5.2 - Internal Resistance, Power & Combining Resistors
5.2 - Internal Resistance, Electrical Power, Resistors in Series & Parallel 1
Internal ResistanceAll cells are made of materials that haveresistance, this is called InternalResistance (r)If the cell is connected to an ExternalResistor (R) then some of the energy isconverted from electrical energy to heatenergy inside the cell.The PD available for the circuit is EMF (E)therefore less than the EMF of thecell.This picture shows that some ofthe EMF is lost before the chargeseven leave the battery. PD (V) 2
Internal Resistance (cont) IUsing Ohms Law: E= Ir + IR I For the main Resistance in the circuit: V=IR Missing volts: Ir 3
More Internal ResistanceIf you connected a wire directly from one end of the Ibattery to the other then the Resistance (R) wouldbe very small.The Current (I) that flows would be enormous. IThis would cause loads of heat to be wasted by thecurrent as it moves through the Internal Resistanceof the battery.The battery would get very hot and run out ofenergy very fast. This is called a short circuit.Try it with the PhET Circuit Construction Kit:http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc or click on the pictureOR connect some steel wool across a battery. 4
Electrical Power (P)Power means the energy per second (Watts or Js-1)Remember some definitions:By substitution: 5
More Electrical Power Equations (P) Using: and Ohms Law: We can find the Power DISSIPATED (converted to heat) by a resistor. By substituting from Ohms Law: V=RI OR I=V/R 6
Combining Resistors1. Series Circuits Putting two resistors together is like putting two separate flights of stairs one after the other. The stairs are harder to climb 7
Combining Resistors2. Parallel CircuitsBecause there are more ways of getting round the circuit this is likemaking the stairs wider. The Resistance is less. 8
See how combinations of Resistors work: http://www.walter-fendt.de/ph14e/combrlc.htm or click on the picture Add Resistors and then highlight areas of the circuit to see the total resistance. Impedance is another word that can be used for Resistance 9
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