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Physics Electr Field
Physics Autumn Y13
| Question | Answer |
|---|---|
| What is a field? | A region of infuence i.e. region where a force can act on an object |
| What do gravitational fields act upon? | Masses |
| What do electric fields act upon? | Charges |
| Electric field lines | Follow path of positive ion (WWAPID). Enter or leave surfaces at right angles. Density shows strength of field. Electric field inside conductor is zero: lines only start at the surface. Charges move due to p.d. inside not difference in field |
| Drawing field lines | MAKE SURE THEY STOP AT SURFACE Need plenty (5 or 6). *Make sure equally spaced* |
| Point between two positive charges | Called a neutral point - no field |
| What is a uniform electric field? | Electric field between 2 parallel plates |
| Edge effect | Not always added - Q specifies. The outer line at the edge of the charged plates curves out slightly before going back in |
| Type of field produced by point charges and uniformly charged spheres | Both make radial fields - identical past where the sphere's surface would be. Only difference is they dont go to the centre of the sphere (which is denoted with small charge symbols around the circumference) |
| Describe the field shape from a conductor with constant charge | Equally spaced and parallel |
| Positively charged sphere fixed above negative plate. How would the electric field change as the sphere charge increases? | Greater density of field lines (more, and close together). Also lines will be slightly flatter going out |
| What is k for the F = Qq/4 pi e r^2 equation | 1/(4*pi*permittivity) Unit is mF^-1 It has a constant value, so if a question asks you about calculating it, dont just do the unit |
| Proportionality between F and r | F proportional to 1/r^2 Inverse square proportionality |
| What is the F = Qq/4 pi e r^2 equation used for | Force between 2 point charges. Always a N3L pair. Make sure to do +ve or -ve sign. -ve means attraction, while +ve shows repulsion Even if it says 'maximum force', give the negative seemingly Watch out if it says magnitude |
| What is epsilon subscript zero? | Permittivity of free space (how easy it is to set up/hold an electric field in a vacuum) |
| Weight of electron | mass x 9.81 |
| Whats one major difference between electric and gravitational fields produced by particles? | Electric can be attractive or repulsive but gravitational only attractive |
| What is E in the formula booklet | For this topic, its electric field strength. Energy is always W (work done) |
| What is electric field strength? | The force per unit of *positive* charge E = F/Q *Vector* quantity |
| Unit for electric field strength | NC^-1 or Vm^-1 |
| Electric field strength from isolated charge equation | The fraction with one Q |
| Electric field strength between 2 parallel plates | It is uniform. aka a potential gradient E = voltage across plates/distance between plates Uniform between the plates and at right angles |
| For object floating in electric field | Specify force X must be equal and opposite to force Y The electric field strength is still the same as everywhere else in the field even though halfway between the plates: uniform strength |
| Basic overview of the practical to investigate coulomb's law | 2 conductive spheres held above each other, with bottom one connected to digital scales and top one fixed. Flying lead connected to the positive 5000V terminal touched to both spheres. Distance between centres of spheres taken (taking as point charges) |
| How to hold spheres in coulomb's law experiment | Insulating rods (also using one for the flying lead) |
| Digital scales for coulomb's law experiment | Very high resolution - they had +- 0.001g ones |
| How to find emf in circuit | IV graph across cell crosses axis - this is the emf. MUST mention that current is zero there |
| Millikan oil drop experiment overview | Oil spray droplets suspended by uniform electric field by changing electric field strength. Trying to find charge of an electron. |
| Millikan oil drop experiment calculations | E = V/d F subscript E = Eq = Vq/d F subscript g = mg Vq/d = mg when stationary mgd/V = q We assume we know the mass of the droplets rather than how they worked it out with velocities |
| Forces acting upon oil droplet | Weight, possible force due to electric field, drag force, buouancy In our electron gun projectile motion calculations though everything is negligible compared to electric field |
| Results of Millikan experiment | From many repeats the charge appeared to be quantised. q = Ne He estimated it to be 1.59 x 10^-19, which is off due to his number for air viscosity being wrong Controversies removing results not valid - he just removed any not in control setting |
| General usage nowadays for electric and magnetic fields | Electric fields generally for accelerating particles while magnetic generally for steering |
| How to describe the acceleration due to an electron gun | CONSTANT |
| p.d. when two plates are charged +400 and -400 | 800 p.d. |
| Electron moving perpendicular to electric field | Electric force always in same direction. Like projectile motion split horizontally + vertically. g often ignored as it is so minor Most Qs it wont collide with the plates. |
| Proving the shape of the electron trace (using Sx and Sy for displacements) | Horizontally no force so Sx= v*t, so Sx/v = t Vertically, F = V*e/d, so acceleration = V*e/d*m Sy = u*t + 0.5*a*t^2 = 0.5 * E * e * t^2 /m Sy = (E*e*x^2)/(2*m*v^2) Sy proportional to x^2 - therefore parabola shape |
| Constant E in uniform field | Remember it- stop using d for distance of particle - its only based on plate separation |
| Properties of all EM waves | Transverse, travel at 2x10^8 in a vacuum, can be polarised, etc |
| How is electric field strength defined | Force per unit charge |
| Electron moving parallel to electric field | Calculating speed of proton as it hits the other plate has 2 approaches. Suvat is long Work done = charge x voltage Work done = KE gained by proton Irrespective of plate separation - just caused by energy per unit charge |
| When electron fired into hole in 2 plates going against the direction of the electric field | Will return back to its original speed as it returns back to the start point - like throwing a ball |
| Where you have plates A and C at -400V and +200 V with a mesh between them at +400V | For the speed gained between A and C, just use 600V |
| Electrical potential energy graphically | 2 equally charged particles brought together, F proportional to 1/r^2 Area under graph going from r on the x axis to infinity is the EPE, where a force is applied so the charges are brought a distance r apart F x d = E, so area represents work done |
| Charges exerting force on others | Electrical charges exert forces on any other charge and potential energy arises from any collection of charges. Equally charged particles where one is fixed in space - as one is brought closer, repulsive force so potential energy |
| Definition of EPE | The work done to move a charge FROM INFINITY to a distance r from a second charge Energy = Qq/(4*pi*permitivity*r) This is the EPE stored by 2 charges Q,q separated by distance r No r^2. When r = infinity, E = 0 |
| Definition of electrical potential at a point | Electrical potential (V) at a point is the work done per unit charge in bringing a *positive* charge from infinity to that point. V = EPE/q = Q/(4*pi*permitivity*r) |
| Electrical potential sign | Can be positive or negative dependent on the charge but it is a scalar quantity |
| Electrical potential energy around a charge | Around any charge, we can calculate the electrical potential of points around it |
| Equipotentials | Join places of equal potential. At right angles to field lines. |
| Equipotentials for point charges | Equipotentials further away are further apart (V = Q/4pier, so V proportional to 1/r) Have a potential of zero at infinity |
| Equipotentials in uniform field | Equally spaced lines parallel to plates. Potential increases in opposite direction to field (direction of WWAPID) |
| Potential gradient | Change of potential per unit distance in a given direction. In uniform field, it is constant and equal to V/d |
| Charge moving along equipotential | As long as moving along equipotential no work done. W = QV and V is potential *difference* so if no difference no work done. No movement in direction of field KE and PE constant |
| Foil-wrapped sphere diameter 22cm charged with positive electrode of 5kV source. Find charge | V = Q/2*pi*e*r r is the distance between the outside of the sphere where V = 5000 and the centre where we take charge origin from |
| Two possible units for Eo | C^2/N/m^2 aka F/m |
Created by:
Pyrogearos2