Science 23203 – M3 – Level 1 – Electricity
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By the end of this lesson you will be able to:
 Calculate resistors in series
 Calculate resistors in parallel
 Calculate resistors in combination (series and parallel)
 Read resistors color code
Resistors in Series
Resistors in series cause
to the current.
equals the
Also called:
of all resistances.


Example:
1) Find the combined resistance of the following circuit.
R1
R3
2) Find the currents in the ammeters if the current in R3 is 2A
3) Find the voltage on R1 , R2, R3
R2
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Class Activity
1) From the following circuit, find:
a) the combined resistance
b) the current flowing through R1 and R2
c) the voltage across R1
d) the voltage across R2
Electric symbol
Electric symbol
R1 = 10Ω
2) From the following circuit, find:
a) the combined resistance
b) the current flowing through R1, R2, R3
c) the voltage across R1, R2, R3
24V
R2= 6Ω
R3= 8Ω
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Resistors in parallel
The combined resistance is
than the smallest resistor.
Good way to find mistakes
4v
1
1
1
1 1
=
+
= +
RT
R1 R2 6 2
R1 = 6 Ω
1
2+6 8
2
=
=
=
RT
2 × 6 12 3
RT =
3
Ω = 1.5Ω
2
Attention !!!
1
≠ RT
RT
R2 = 2 Ω
Smaller the smallest
resistor:
Example
1) a) Find the combined resistance in the following circuit:
20 V
= 30Ω
= 60Ω
b) Find the voltage across R1 and R2
c) Find the currents IT, I1, I2

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*** If there are ONLY 2 resistors in parallel we can use this shortcut
If there are only 2 resistors:
RT =
R1 × R2
R1 + R2
1
1
1
R + R2
= 1
=
+
R1 xR2
R1 R2
RT
RT =
R1 xR2
R1 + R2
Class Activity
Find the efficient resistance in the following circuit
3v
R1 = 500 Ω
R2 = 1200 Ω
Steps to follow when solving mixed circuit
A real circuit is usually a mix of series and parallel in one circuit
 Divide the circuits to small parts.
 Start with parts that do not affect other parts.
 Simply the circuits as you go.
Example:
Find the combined resistance
Science 23203 – M3 – Level 1 – Electricity
Class activity
1) From the following circuit, find:
(Assume that the switch is closed)
A. the combined resistance
B. what is the reading in the ammeter?
C. the voltage across R1, R2, R3, R4
D. the current in R1, R2, R3, R4
E. What will be the reading in the ammeter,
if the switch is open?
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60 V
A
R1=10Ω
R3=20Ω
R2=20Ω
R4=40Ω
2) The following diagram shows 4 identical bulbs connected to a battery.
Which bulbs will continue to work if:
A. Bulb L2 will be removed from the circuit?
B. Bulb L4 will be removed from the circuit?
L1
L2
L3
L4
C. Bulb L1 will be removed from the circuit?
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3) From the following circuit,
Calculate the combined resistance between points A and B when:
S1
R2 = 3Ω
A
R1 = 6Ω
a) S1, S2 and S3 are closed
b) S2, S3 are closed while S1 is open
c) S1, S3 are closed while S2 is open
Summary
Circuits
Only 2 resistors
R3 = 6Ω
S2
S3
B
Science 23203 – M3 – Level 1 – Electricity
Homework
1) For the following circuit:
i. What is the total resistance of the circuit?
ii. Calculate the value of I.
iii. Calculate the values of V1 and V2.
iv. In which resistor is the greater current?
2) For the following circuit:
i. What is the total resistance of the circuit?
ii. Calculate the value of I.
iii. Calculate the values of I1 and I2.
iv. In which resistor is the greater current?
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Science 23203 – M3 – Level 1 – Electricity
3) For the following circuit:
i. What is the total resistance of the circuit?
ii. Calculate the value of IT.
iii. What is the p.d across the V1, V2, V3 ?
iv. Calculate the values of I1 and I2.
4) Find the reading of the ammeter and voltmeter (V2).
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IT
>
I3
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5) Find the current through R3
R2=4Ω
R1=3Ω
R3=6Ω
6) Which of these bulbs will turn on after closing the switch?
(May be more than one answer).
a
b
c
7) Which switch do you need to close for the bulb to lit?
a. S1 only
b. S2 only
c. S1 and S2
d. This lamp will never light.
8) Find the combined resistance in the following
A.
B.
C.
D.
d
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Extra Practice
(Only for excellent students >>> Lazy students stay away)
Find the combined resistance of the following circuits
Give names to the resistors before you start (R1, R2, R3,...)
6Ω
1)
2)
3)
12Ω
6Ω
Science 23203 – M3 – Level 1 – Electricity
Extra Exercise – Resistors (Thai Style)
Find the combined resistance between the points A and B in the following circuits.
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Resistors Color Code
Resistors have a color code to know their value.
Example:
+10% - 27000 + 2700 = 29700 Ω
-10% - 27000 – 2700 = 24300 Ω
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Class Activity
1) What is the value of the following resistors?
Resistor
Value
red, red, black, gold
red, black, red, gold
green, blue, brown, gold
brown, black, yellow, gold
2) Complete the following table:
First
color
Second
color
Third
color
Forth
color
red
violet
orange
silver
brown
green
blue
gold
brown
black
1 Kilo = 1,000
1 Mega = 1,000,000
Tolerance
120 KΩ
± 10 %
black
±5%
------
770 KΩ
------
1.5 MΩ
silver
9Ω
(3 zeros)
(6 zeros)
3) Find the combined resistance if all the resistors are:
Red, Black, Red, Gold
Value
± 20 %
Science 23203 – M3 – Level 1 – Electricity
Homework
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Find the combined resistance if the following circuits using the color code system
Give names to the resistors before you start (R1, R2, R3,...)
1)
brown, black, red, silver
red, green, red, silver
yellow, black, red, silver
red, green, red, silver
2)
brown, black, red, gold
yellow, black, red, silver
3)
red, black, red, silver
yellow, black, red, silver
yellow, black, red, silver
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4) Find the P.D across the “red, green, red” resistor in the following circuit, if the
e.m.f is 12V.
Don’t forget to give names to the components!
red, black, red, silver
yellow, black, red, silver
brown, black, red, silver
red, green, red, silver
Show your working
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