Combinations of resistors

1
Combinations of resistors

• Resistors in series
• V V1V2V3 IR1 IR2 IR3
• Req V/I R1 R2 R3

R1
R2
R3
V
2
Combinations of resistors

• Resistors in parallel
• I I1I2I3 V/ R1 V/ R2 V/R3
• I V/Req
• 1/Req 1/ R1 1/ R2 1/R3

R1
R2
R3
V
3
Combinations of resistors

• Resistors in parallel
• 1/Req 1/ R1 1/ R2 1/R3
• Req R1R2R3 / (R1R2 R2R3 R1R3)
• Notice Req is smaller than any one of them!

R1
R2
R3
V
4
Demos Bulbs in series and in parallel
5
Resistivity

• Current through the rod
• I sAV/d
• where A is a cross section area of the rod, d is
  its length and V is the applied voltage.
• Inverse of s is called resistivity ?
• ? 1/s
• R V/I ?d/A

6
Battery and Resistor Circuit Simulation

• http//phet.colorado.edu/new/simulations/sims.php?
  simBatteryResistor_Circuit

7
Why the lights in the car dim when we start the
engine (when the starter motor is running?

• Starter motor and lights are connected in
  parallel so adding running the motor should not
  change the voltage on the lights.

8
Non Ideal Battery

• Electromagnetic force (emf) of a potential source
• Terminal voltage
• Internal resistance of the battery
• Notice it is always connected in series with the
  load!
• What is the terminal voltage of a 1.5 V battery
  with internal resistance 0.2 O loaded by a 0.4 O
  resistor?