20.3 Electric Circuits

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20.3 Electric Circuits

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What we have learned so far…

Electric chargesStatic electricityCoulomb’s LawElectric currentInsulators and

ConductorsResistance and

VoltageOhm’s Law



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What we are going to learn…How do we keep track of the flow of electrons?How do series and parallel circuits differ?How do you calculate electric power and electrical

energy?What devices make electricity safe to use?



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Circuit DiagramsRemember that, in order for electricity to power our TVs

and computers, electrons have to be moving through them.

An electric circuit is a complete path through which charge can flow.

Wires in a house form a complex network of circuits, and each wire must allow electrons to flow through it in order to be useful.

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Circuit DiagramsThe idea of electric circuits is very similar to roads.

In order for them to be useful, they have to allow cars to travel to different places.

Also, dead ends do not allow for a lot of traffic flow.

Disconnected wires are like dead ends, they don’t allow electrons to flow past them. QuickTime™ and a

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Circuit Diagrams

If you want to know possible routes to get to your friends house, you would look at a road map (or use some computerized road map that would tell you where to go).

Electricians have to know where the electrons can flow.

In order to do that, they use something like a road map - a circuit diagram.



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Circuit DiagramsA circuit diagram uses symbols to represent parts of a

circuit, including a source of electrical energy and devices that are run by the electrical energy.

It essentially maps out where the electrons can flow and what they might encounter.

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Circuit DiagramsA circuit diagram includes many symbols:

Anything that uses electricity is seen as a resistor. This symbol could represent a light bulb or a computer.

A battery is represented by the following symbol. Note that current “flows” from the positive terminal to the negative terminal.

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Circuit DiagramsSwitches are important! (Like light switches)

A switch is like a draw bridge. When the bridge is up, no cars can travel.





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Circuit DiagramsWhen the switch is up, no electrons can flow, so no

electricity.When the switch is down, the circuit is complete and

then electrons can flow. This is then called a “closed circuit.”

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Types of Circuits

There are basically two types of circuits: series and parallel.

Once again, each of them act like roads for electrons.

To understand them, we can think about cars on a road.



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Series CircuitsIn a series circuit, charge has only one path through

which it can flow.

So the electrons only have one road to drive on.

If anything happens to just one section of the path, all of the electrons will feel the effects!

So, if one section of the path is blocked, all the electrons will be blocked from traveling through the path. QuickTime™ and a

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Series CircuitsAnd if electrons can’t travel, there is no electric current.When a light bulb burns out, it essentially blocks the

path.So, for two light bulbs “in series,” if one of them burns

out, then the other one will not work either.

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Series CircuitsIf you add more light bulbs to the series circuit, what

does that do to the electrons?

Adding a light bulb is like adding something difficult for electrons to travel through.

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Series CircuitsThe more resistance you add, the more difficult of a time

the electrons have to travel through the whole path.

As a result of more resistance, the electrons have to go slower (less current), and each bulb shines less brightly.

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Parallel Circuits

Imagine what would happen if circuits in your home were wired in series…

If a light bulb burned out, the television would turn off!

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Parallel Circuits

To avoid this problem, circuits in the home are mostly wired in parallel.

A parallel circuit is an electric circuit with two or more paths through which charges can flow.

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Parallel CircuitsThis is like having two roads to drive on.If one of the paths gets blocked, the electrons can still

drive on the other unblocked road.So if one element stops functioning in a parallel circuit,

the rest of the elements still can operate.



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Parallel CircuitsAlso, the more parallel paths you have, the less

resistance electrons feel.More parallel paths mean there are more roads to for the

electrons to take, which means an easier time for the electrons to travel.

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Power and Energy CalculationsThe purpose of electricity is to power other appliances.

Recall that power is the rate of doing work.

The rate at which electrical energy is converted to another form of energy is electric power.

The SI unit for power is the “watt” (W).

1 W = 1 joule / second

We usually measure power in kilowatts (kW) for convenience.



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Power and Energy CalculationsElectric power can be calculated by multiplying voltage

by current.Electric Power:

P (watts) = I (amps) X V (volts)Every time you turn on a 75-watt light bulb, you use

electric power.

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Calculating PowerAn electric oven is connected to a 240-volt line,

and it uses 34 amps of current. What is the power used by the oven?

Finding Electrical EnergyTo find the electrical energy used by an appliance,

multiply power by time.

E = P x t

Most electric power companies usually determine the cost of your electric bill using kilowatt-hours as a unit of energy.



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Electrical SafetyElectricity can cause serious damage to your health, so

it is important to know how to avoid electric shock.

Inspectors check all new houses to make sure electrical wiring is installed safely.



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Electrical SafetyAll wires must be able to carry the maximum expected

current.The amount of current depends on the number of

devices that are in the circuit. The more devices turned on, the more current you get.

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Electrical SafetyIf the current exceeds the circuit’s safety limit, the wire

may overheat and start a fire.In order to prevent this, we use fuses and circuit

breakers.A fuse is a device that is inserted

into a circuit. The fuse has a wire that is designed to melt if too much current passes through it.



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Electrical SafetyThis is called “blowing a fuse,” and this event breaks the

circuit so that electricity doesn’t flow.The fuse has to be replaced before the circuit can be

used again.

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Electrical SafetyCircuit breakers are like fuses, and in most houses,

they replace the need to have a fuse.A circuit breaker is a switch that opens when current in a

circuit is too high.The circuit breaker must be reset before the circuit can

be used again.



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Personal Safety

Touching any open wire is dangerous, so wires are usually put inside the walls of the building.

Also, we use three-prong plugs to prevent shocks.

The third prong connects to ground, so if a short circuit develops, the charges flow through the grounding wire instead of your body.

The transfer of excess charge through a conductor to Earth is called grounding.

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Short CircuitsIn a short circuit, current finds a short path through the

circuit with less resistance than the full path.

This means that current will flow excessively, which will result in overheating, fire or explosions.

If you are close to an object that is “shorting out,” there is a chance for a very nasty shock.



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Effect of Current on the Body

1 mA = slight tingling sensation

5 mA = slight shock

5-30 mA = Painful shock; loss of muscular control

50-150 mA = Extreme pain; severe muscular contractions. Breathing stops.

1-4.3 A = Nerve damage; heart stops, death is likely.

10 A = Severe burns; heart stops, death is probable.

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20.3 Electric Circuits