1. Circuits & Circuit Diagrams

2. Circuits and Circuit Diagrams
Circuit = Complete path along which electrons can flow
When drawing circuits…use the following symbols in your diagrams
Wire/Conductor
Resistor (light bulbs, fans)
Battery
Switch

3. Ohm’s Law Resistance = Voltage/ Current R = V / I Units
Resistance = ohms (Ω)
Voltage (Potential Difference) = volt (V)
Current = ampere (A)

4. Example #1:
A 30.0 V battery is connected to a 10.0 Ω resistor. What is the current in the circuit?
I = V/R
I = 30.0 V / 10.0 Ω
= 3.00 A

5. Series Circuits Single path for electrons to flow
If any resistor (bulb) in the circuit is removed or burnt out, then NO BULBS will light – the circuit is not complete.
Electric current is the same through each device / resistor / bulb. (I = V/Rtotal)
Total resistance to current in the circuit is the sum of the individual resistors along the circuit path (Rtotal = RA + RB)
9.0 V
2.0 Ω
3.0 Ω A B

6. Series Circuits
Voltage drop across each device depends directly on its resistance (V = I x R)
Total voltage divides among the individual electric devices in the circuit (Rtotal = RA + RB)

7. Parallel Circuits Multiple paths for the current to flow; Branches
If any resistor (bulb) in the circuit is removed or burnt out, then the other bulbs will light as long as there is an unbroken path from the battery through that bulb and BACK to the battery.
Total current equals the sum of currents in branches
As the number of branches is increased, overall resistance of the circuit is decreased
think about driving on a 4 lane highway – little resistance to the flow of traffic
now consider an accident that blocks three of the lanes…a reduction to only one lane INCREASED the resistance
opening all lanes DECREASED the resistance

8. Combining Resistors
Total Resistance or Equivalent resistance in series = sum of all individual resistances
Rtotal = RA + RB
Parallel – the inverse of the total resistance is the sum of the inverse of the resistors
1/Rtotal = 1/RA + 1/RB

9. Equations 1/Rtotal= 1/R1 + 1/R2 + … Series R= R1 + R2 + ….. Parallel
I=V/Rtotal
R= R1 + R2 + …..
Parallel
I=V/R
1/Rtotal= 1/R1 + 1/R2 + …

10. Measures the rate at which energy is transferred
Power
Measures the rate at which energy is transferred
Power = Current x Voltage
P = IV
The unit of power is the Watt

11. Example #2:
A 6.0 V battery delivers a 0.50 A current to an electrical motor. What power is consumed by the motor?
P = IV
P = (0.50 A)(6.0 V)
P = 3.0 W

12. Practice… Req = 2 Ω + 3 Ω = 5 Ω IA = V/Rtotal……9 v / 5 Ω = 1.8 Amps
2.0 Ω
3.0 Ω A B
Req = 2 Ω + 3 Ω = 5 Ω
IA =
V/Rtotal……9 v / 5 Ω = 1.8 Amps
IB =
VA = IA x RA = 1.8 Amps x 2 Ω = 3.6 volts
add these and you should get
the voltage
supplied by the
battery, 9 volts
VB = IB x RB = 1.8 Amps x 3 Ω = 5.4 volts

13. Practice… Req => 1/ Req = 1/2 + 1/3 = 0.833,
9.0 V A B
2.0 Ω
3.0 Ω
Req => 1/ Req = 1/2 + 1/3 = 0.833,
but remember 1/ Req = (rearrange and solve for Req)
so Req = 1/0.833 = 1.2 Ω
IA = V/RA = 9.0 V / 2 Ω = 4.5 Amps
In a parallel circuit
these are NOT identical.
IB = V/RA = 9.0 V / 3 Ω = 3 Amps
VA = IA X RA = 4.5 Amps x 2 Ω = 9 v
these should EACH equal the
voltage being supplied by the
battery, 9.0 volts.
VB = IB X RB = 3 Amps x 3 Ω = 9 v