Chapter 35: Electric Circuits

Conceptual PhysicsBloom High School

35.1 A Battery & a Bulb

• Circuit- a complete path that the electrons take– Electrons flow from the battery (dry cell) from the

negative end and travel to the positive end– Electrons are provided by the battery, wires, components

• Water as analogy– Battery=pump– Wires=pipe or hose– Bulb=device that operates

• http://faraday.physics.utoronto.ca/IYearLab/Intros/DCI/Flash/WaterAnalogy.html

35.2 Electric Circuits• Switch- allows for a gap in a circuit

– Gaps cause the electrons to stop flowing– Turns the flow on and off

• Water analogy differences– Closing a circuit = turn the flow on– Opening a faucet = turning the flow on– Wires can’t “leak” like a pipe or hose

• Series- devices that are connected in a row– Allows for a single path for the electrons

• Parallel- devices are connected by their own ‘branch’– Allows for several paths an electron can take

35.3 Series Circuits• Rules of Series Circuits• 1. Current flows equally through each device.• 2. The total resistance of the circuit is equal to the

sum of the individual resistances.• 3. The current is equal to the voltage of the source

divided by the total resistance. (Ohm’s Law)• 4. Ohm’s Law also applies to each individual device

based on its resistance.• 5. The total voltage of the circuit is equal to the sum

of the individual voltage drops across each device.

Current flows equally through each device.

The total resistance of the circuit is equal to the sum of the individual resistances.• The total resistance of this circuit is 18kW.

The current is equal to the voltage of the source divided by the total resistance. (Ohm’s Law)

Ohm’s Law also applies to each individual device based on its resistance.

The total voltage of the circuit is equal to the sum of the individual voltage drops across each device.

Series Circuits• Disadvantages

– If one device fails, the whole circuit fails• Old Christmas lights

– You don’t need to use everything to use one thing• Microwave + Toaster + TV

Parallel Circuits• Rules of Parallel Circuits• 1. Each device connects the same two points of

a circuit. The voltage is equal in each ‘branch.’• 2. The total current is divided among the

branches.– a. Path of least resistance is always taken.

• Current inversely proportional to resistance• Ohm’s Law applies separately to each branch

Parallel Circuits• Rules, part 2• 3. The total current is equal to the sum of

the currents.• 4. As the number of branches increases,

the overall resistance decreases.– Overall resistance is less than the resistance of

any one branch.

Each device connects the same two points of a circuit.

The total current is divided among the branches.

The total current is equal to the sum of the currents.• Physics Physlet P.30.2

As the number of branches increases, the overall resistance decreases.

35.5 Schematic Diagrams• Schematic

diagrams- uses symbols to represent parts of a circuit

35.6 Combining Resistors in a Compound Circuit

• Is series, the resistances are added up

• In parallel, the addition is more complicated– 1/R=1/R1+1/R2+1/R3…

• Add parallel resistances first– Make the circuit appear to be

series• Add series resistances

35.7 Parallel Circuits & Overloading

• Homes are fed with parallel circuits from utility companies• More appliances added allows for lower resistance for the current

– Greater current can occur in the wires

• Overloaded- higher amount of current in wires than is safe– Fire may result

• Short Circuit- a new, shorter path is offered and often bypasses regular resistance

• Fuses- rated to self-destruct (melt) at a certain current level (amperage)

• Circuit breakers- designed to turn off at a certain amperage

Circuit Breakers• Increasing current boosts

an electromagnets force.• When the current jumps

to unsafe levels, the electromagnet pulls down a metal lever.

• The linkage tilts the moving contact away from the stationary contact to break the circuit.

Fuses• Internal wire designed to melt

at a prescribed amperage• In older homes, look like a light

bulb base• Most car fuses look similar to

this• Other types of fuses