|5.6||ELECTROMOTIVE FORCE AND INTERNAL RESISTANCE|
5.1 ELECTRIC CURRENT
CURRENT = rate of flow of charge,
Conventional Current direction is + to -.
(Amps = Coulombs / second)
Charge carriers in metals are ELECTRONS, in liquids
and gases you can have both –ve and +ve charge carriers
POTENTIAL DIFFERENCE V = W / Q
eg. the pd across a lamp = the number of Joules of
electrical energy changed into light energy when ONE
Coulomb of charge flows through the lamp
1 volt = 1 Joule PER Coulomb, 1V = 1J/C.
RESISTANCE R = V / I units are OHMS W
Supercoductor – a material’s resistance becomes zero
when cooled below a certain critical temperature.
Applications of superconductors to include production of strong magnetic fields,
and reduction of energy loss in transmission of electric power.
RESISTIVITY = resistance of a 1m length of material of
cross-sectional area 1m2 , r = RA / l units W m
V / I characteristics are graphs of pd against current
Learn characteristics for resistor, filament lamp,
A resistor A filament lamp A diode
Ohm’s Law, V proprotional to I gives constant slope on graph,
applies to constant resistance ‘Ohmic conductors’.
Lamp, diode, thermistor are Non-Ohmic devices.
You should be able to explain how temperature
affects resistance in a lamp filament and in a thermistor
Applications of thermistors to include temperature sensors.
Note graphs may be plotted either way up!!
Ammeters have low resistance (Ideal = 0), Voltmeters have high resistance (Ideal=∞)
RETURN TO CONTENTS
SERIES CIRCUITS : Current same everywhere,
Pds across components add up to the supply
voltage (Kirchoff 2), R = R1 + R2 + R3
PARALLEL CIRCUITS : PD same across each
item in parallel, Current splits up at junctions
(Kirchoff 1) I = I1 + I2 + I3
1 / R = 1 / R1 + 1 / R2 + 1 / R
NB quick rule for 2 in parallel R = PRODUCT / SUM
POWER = VI = I2 R = V2 /R units are Watts
ENERGY = POWER X TIME = ItV units are Joules
Heating effect– when charge carriers pass through
a material they collide with its atoms causing the atoms
to gain energy, this results in a temperature rise.
Conservation of charge (Kirchoff1): total current going in to a junction = total current coming out of that junction
Conservation of energy (Kirchoff2): in a series circuit the sum of the pds
across all of the components = the pd across the supply.
5.5 POTENTIAL DIVIDER
POTENTIAL DIVIDER: A set of resistors in series which divide the supply voltage
between them, eg. to find the voltage across R2 :
Vout = R2___ X Vin
R1 + R2
5.6 ELECTROMOTIVE FORCE AND INTERNAL RESISTANCE
ELECTROMOTIVE FORCE (EMF, E) of a supply is the
pd across its terminals when no current is drawn from it.
Also, EMF is the number of Joules of source energy
(chemical) converted to electrical energy when 1
Coulomb of charge flows. EMF = ENERGY / CHARGE
INTERNAL RESISTANCE r = the resistance of the
voltage source itself, and r = emf/maximum current
TERMINAL PD = the pd across the source whilst
current is drawn from it (always less than the EMF)
LOST VOLTS = the pd across the internal resistance
= EMF – TERMINAL PD
EMF E = I (R + r) or, E = V + Ir
RETURN TO CONTENTS