Series and Parallel Circuits

Ohm’s Law

I = V / R

Georg Simon Ohm (1787-1854)

I = Current (Amperes) (amps)

V = Voltage (Volts)

R = Resistance (ohms)

How you should be thinking about electric circuits:Voltage: a force that pushes the current through the circuit (in this picture it would be equivalent to gravity)

Resistance: friction that impedes flow of current through the circuit (rocks in the river)

How you should be thinking about electric circuits:

Current: the actual “substance” that is flowing through the wires of the circuit (electrons!)

How you should be thinking about electric circuits:

Would This Work?

Would This Work?

Would This Work?

The Central Concept: Closed Circuit

Simple Circuits Series circuit

All in a row 1 path for electricity 1 light goes out and

the circuit is broken

Parallel circuit Many paths for

electricity 1 light goes out and

the others stay on

CircuitsCan either be series or parallel.

SeriesCurrent only takes one path for electrons

Current flows through every part of the circuit

Lights in a Series

SeriesIf you add a resistor (like another light):Total resistance goes UP since all the current has must go through each resistor.

Adding Resistors to Series:Current in the circuit will go DOWN (lights will dim)

If you remove a light bulb or one burns out—all go out!

Current in Series

Current is the same at all points

Voltage in Series

Voltage is reduced by each resistance – voltage drop

Resistance in SeriesAdd up all resistors to get totalTotal resistance will go up because all of the current must go through each resistor.

Sample Problem #1 Draw a series circuit with two 1.5 V

batteries, 3 resistors, and a current of 0.5 A.

1. What is the total voltage of the circuit?

2. What is the resistance of each resistor?

3. What is the voltage drop across each resistor? Label on your circuit.

Parallel CircuitsHas at least one point where current divides

More than one path for current to flow

Paths are also known as branches

Lights in Parallel

Parallel:If you add a resistor:

Total resistance goes down

Total current goes up when you add another path

Removing a Light BulbIf you remove a light bulb or one burns out, the others stay on because the circuit is still closed.

Current in Parallel Current flows into a branching point, the same total current must flow out again

Current depends on resistance in each branch

Voltage in Parallel

Voltage is the same across each branch – because each branch is on the same wire

Resistance in ParallelCalculate current in each branch based on resistance in each branch by using Ohm’s Law

Practice problem #2

Draw a parallel circuit with two resistors (one on each branch) and a 12 V battery.

1. What is the voltage through each resistor?

2. What is the current flowing through each branch?

Toll Road—Circuit Analogy

Toll Booth ExplanationAdding toll booths in series

increases resistance and slows the current flow.

Adding toll booths in parallel lowers resistance and increases the current flow.

Batteries in Series and Parallel:

In series—The voltage is increased.

In parallel—No change in voltage; these batteries will last longer!

One More FINAL Thing:

Two Types of Current:DC—Direct Current—produced by solar cells and chemical cells (batteries)

Current only flows in one direction.

2nd type of current:AC—Alternating CurrentCurrent flows back and forth (alternates)

Found in homesGenerators produce AC current