ELECTRICAL FORCES

Charges: Like air molecules (following the wind analogy). Hence electrical energy is like wave motion.Note: where there is a potential difference, charges do not necessarily flow/ current not necessarily generated.Electric current: Rate of flow of electric charge (wind analogy, in layman terms ‘charge flow’, hence cannot say current flows = ‘charge flow flows’ ^-^ Also neither can current be ‘used up’ by devices. Whether a current exists or not does not depend on whether there are charges – all circuits have charges!, but whether the charges can move)Current depends on number of cells in circuit and the number and nature of other components In an electric field (potential difference applied across conductor), electrons in conductor are forced to accelerate (in direction opposite to electric field). This flow of electrons constitutes an electric current.When there is a current, electrons collide with lattice ions, increasing temperature of material, hence resistance of material, thus work needs to be done in moving electrons. * When a heating element is first switched on, initial current higher as the element is cold and thus has a smaller resistance compared to when it is heated. As it is heated, its resistance increases, current drops.Electrical resistance: Ratio of the potential difference to the currentOhm’s Law: The current through a conductor is directly proportional to the potential difference across it provided the temperature and other physical conditions remain constant.Ohmic conductors: Conductors that have a constant resistance at constant temperature, current and potential difference are proportional.Resistivity: At constant temperature, three factors; length, cross-sectional area, material; affect the resistance of a wire.Resistors: Drop the potential when a current flows through it.Electrical conductor: Material which allows flow of charge through it.Electrical insulator: Material which does not allow flow of charge through it.Flow of charge = Flow of electronsElectroscope: Determines if an object is charged, magnitude of displacement of gold foil indicates magnitude of charge.Conservation of Charge: Total charge of an isolated system cannot change.Coulomb’s law for electric force: The electrostatic force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them.Charges suspended from springs:Know how to find point where electric field due to two opposite charges equal to zero/ cancels out:Electric field strength: The force per unit charge experienced by a small positive test charge q in an electric field. A vector quantity,.(Test charge needs to be small so that it does not disturb charge(s) being considered.)Electric field inside a metal/sphere is always zero.Electric field lines never cross, they are vectors.Density of electric field lines indicates magnitude of field, get stronger closer to the charge.Electric field lines always travel from regions of high potential to low potential.Electric field strength of charged plates:(velocity in direction of electric field) To derive:(use projectile motion concepts when dealing with movement of charges between parallel plates, just that acceleration is determined by the electric field strength)OR use Conservation of energy method:IE = FE where Total Energy = P.E. + K.E. = Ep + KE = Vq + ½ mv2(field lines at sides should be curved)Electrical potential energy: Energy that a charge has a result of its position in an electric fieldElectric potential (at a point in an electric field): Work done in moving a unit positive test charge from infinity to that point in an electric field/ between 2 points. (Route taken does not affect amount of work done)Work done depends on value of charge, independent of speed or path taken. A scalar.Units: 1V=1JC-11 electronvolt: Work done when a charge equal to 1 electron charge is moved across a potential difference of 1 volt.The electronvolt is a unit of energy, not potential difference. Electric potential at infinity = 0For moving charge in electric field: Loss in PE = Gain in KE = Work done = VqGraph of Electric potential against distance: -Gradient = Electric field strength.Electric equipotential surfaces: Points that have the same electric potential, concentric circles around the charge, at the same distance from the charge.Parallel plates: Equipotential surfaces are planes parallel to the platesMoving a charge along equipotential surface, no work done as potential difference between the two points is zero.Electric field lines are always normal to the equipotential surfaces. X and Y are two points in an electric field, potential Vx and Vy respectively, where Vx>Vy. Small positive test charge placed at X.Work done per unit charge by electric field as charge moves from X to Y is +(Vx-Vy). Electric power: Electromotive force:- EMF of a cell: Energy per unit charge supplied by an electric source. OR Electrical power supplied by the source per unit current delivered by the source . Electrical energy generated - Potential difference between terminals of a cell Energy per unit charge dissipated in the circuit external to the electric source Electrical energy converted to other forms of energy KNOW HOW TO DIFFERENTIATE EMF OF A CELL AND PD BETWEEN ITS TERMINALSSeries: I1 = I2, V = V1 + V2 +… ,Parallel: V1 = V2, I = I1 + I2 +… ,Brightness of a bulb depends on power (current AND voltage)E.g. Doubling number of batteries doubles the voltage, since V=IR, also roughly doubles the current. Hence P=2V(2I)=4VI, hence brightness is quadrupled.E.g. For two bulbs in series, resistance is doubled, since R=V/I hence current is halved, while each bulb only gets half the voltage. Thus brightness of each bulb is a quarter as bright as a single bulb. Regardless series or parallel arrangement, power of devices always adds up! For battery of emf E and internal resistance r, with ammeter and variable resistor, by varying resistor and measuring corresponding current, possible to find value of emf E and internal resistance r, using V=IR where R=(Rvariable resistor + r) To measure voltmeter across resistor in parallel, find total resistance of parallel section of circuit: . Then find current flowing throughout whole circuit using emf of battery and total resistance of circuit (including series portions). Voltage is same throughout for parallel, henceV = (current throughout whole circuit)(total resistance of parallel section of circuit)

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