UCSD Summer Session
Physics 2B Formula Sheet
UNIT 3 –Resistor & Network (combo) Supplement
Three macroscopic parameters completely define how a resistor behaves in a circuit:
the Resistance R, the applied Potential V, and current I. They are always related by
V = IR
Ohm’s Law
We connect Resistors together into networks in different ways. The basic building blocks are
SERIES and PARALLEL. These in turn can be joined to make more complicated circuits.
In these configurations, one equivalent capacitor REquiv can replace them all, and we can use
the same equation for the equivalent. When we do so, we may still be interested the R, V, and
I of each of the individual elements. The following table summarizes the characteristics for
the two types of networks.
Of course, when we combine these into more complicated networks, the same rules apply.
Supplementary Resistor Circuit Table
Parallel
R1
Schematic Diagram
R3
R1
R2
R3
RSeries = R1 + R2 + R3 +"
Equivalent Shortcut for
Identical Resistors
1
1
1
+
+ +"
RParallel R1 R2 R3
1
RParallel = × R
n
Potential (Voltage)
V1 = V2 = V3 " = Vcircuit
V1 + V2 + V3 + " = Vcircuit
Current
I1 + I 2 + I 3 + " = I circuit
I1 = I 2 = I 3 " = I circuit
Ratios
I1 R1 = I 2 R2 = I 3 R3 " = I cir Rcir
V
V1 V2 V3
=
= " = circuit
R1 R2 R3
R parall
Equivalent Resistance
1
R2
Series
=
RSeries = n × R
The Potential and Current lines are statements of Kirchhoff’s Laws.
The Ratios Line are the Potential & Current equalities substituting Ohm’s Law.
In parallel, the ratio of currents is the inverse ratio of the resistances.
In series, the ratio of the voltages is the direct ratio of the resistances.
Remember: Ohm’s Law must be used carefully! It applies only to one device at a time.