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Electricity and magnetism Core / HL 5 - SL ● ● Electric field: area around a charge where a small test charge will experience a force Electric field strength: force per unit charge experienced by a small test charge when placed in an electric field E = Fq ● Coulomb's law: any two charge will experience a force which is proportional to the product of their charges and inversely proportional to the distance between them squared → qQ r2 where 2 −1 −2 C N m ) F =k 8.85×10^(−12) ● k= 1 4πε0 (ε0 is called the electric permittivity of vacuum and is Relationship between electric field strength and Coulomb’s law: E = F q and F = k qQ r2 , so and ELECTRIC CURRENT: ● ● Electric current is the flow of electrons (in conductors) or ions (in solutions) Δq Current (I) is the rate of flow of charge and is measured in Ampere (A) → Δt ● In a conductor, I = nAvq ● The potential difference V between two points is the work done per unit charge to move a point charge from one point to the other. V is measured in Volt (V) and V = Wq ● ● Whenever there is a potential difference there has to be an electric field. We define the electron volt as the work done when a charge equal to one electron charge is taken across a potential difference of one volt → 1eV=1.6 ×10−19 C×1V=1.6×10−19 J RESISTANCE AND ITS RELATIONSHIP WITH I AND V: ● ● The electric resistance R of a conductor is defined as the potential difference V across its ends divided by the current I passing through it. R = VI Ohm’s Law → I∝V ● ● ● It is found from experiment that the electric resistance R of a wire (at fixed temperature) is proportional to its length L a nd inversely proportional to the cross-sectional area A: R = ρ AL The constant ρ i s called resistivity and depends on the material of the conductor and the temperature Electric Power: work done per unit time. P = V I = I 2 R = V2 R CIRCUITS: ● ● IN SERIES: R= R1 + R2 + R3 ● I= I 1 = I 2 = I 3 ● V= V 1 + V 2 + V 3 ● ● IN PARALLEL: 1 1 1 1 R = R + R + R ● I= I 1 + I 2 + I 3 ● V= V 1 = V 2 = V 3 1 2 3 EMF: ● ● Electromotive force: the emf of a cell is the work done per unit charge in taking the charge from low (-) to high (+) potential emf= I (R + r) MAGNETIC FIELDS: ● Grip rule: Grip the wire with the fingers of the right hand in such a way that the thumb points in the direction of the current. Then the direction in which the fingers curl is the direction of the magnetic field vectors. ● There is no magnetic force on a moving charge if the charge moves along the field direction. ● If the magnetic force is F when a charge q moves with speed v making an angle θ with the direction of the field, then the magnitude of the field, B, also called the magnetic flux density, is ● ● ● A charge q moving with speed v in a region of magnetic eld of magnetic ux density B will experience a magnetic force F given by → F=qvBsinθ The formula for the magnetic force on a length L (L is that length of the wire that nds itself in the region of the field) is: F=BILsinθ To find the direction of the force, use the right-hand rules