1. Series & Parallel Circuits
Important: difference between Series & Parallel
Make sure you have correct units
Voltage (V) = Volts, V
Current (I) = Amps, A
Resistance (R) = Ohms, Ω
Power (P) = Watts, W

2. Series & Parallel Series Circuits Parallel Circuits Only ONE path
Same current flows through each component
Voltage drops across each resistor
Voltage adds up around loop: (Vtotal = IR1+IR2+IR3)
Resistors add: (REQ=R1+R2+R3)
Vtotal = I REQ
MANY paths
Same Voltage across each path
Currents add up around circuit: Itotal= I1 + I2 + I3
Itotal = V/R1 + V/R2 + V/R3
Resistors add inversely: 1/REQ = 1/R1 + 1/R2 + 1/R3
If 2 resistors, use: REQ = (R1)(R2) / (R1+R2)
Itotal = V / REQ

3. Series Circuit Two resistors ADD in a series circuit
Rtotal = 4Ω + 8Ω = 12 Ω
Current is SAME everywhere
I = V / Rtotal = 6V / 12 Ω =0.5Amps (everywhere)
Voltage in each resistor = IR
For R1: V = (0.5A)(4 Ω) = 2 V
For R2: V = (0.5A)(8 Ω) = 4 V
Power = V*I = (6V)(0.5A)= 3Watts

4. Parallel Circuit
Two resistors/lamps ADD INVERSELY in a parallel circuit
1/Rtotal = (1/10Ω) + (1/5Ω) = 3/10 Ω → Rtotal = 10/3 Ω
Voltage is SAME across each path
Itotal = V / Rtotal = 12V/ (10/3Ω) = 3.6 Amps (total through battery)
Current, each lamp = V/Rlamp
For R1: I = (12V)/(10 Ω) =1.2A
For R2: I = (12V)/(5 Ω) = 2.4 A

5. Parallel Circuit For R1: I = (12V)/(10 Ω) =1.2A
Power = V*I = (12V)(3.6A)= 43.2 Watts (total)
Power, each lamp: Plamp=VIlamp
Lamp 1 (or R1):
P1= (12V) (1.2A)= 14.4 Watts
Lamp 2 (or R2):
P2= (12V) (2.4A)= 28.8 Watts
Brightness = power
Add in parallel, each lamp stays the same brightness
Add 5 Ω in series, total resistance goes up or down?
When resistance goes up, current goes down, and bulbs get dimmer.
Parallel Circuit

6. Ohm’s Law & Power
V=IR
Use to calculate voltage, current, or resistance in circuits.
Power, P = VI = V2/R = I2R
Use to calculate power (energy used per unit time)