Introduction

CIRCUITS

HOW  CAN YOU GET CHARGES TO FLOW ??

“PUMPING” CHARGES

·         To make Charge carriers flow through  a resistor 

·         Need to establish a potential difference between the ends device

·         Connect each end of the resistor to one plate of charged capacitor

But..

·         The flow of charge acts to discharge the capacitor

·         Bringing the plates to the same potential

·         No longer an electric field in resistor so the flow of charges stop

We need a “Charge pump” To produce a steady flow of charge

·         Charge pump –device that doing work on charge carriers so that the potential differences between a pair of terminals are maintains ---

This device called as emf device(electromotive force)

·         The motion of charge carriers in terms of the required energy(the emf devices supplies the energy for the motion via the work it does.

·         Common emf ----battery,electric generator,solar cells,fuel cells,thermopiles

                       ----electric eels..

                       ------they do the work on charge carriers

                       ------maintain a potential difference between their terminals

WORK , ENERGY , EMF

Simple circuit:

--- Consider emf as the battery.. ( positive terminal --- higher elecrtric potential than                          negative terminal)

--- if emf not connected to a circuit ,internal chemistry of the device does not cause any net flow of charges within it)

--- within emf , +ve charge carriers move from region negative terminal àpositive terminal)

--- but the electric field between the terminals is opposite from this motion (positive terminal à negative terminal)

--- there must be some source of energy within device enabling it to do work on the charges forcing them to move.(maybe chemical in battery or mechanical forces in electric generator)

View of work and energy transfer

--- definition emf :

in words:work per unit charge

                                  :moving charge from its low-potential terminal to its high

                                   Potential terminal ( -ve to +ve )  --- unit volt.

Ideal emf : lacks any internal resistance to the internal movement of charge  from terminal to terminal.

                 :potential difference between terminal = emf

Real emf : has internal resistance to the internal movement of charge.

                :when connected to the circuit

                -if there is no current through it , potential difference = emf

                -if there is current through it , potential difference not same with emf

              

Circuit containing :

--- two ideal rechargeable batteries A and B

--- resistance R

--- electric motor M

Process occur :

1)   Batteries are connected to send charges around the circuit in opposite direction

2)   Actual direction of current determined from battery with larger emf (B)

3)   Chemical energy in Battery B decreasing because  energy  transferred to the charge carriers passing through it

4)   Chemical energy in Battery A  increasing because current in it is directed from positive terminal to negative terminal.

5)   Battery B charges battery A , providing energy to motor M and energy dissipated by resistance R

                                                                                                                                                  

 Fazihan