PHY122_L05 - PHY122 Physics for the Life Sciences II PHY122...

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HY122 hysics for the Life Sciences PHY122 Physics for the Life Sciences Physics for the Life Sciences II II Lecture 5 Currents and Resistance Note: Clicker Channel 21 Lecture 5 1 09/13/2011
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Current Current = = Flow of Charges Flow of Charges In the preceding notes, we saw that the static electric field E inside a conductor must always be zero – if not, the mobile charges will re-arrange them- selves such as to cancel the electric field E : + + + + + + + - - - - - - + + + + + + - - - - - - E=0 E E –note that only a small fraction of the mobile char- ges (electrons) need to rearrange themselves in or- - + - E inside = E + E = 0 ndré- arie mpère der to cancel even a strong external electric field… If a net electric field exists inside a conductor (i.e. a Vol- tage gradient : E x = Δ V/ Δ x , …) an electric current I will flow: André Marie Ampère (1775 – 1836) – Current: I ≡Δ Q/ Δ t , units: A (mpere) = C/s – this current dies out very quickly, unless the build-up of charges at the ends is prevented by continuously delivering ( source- ing) and 09/13/2011 Lecture 5 2 removing ( sink -ing) them at the ends! – A well-charged capacitor, or a BATTERY may provide such… V
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A charge of 3.0 Coulombs flows though a wire in a time of 0.20 s; the current is … A 15 168 62 13 1 15.000 1 60 1 Matched Acceptable Value: 169, Within Range: 169 Acceptable Value: 15, Acceptable Range: {14.9, 15.1} 09/13/2011 Lecture 5 3
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Currents and Current Conservation n Water flow (water current): – goes from high to low; •f r om places with high gravitational potential to places with lower potential – it can be “pumped up” by other sources of energy in water pumps … •i t s potential energy can be stored in a reservoir behind a dam – when ever it gets to a junction, the total amount of flow (water mass/second) is conserved • the amount of flow (mass/second) into a junction equals the out flow All this translates to charge flow = electrical current – current flows from high electrical potential to lower potential Q I t – it can be “ pumped up ” inside a battery or generator … • it can be stored in a capacitor – when it gets to an electrical junction , the total flow is conserved : • the current that goes in = the current that comes out • With convention in-going current is positive, outgoing is negative: 09/13/2011 Lecture 5 4 0 i i I Kirchhoff’s junction law
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Electromotive Electromotive Force Force –em f emf We maintain a constant electric field
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PHY122_L05 - PHY122 Physics for the Life Sciences II PHY122...

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