Equivalent Resistance

1. Equivalent Resistance

2. Types of Circuits:
 There are 2 types of circuits:
 1. Series Circuit
 2. Parallel Circuit
SERIES
Trick to help identify type of circuit:
Series:Pen follows wire
Parallel: Pen can go in more than one direction
PARALLEL

3. Equivalent Resistance - Req
 Equivalent Resistance (Req) is the TOTAL
resistance of a circuit
 Symbol: Req
 Units: Ω Ohms
 Formula:
Series:
Req = R1 + R2 + R3 + …
Parallel:
...
111
R
1
321e RRRq
++=
q

4. 5 Ω
3 Ω
10 Ω
Type of circuit: Series
Formula: Req = R1 + R2 + R3
Req = 10 + 5 + 3
Req = 18 Ω
What is the equivalent resistance
(Req) of the circuit below?
R eq

5. 30 Ω
75 Ω
Type of circuit: Series
Formula: Req = R1 + R2
Req = 75 +30
Req = 105 Ω
What is the equivalent resistance
(Req) of the circuit below?
Req

6. R2
30 Ω
R eq
75 Ω
Type of circuit: Series
Formula: Req = R1 + R2
75 = R1 +30
R1 = 75 – 30 = 45 Ω
What is the resistance (R1) in Resistor
1 in the circuit below?
R 1

7. Type of circuit: Parallel
Formula:
What is the equivalent
resistance (Req) of the
circuit below?
10 Ω
15 Ω
21
11
Re
1
RRq
+=
1 ÷ 0.1666 = 6Ω
15
1
10
1
Re
1
+=
q
0666.01.0
Re
1
+=
q
1666.0
Re
1
=
q
Ω= 6Req
R eq

8. Type of circuit: Parallel
Formula:
What is the equivalent
resistance (Req) of the
circuit below?
321
111
Re
1
RRRq
++=
1 ÷ 0.3333 = 3Ω
15
1
5
1
15
1
Re
1
++=
q
0666.02.00666.0
Re
1
++=
q
3333.0
Re
1
=
q
Ω= 3Req
15 Ω
5 Ω
15 Ω
R eq

9. KEY POINTS
321 R
1
R
1
R
1
Req
1
++=
 Circuit Types: Series and Parallel
 The Req of a series circuit is the sum
of the resistances.
 The Req of a parallel circuit is LESS
than any of the resistors.
 Formula for Req (equivalent resistance)
 Series – Req = R1 + R2 + R3
Parallel -

10. Activities
 Worksheet #10
