r1 = distance from Q1 to point of interest, same for Q2. Put in the Q values, cross multiply to get eq between r's. If you don't have r values, make equation with sum of r's and sub original into it |
| Solving where the third object won't experience zero net force with different charges | The centerpoint won't have zero netforce. There are two different areas you could use, but only one will be zero at once. Set up an equation between the r's and eliminate the area that doesn't correspond to the right distances. |
| How to solve for q3 when you are given other charges and their locations | Remember that like charges repel and unlike attract. Use the f = kq1q2/r1^2 to first find relationship between Q1 and Q3, then Q2 and Q3. To find Fnet, you need to do F1 + F2. First find the total x component, then y, then add together with Pythag |
| The angle used for triangle and square problems is usually... | 45 |
| Uniform Electric Field | Created between two oppositely charged, closely spaced parallel plates where the electric field strength (E) is constant in both magnitude and direction. The lines are directed from the positive to the negative plate. |
| The episilon in the uniform electric field equation is also equal to... | q/A |
| How to determine the magnitude of force acting on the particle and the rate of acceleration in an electric field problem... | You can use net F = qE to find the net force where q is the charge and E is the magnitude of the electric field. You can use F = ma once you have an answer to get the value for acceleration. |
| Electric Field Inside a Conductor | The inside has no net electric field while the electric field exists only outside the conductor |
| Explanation for Car | If you place a conducter between two oppositely charged plates, charges rearranged on its surface so the electric field inside becomes zero. During lightning, charge stays on the outside of the car (a conductor), shielding the inside so people stay safe. |
| Faraday's Cage Effect | Conductive enclosure that blocks external electric fields by distributing on the surface. Dr. Brahmia did not get hurt while wearing the suit and getting close to the electrosphere! |
| Gauss's Law | the electric flux through a closed surface is equal to the net charge enclosed by the surface divided by episilon 0, the permivitiy of free space. FORMULA GIVEN ON SHEET. |
| Electric Flux | the amount of electric field passing through a surface, which is maximum when the surface is perpendicular to the field and zero when parallel. If the surface is angled, you may have partial flux. E * A = maximum flux |
| Tip about Electric Flux | remember that only the perpendicular portion counts, even if you're solving an angle for partial flux. E * At = flux |
| How to solve problems where it asks for the charge on each sphere after a negatively charged rod is brought close to two spheres that are touching? | The negative charge will move as far away as possible due to repulsion, while the positive charges are attracted to the rod. The closer sphere will be positively charged and the further will be neg. Furthermore, the further has more e- and greater mass. |
| If the rod coming closer to the sphere is positively charged... | The opposite would happen with the closer being negative and the further being positive. Remember that if the rod is moved away first, the charges will go back to normal and the two spheres will be in a neutral state. |
| How to determine the magnitude and direction of the electric field at a certain spot? | Draw arrows starting from x and going away or towards the charges. Then set up an equation with k|q1|/r^2, solve for r, then E1, and E2 (they may be equiv if they have the same charges). Then find x and y components, Pythag |
| If your angle for a pythag is not perfectly square and equal to 45, you can... | Find it using Pythag |
| Finding the angle and tension of the hanging thread when given charge, distance, and mass | Draw a free body diagram with T, mg, and Fe. Remember that a hanging mass is in static equilibrium with Fnet = 0. Split the tension into components and set equal to 0. Solve for Fe, find the ratio of the equations, and then do tan for angle. Sub T back in |
| Find magnitude of net electrostatic force exerted on a point charge across a square | Find like and unlike charges, draw your vectors originating from the charge of interest, divide the diagonal vectors into components - 45 degrees for square - then do kq1q2/r for each of the charges & Q of interest. Find Fx and Fy, add, pythag |
| When given two surfaces through which does the greatest electric flux pass? | The equation is ET * delta A = Q/episilon 0, so you just have to add up the charges and put them over episilon 0. If they both have the same sum of charges, they both have the same electric flux. |
| A negatively charged rod approaches two spheres, one of which is repelled and the other attracted. After they touch, they both repel. What were the initial charges and which sphere had a charge with larger quantity? | A was initially negative and B was initially positive. After touching, both spheres are negative because they repel. A has the larger charge magnitude, because it already had electrons. |
| When presented with a capacitor situation and a hanging ball, how would you find the magnitude of the charge on each plate? | Make a free body diagram with T, Fe, and Mg. Set up equations for the components of T and set equal to 0 for static equilibrium. Find the ratio of the equations and solve for E, then put that E into epsilon 0 equations to get Q. |
| Tip about Fnet | If you're solving for Fnet, then even if the charges are both attracting, you would still subtract the two to get Fnet if they were pulling in different directions. |
| A proton and electron are separated by a certain distance. How would you find the magnitude of the force on the proton? | Use the F = k|q1||q2|/r^2 formula, remember that protons and electrons have the same charges but opposite. |
| Three charges are brought into contact with each other, then separated. What is the final charge on each subject? | Add up the charges then divide by 3 |
| How to find the amount of electrons that make up the final charge on each object? | You take the final charge and then divide by electron charge |
| When they ask for a charge in the corner, they mean... | You can consider the corner to be positive. Attracts negative, repels positive |
| What would you use as radius and angle for an equilateral triangle? | 60 for the angle and standard measurements for the sides |
Created by:
smurtab