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Equivalent Resistance, Volts,
Amps
Volts and Amps in a Series Circuit

In a series circuit, the Amps remain
constant throughout the whole circuit
Amps =
2A
Amps =
2A
Amps =
2A
Volts and Amps in a Series Circuit

However, in a series circuit at each resistor the volts
change. When you add up all the voltage at each
resistor, you end up getting the total volts going
Volts3
through the circuit.
Volts
total =
7V
= 2V
Volts2
= 3V
Amps =
2A
Volts1
= 2V
Amps =
2A
Amps =
2A
Volts and Amps in a
Parallel Circuit

In a parallel circuit, each line of wire has a
different amount of amps. When added the
amps together, you get the total amount of
amps in the entire circuit.
Amps3
= 1A
Amps2
= .5A
Amps3
= .5A
Amps tot = 2A
Volts and Amps in a
Parallel Circuit

However, in a parallel circuit if you have a total
of 3 volts in the circuit, each resistor will only
have 3 volts
Volts3 =
3V
Amps3
= 1A
Volts2 =
3V
Amps2
= .5A
Amps1
= .5A
Volts1 =
3V
Amps tot = 2A
Volts
Total =
Equivalent or Total Resistance - Req

Equivalent Resistance is the TOTAL
resistance of a circuit



Symbol:
Units:
Req or Rt
Ω Ohms
Formula:
Series:
Req = R1 + R2 + R3 + …
Parallel:
1
1
1
1



...
Re q R1 R 2 R3
What is the equivalent
(Req) of the circuit below?
5W
3W
10 W
Type of circuit:
Series
Formula:
Req = R1 + R2 + R3
Req = 10 + 5 + 3
Req = 18 W
resistance
What is the equivalent
(Req) of the circuit below?
75 W
30 W
Type of circuit:
Series
Formula:
Req = R1 + R2
Req = 75 +30
Req = 105 W
resistance
What is the equivalent
resistance (Req) of the
circuit below?
10 Ω
15 Ω
Type of circuit:
Formula:
Parallel
1
1
1
 
Re q1 R11 1R2

1 ÷ 0.1666 = 6Ω

Re q 10 15
1
 0.1  0.0666
Re q
1
 0.1666
Re q
Re q  6W
What is the equivalent
resistance (Req) of the
circuit below?
15 W
5W
15 W
Type of circuit:
Parallel
Formula:
1
1
1
1
 

Re q R1 R2 R3
1 ÷ 0.3333 = 3Ω
1
1 1 1

 
Re q 15 5 15
1
 0.0666  0.2  0.0666
Re q
1
 0.3333
Re q
Re q  3W
KEY POINTS

Symbols of circuit parts:



Voltmeter, Ammeter, Power Supply,
Resistor
Two types of circuits (series and
parallel)
Formula for Req (equivalent resistance)
Series – Req = R1 + R2 + R3
1
1
1
1



Req R 1 R 2 R 3
All the Information you need
Simulations


Open up applications for simulation
Or go to
http://phet.colorado.edu/en/simulat
ions/category/physics
Colour coding on resistors


Resistors are coated with
ceramic.
They have color coded bands
to indicate resistance
1st & 2nd band
first two digits of
resistance
3rd band
Number of zeros to add
after the first two
numbers.
I.E. 101, or 2, or 3….
Resistor
Example: If you have a resister that is red,
orange and yellow
First colour is red, thus
first digit is 2 and the
second colour is orange,
thus the second digit is 3
= 23
The third colour is yellow,
and this digit is 4 we
therefore find 104 =
10000
Black
0
Brown
1
Red
2
Orange
3
Yellow
4
Green
5
Blue
6
Violet
7
Gray
8
White
9
This resistance would be 10000 X 23
= 230 000 W