1. Equivalent Resistance
Objectives:
3.7 Distinguish the equivalent resistance of series and parallel circuits kept at constant temperatures.
3.8 Determine the equivalent resistance of a series- parallel circuit kept at constant temperature.
3.9 Evaluate the significance of the error in the measurement of electrical resistance.

2. Equivalent or Total Resistance - Req
Equivalent Resistance is the TOTAL resistance of a circuit
Symbol: Req or Rt
Units: Ω Ohms
Formula:
Series:
Req = R1 + R2 + R3 + …
Parallel:

3. What is the equivalent resistance (Req) of the circuit below?
Example #1:
What is the equivalent resistance (Req) of the circuit below?
5 W
3 W
10 W
Type of circuit: Series
Formula: Req = R1 + R2 + R3
Req =
Req = 18 W

4. What is the equivalent resistance (Req) of the circuit below?
Example #2:
What is the equivalent resistance (Req) of the circuit below?
30 W
75 W
Type of circuit: Series
Formula: Req = R1 + R2
Req =
Req = 105 W

5. What is the equivalent resistance (Req) of the circuit below?
Example #3:
What is the equivalent resistance (Req) of the circuit below?
10 Ω
15 Ω
Type of circuit: Parallel
Formula:
1 ÷ = 6Ω

6. What is the equivalent resistance (Req) of the circuit below?
Example #4:
15 W
5 W
Type of circuit: Parallel
Formula:
1 ÷ = 3Ω

7. 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
Parallel -