PHY122_L07 - 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 7 Currents and Resistance Note: Clicker Channel 21 09/20/2011 1 Lecture 7
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Summary: Currents n Analogy: water flow = charge flow = electrical current: Q I – current flows from high electrical potential to lower potential •c h a r ge can be “ pumped up ” inside an emf t • charge can be stored in a capacitor Kirchhoff’s junction law: an lectrical junction the tal low is onserved – in 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, ut oing current is negative: nction 0 i i I out-going current is negative: Kirchhoff’s loop law is equally intuitive: •i n a n y closed loop , the sum of all potential teps is zer 2 3 V VV  junction steps is zero … closed loop 0 i i V 1 7 6 5 4 7543 0 VVVV  123456 0 1276 0 Note: signs of Δ V i depend on the direction ! 09/20/2011 2 Lecture 7
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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 – solar cell (photo-voltaics), … Note: a capacitor is a storage device, not an emf! 09/20/2011 3 Lecture 7
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Resistance Resistance R and Resistivity ρ : –The proportionality constant ρ is dependent on material, temperature, V R I linear vs. VI V I L A and typically not even constant with voltage… –U n i t s : • Resistance: [ R ] = V/A Ohm = • Resistivity: [ ρ ] = (V/m)/(A/m 2 ) = Vm/A Ohm ·m = m Ohm’s “Law” : Δ V = I R R R (a thin wire or film wrapped Combining Resistors: Series: R 1 R 2 a b c (p p on an insulating rod) ab ac
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PHY122_L07 - PHY122 Physics for the Life Sciences II PHY122...

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