Transcript Unit 3 Day 5: EMF & Terminal Voltage, & DC Resistor Circuits

Unit 3 Day 5: EMF & Terminal Voltage,
& DC Resistor Circuits
• Electromotive Force (EMF)
• Terminal Voltage
• Internal Resistance
• Series, Parallel, and SeriesParallel Resistor Networks
• Kirchhoff’s Current & Voltage Laws
EMF vs. Terminal Voltage
• For current to flow through a circuit, we need a device to
supply the electrical energy, ie: a battery
• A device that supplies electrical energy to a circuit is
called the source of what is referred to as the
Electromotive Force or EMF (  )
• EMF is a misnomer because the battery does not deliver
a force in Newtons
• The potential difference ΔV=Vab , is measured across the
terminals of a battery
Internal Resistance
• The battery is not a constant source of current because
of internal losses within the battery
• The chemical reaction that produces the electrical
energy also produces heat, and may be modeled as a
resistor internal to the battery. This is called the internal
resistance “r”
Battery Circuit
Vab  V    I  r
where I 

Rr
• The terminal voltage is always smaller than the EMF
Resistors in Series


Req  R1  R2  R3
V
I
Req
• The current is the same through each resistor
V1  I  R1 V2  I  R2 V3  I  R3
• Kirchhoff’s Voltage Law states:
V  V1  V2  V3
Series Circuit
• Three lamps connected in a daisy-chain fashion
can be considered as three resistors in series
Resistors in Parallel
1
1
1
1
 

Req R1 R2 R3
I
V
Req
• The voltage across each resistor is the same as the
battery voltage
V  V1  V2  V3
• Kirchhoff Current Law states:
I  I1  I 2  I3
where I1 
V1
V
V
I 2  2 I3  3
R1
R2
R3
Parallel Circuit
• Three lamps connected across each other can be
modeled as three resistors in parallel
R1R2
• For only 2 resistors in parallel, Req becomes: Req 
R1  R2
Series-Parallel Resistor Networks