PHYSICS STUDY GUIDE
CHAPTER 13: ELECTRIC CIRCUITS
TOPICS:
 Electric current
 Resistance
 Ohm’s Law
 Circuits in series
 Circuits in parallel
1. ELECTRIC CURRENT
PHYSICAL QUANTITY
Electric Current ( I )
DEFINITION
Amount of electric charge (q) per unit of time (t).
MATH MODEL
I=
UNITS
Ampere ( A )
q
t
2. RESISTANCE
PHYSICAL QUANTITY
Resistance ( R )
DEFINITION
Property that determines how much current will flow.
UNITS
Ohms (  )
3. OHM’S LAW


MATH MODEL
I=
Electric current (A)
DEFINITION
Electric current ( I ) is directly prportional to voltage (v).
Electric current ( I ) is indirectly prportional to Resistance (R).
v
R
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
Electric current vs. voltage graph:
0
5
10
15
voltage (v)
20
25
30

Shape: diagonal straight line

As voltage increases, so does the
electric current.

The slope of the graph is:
slope =
1
R
4. ELECTRIC CIRCUITS
PHYSICAL QUANTITY
CIRCUITS IN SERIES
CIRCUITS IN PARALLEL
Stays the same
IT = I1 + I2 + I3 +. . .
VT = V1 + V2 + V3 +. . .
Stays the same
RT = R 1 + R 2 + R 3 + . . .
1
1
1
1
=
+
+
+ ...
RT
R1 R 2
R3
PT = P1 + P2 + P3 +. . .
PT = P1 + P2 + P3 +. . .
ELECTRIC CURRENT
VOLTAGE
RESISTANCE
POWER
5. CIRCUITS IN SERIES
R1
R2
R3
VT
I
(A)
V
(v)
R
()
P
(w )



LIGHT BULB
1
0.34 A
LIGHT BULB
2
0.34 A
LIGHT BULB
3
0.34 A
Measured with
ammeter
Measured with
ammeter
Measured with
ammeter
TOTAL
0.34 A
56 v
40 v
24 v
120 v
Measured with
voltmeter
Measured with
voltmeter
Measured with
voltmeter
Measured with
voltmeter
164.7 
117.6 
70.6 
352.9 
Calculated
using Ohm’s
law
Calculated
using Ohm’s
law
Calculated
using Ohm’s
law
Calculated
using Ohm’s
law
19 w
13.6 w
8.2 w
40.8 w
Calculated
using P = I ·
V
Calculated
using P = I ·
V
Calculated
using P = I ·
V
Calculated
using P = I ·
V
Stays the same in electric
circuits connected in SERIES
VT = V1 + V2 + V3
Sum of the individual
voltages
VT = 56 v + 40 v + 24 v
VT = 120 v
RT = R 1 + R 2 + R 3
Sum of the individual
resistances
RT = 164.7 + 117.6 +
70.6
RT = 352.9 
PT = P 1 + P 2 + P 3
Sum of the individual powers
PT = 19 W + 13.6 W +8.2 W
Brighter bulb: Light bulb 1
Greater power, brighter bulb
P=I2·R (Electric current is constant, greater resistance, brighter bulb)
PT = 40.8 W
6. CIRCUITS IN PARALLEL
LIGHT BULB
1
I
(A)
V
(v)
R
()
P
(w )
LIGHT BULB
2
LIGHT BULB
3
TOTAL
IT = I1 + I2 + I3
Sum of the individual
electric currents
0.500 A
0.625 A
0.750 A
Measured with
ammeter
Measured with
ammeter
Measured with
ammeter
120 v
120 v
120 v
120 v
Measured with
voltmeter
240 
Measured with
voltmeter
192 
Measured with
voltmeter
160 
Measured with
voltmeter
64 
Calculated
using Ohm’s
law
Calculated
using Ohm’s
law
Calculated
using Ohm’s
law
Calculated
using Ohm’s
law
60 w
75 w
120 w
225 w
Calculated
using P = I ·
V
Calculated
using P = I ·
V
Calculated
using P = I ·
V
Calculated
using P = I ·
V
1. Mathematical model
1
1
1
1
=
+
+
RT
R1
R2
R3
1.875 A
IT = 0.500A + 0.625A +
0.750A
RT = 1.875 A 
Stays the same in electric
circuits connected in
PARALLEL
See below
PT = P 1 + P 2 + P 3
Sum of the individual powers
PT = 60 W + 75 W + 120 W
PT = 225 W
4. Add the fractions
1
15
=
RT
960
2. Substitutions
1
1
1
1
=
+
+
RT
240 192 160
5. Cross multiply
960  = 15 · RT
3. Find the common denominator
1
4
5
6
=
+
+
RT
960 960 960



6. Solve for RT
RT = 64 
(this number must match RT in the chart above).
Brighter bulb: Light bulb 3
Greater power, brighter bulb
P=V2 / R (voltage is constant, greater resistance, dimmer bulb)