CircuitCircuit

A complete path of A complete path of conductors from the conductors from the negative terminal of the negative terminal of the voltage source back to voltage source back to the positive terminal of the the positive terminal of the voltage source.voltage source.

Two main types of circuits:Two main types of circuits:SeriesSeriesParallelParallel

What’s wrong in this picture???

What is a circuit? Would the light bulb still light up if the filament were broken? No! The electrons must have a complete circuit to flow through. A circuit is any path along which electrons can flow. For the continuous flow of electrons, there must be a complete circuit with no gaps

CircuitsCircuits

SeriesSeries One path. All current One path. All current

flows through all parts flows through all parts of the circuit.of the circuit.

““All or nothing”All or nothing” If there is a break in If there is a break in

the circuit, open the circuit, open circuit, no current will circuit, no current will flow.flow.

ParallelParallel More than one path. More than one path.

Current splits and Current splits and travels through travels through different paths.different paths.

““Selective”Selective” If there is a break in If there is a break in

one branch of the one branch of the circuit, current can circuit, current can still flow through other still flow through other branches.branches.What’s wrong in this

picture???

Circuit ElementsCircuit Elements

We have already talked about someWe have already talked about some properties of an electrical circuit…properties of an electrical circuit… CurrentCurrent, which is the flow of electrons., which is the flow of electrons. ResistanceResistance, which is the slowing of electrons., which is the slowing of electrons. Voltage,Voltage, which produces the flow of electrons. which produces the flow of electrons. These properties have different parts of aThese properties have different parts of a circuit, or circuit, or circuit elementscircuit elements..

Series CircuitsSeries Circuits

When items are connected in series, they provide onesingle path for the current to flow through For example, when the current travels out ofthe wall, through the filament, and back into the wall, the light bulb has been connected inseries

Series CircuitsSeries Circuits There are four important characteristics of There are four important characteristics of

aa series circuit:series circuit: 1. Current has a single pathway through 1. Current has a single pathway through

the circuit, sothe circuit, so current through each device is the same.current through each device is the same. 2. Current is resisted by the first device, 2. Current is resisted by the first device,

then the second,then the second, then the third and so on, so the total then the third and so on, so the total

resistance is theresistance is the sum of the individual resistancessum of the individual resistances

3. The current in the circuit is equal to the3. The current in the circuit is equal to the voltage from the voltage source divided by voltage from the voltage source divided by

thethe total resistance in the circuit (Ohm’s Law,total resistance in the circuit (Ohm’s Law, duh!)duh!) 4. Ohm’s Law applies separately to each 4. Ohm’s Law applies separately to each

device,device, so the voltage drops across each device,so the voltage drops across each device, according to its resistanceaccording to its resistance

Series CircuitsSeries Circuits Series circuits have some Series circuits have some

pretty obvious disadvantagespretty obvious disadvantages If several light bulbs are If several light bulbs are

connected in series, and one connected in series, and one goes out, what happens to goes out, what happens to the circuit?the circuit?

This idea may be familiar toThis idea may be familiar to you through some you through some

traumatizing Christmas-tree traumatizing Christmas-tree decorating experience.decorating experience.

Parallel CircuitParallel Circuit

•When items areconnected in parallel,they form separatebranches, any one ofwhich the electronscan flow through.

• In a parallel circuit, allof the electrons DONOT flow through allof the circuitelements

Parallel CircuitParallel Circuit

There are four important characteristics of parallel There are four important characteristics of parallel circuits:circuits:

1. Each device connects the same two points in a 1. Each device connects the same two points in a circuit, so the voltage is the same across each circuit, so the voltage is the same across each device.device.

2. The total current in the circuit splits up among 2. The total current in the circuit splits up among the different branches (think of the electrons the different branches (think of the electrons getting to chose which branch they go down), so getting to chose which branch they go down), so the current is different across each branch. More the current is different across each branch. More current passes through the branches with lower current passes through the branches with lower resistance (Ohm’sLaw)resistance (Ohm’sLaw)

Parallel CircuitParallel Circuit The total current in the circuit equals the The total current in the circuit equals the

sum of the currents in its parallel sum of the currents in its parallel branches.branches.

4. As the number of parallel branches 4. As the number of parallel branches increases,the total resistance decreases.increases,the total resistance decreases.

Think of checking out at the grocery store. Think of checking out at the grocery store. A certain number of people need to check A certain number of people need to check out (like electrons in a circuit). As more out (like electrons in a circuit). As more check-out lanes open, the people can check-out lanes open, the people can move through more quickly.move through more quickly.

Thus as the number of lanes increases, Thus as the number of lanes increases, the“resistance” decrease.the“resistance” decrease.

Review and CompareReview and Compare

SeriesSeries All electrons pass All electrons pass

through single through single pathpath

Voltage drops Voltage drops different across different across each deviceeach device

Current is the Current is the samesame

through each through each devicedevice

ParallelElectrons pass throughone of many paths

Voltage is the sameacross each device

Current is differentthrough each device

Ohm’s LawOhm’s Law

Current through a conductor is Current through a conductor is proportional to the applied voltageproportional to the applied voltage

Conductor is also known as a resistorConductor is also known as a resistor An ideal conductor is a material whose An ideal conductor is a material whose

resistance does not change with temperatureresistance does not change with temperature

Voltage Current Resist

I R

ance

V

V = Voltage V = Voltage (Volts = V)(Volts = V)

I = Current I = Current (Amperes = A)(Amperes = A)R = Resistance R = Resistance (Ohms = (Ohms = ΩΩ))

Current and Voltage DefinedCurrent and Voltage Defined CurrentCurrent: (the current in electrical circuits) : (the current in electrical circuits) Flow of current from Flow of current from positive terminal to the negative positive terminal to the negative

terminal. terminal. - has units of Amperes (A) and is measured using - has units of Amperes (A) and is measured using ammetersammeters..

VoltageVoltage::Energy required to move a charge from one point to another.Energy required to move a charge from one point to another. - has units of Volts (V) and is measured using - has units of Volts (V) and is measured using voltmetersvoltmeters..

Think of voltage as what pushes the electrons along in the circuit, and current as a group of electrons that are constantly trying to reach a state of equilibrium.

Ohm’s Law continuedOhm’s Law continued

Ohm’s Law continuedOhm’s Law continued

The The total resistancetotal resistance of a circuit is dependant on of a circuit is dependant on the the numbernumber of resistors in the circuit and their of resistors in the circuit and their configurationconfiguration

1 2

1 2

...

1 1 1 1...

total

total

R R R R

R R R R

Series CircuitSeries Circuit

Parallel CircuitParallel Circuit

CircuitsCircuits

SeriesSeries Each resistor added Each resistor added

decreases the decreases the voltage. (Loads share voltage. (Loads share the current.)the current.)

RRTT = R = R11 + R + R22 +R +R33…… As resistors are As resistors are

added, total added, total resistance increases.resistance increases.

ParallelParallel Each resistor gets full Each resistor gets full

voltage. (Each load voltage. (Each load draws own current.)draws own current.)

1/R1/RTT = 1/R = 1/R11 + 1/R + 1/R22…… As resistors are As resistors are

added, total added, total resistance decreases.resistance decreases.

Fuses & Circuit BreakersFuses & Circuit Breakers

Limit the current that runs Limit the current that runs through wires in your housethrough wires in your house

These wires have some These wires have some resistanceresistance

Energy loss by IEnergy loss by I22R converts R converts to heatto heat

Hot wires can start a fireHot wires can start a fire Limit the current with a fuse Limit the current with a fuse

or circuit breakeror circuit breaker