1. DC circuits
Physics 122
12/3/2018
Lecture X

2. Test problem
For resistors connected in series, which of the following will be the same for each of the resistors?
A The current through.
B The potential difference across.
C The resistance of.
D The power dissipated by.
12/3/2018
Lecture X

3. Test problem
For resistors connected in parallel, which of the following will be the same for each of the resistors?
A The current through.
B The potential difference across.
C The resistance of.
D The power dissipated by.
12/3/2018
Lecture X

4. RC circuit
RC circuit contains at least one capacitor (C ) and at least one resistor (R ). +Q C -Q
12/3/2018
Lecture X

5. Electric battery For idealized battery emf – supplied voltage
Real life batteries have internal resistance r
Terminal voltage (supplied to the circuit)
Vab=emf-Ir
12/3/2018
Lecture X

6. Kirchhoff’s rules Junction rule (charge conservation):
At any junction point the sum of all current entering the junction must equal the sum of all currents leaving the junction
Do not disregard signs!
Assign initial signs based on your best guess – if you were wrong the currents will turn to be negative (opposite to initial guess)
Loop rule (energy conservation):
The sum of the changes in potential around any closed path of a circuit must be zero.
Choose a direction in which you circle the loop (random)
For resistor Vab=-IR – negative if current along the loop direction
For resistor Vab=+IR – positive if current opposite the loop direction
For battery Vab – positive if the direction is from – to +
For resistor Vab –negative if the direction is from + to -
12/3/2018
Lecture X

7. Kirchhoff’s rules Loop rule (energy conservation):
The sum of the changes in potential around any closed path of a circuit must be zero.
Choose a direction in which you circle the loop (random)
For resistor Vab=-IR – negative if current along the loop direction
For resistor Vab=+IR – positive if current opposite the loop direction
For battery Vab – positive if the direction is from – to +
For resistor Vab –negative if the direction is from + to -
12/3/2018
Lecture X

8. Series vs parallel - Req
R1=R2R R2=R3=R
Req=3R4R
R1=R2R R2=R3=R
Req=R/3=2R/62R/5
If any of R’s increased the equivalent resistance will increase
in both parallel and series connection.
12/3/2018
Lecture X

9. Series vs parallel - I R1=R2R R2=R3=R Req=3R4R I=V/Req=V/3RV/4R
I1=I2=I3=I
R1=R2R R2=R3=R
Req=R/3=2R/62R/5
I=V/Req=6V/2R5V/2R
I1=V/R1=V/RV/2R
I2=I3=V/R – the same
12/3/2018
Lecture X