Aim: How can we explain parallel circuits?

Do Now:

A series circuit has 3 resistors, 20 Ω, 40 Ω, and 60 Ω. What is the total resistance?

RT = R1 + R2 + R3

RT = 20 Ω + 40 Ω + 60 Ω

RT = 120 Ω

RT in series is always larger than the largest resistor!

Parallel Circuits

•A circuit with 2 or more current loops

Draw 2 resistors and a battery in a parallel circuit

Or…

Or…

Light Bulb Demo

If one goes out, the others remain lit

In a parallel circuit…

• Voltage is constant

• Total current is the sum of the currents through each resistor

And resistance…

Straw Demo

RT in parallel is always smaller than the smallest resistor!

R1 = 10Ω, R2 = 20Ω, R3 = 30Ω, in parallel with a VT = 100V

• Draw this circuit

• Calculate the current though each resistor

R1 = 10Ω R2 = 20Ω R3 = 30Ω

VT = 100V

V I R

1 100V 10 A 10Ω

2 100V 5 A 20Ω

3 100V 3.3 A 30Ω

T 100V 18.3 A 5.5 Ω

Draw a parallel circuit with a battery, 3 resistors, an ammeter reading the current through R1 and an ammeter reading the current through R3. R1 = 2Ω, R2 = 4Ω, R3 = 8Ω, and the ammeter for R3 reads 2.5 A.

R1 = 2ΩR2 = 4Ω

R3 = 8Ω

I3 = 2.5A

V I R

1 20V 10 A 2Ω

2 20V 5 A 4Ω

3 20V 2.5 A 8Ω

T 20V 17.5 A 1.1 Ω