PHY122_L06 - 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 6 Currents and Resistance Note: Clicker Channel 21 Lecture 6 1 09/15/2011
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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: 09/15/2011 Lecture 6 2 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 in conductors only if we provide some sort of charge “pump” in the circuit . .. – In the “pump” the mobile charges are pushed against the electrical force (using mechanical, chemical force, or magnetic force), such that they acquire electric potential energy at the terminals Inside the “pump” charges are pushed from a lower to a i h r p t nti l n l ctric l r c higher potential by a non-electrical force! – the resulting potential difference is the “ emf ”: non-elec f( ) W examples: attery (electro hemical) emf( ) q E elec V  – battery (electro-chemical), – dynamo or generator (magnetic), – van der Graaf generator (mechanical), olar cell (photo oltaics 09/15/2011 Lecture 6 3 – solar cell (photo-voltaics), … Note: a capacitor is a storage device, not an emf!
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Resistance he urrent hrough wire of ross section : The current through a wire of cross section A is: – where n q
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PHY122_L06 - PHY122 Physics for the Life Sciences II PHY122...

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