Chapter 9: CircuitsSeries circuits have only one path

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Parallel circuit – current can take more than one path at a time

Here a second 60 W bulb is added in parallel with the first bulb

Current may now flow through the first bulb or the second bulb.

Both bulbs glow at their intended brightness.Each receives the full circuit voltage of 120 volts.

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Parallel circuit – Another 60 W bulb is added, in parallel to the others

Every load connected still receives the full 120 V circuit voltage.

How is this possible? Each additional bulb causes more current (amperage) to be drawn from the wall outlet.

There is a lot of voltage (“pressure”) in electrical lines coming to your house, just like there is a lot of water pressure in the water pipes coming to you house.

If you open up more faucet taps, then the water pressure forces more water current into your faucet taps.

If you open up more parallel paths for electricity, then the voltage forces more electrical current into your light bulbs.

Danger! Current from the wall outlet increases each time another load is added to the circuit in parallel – and it is too easy to keep plugging in more items in parallel.

You can draw so much current through your wires that they will melt the wire insulation, heat up, and start a fire!

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Houses have circuit breakers or fuse-boxes to prevent this.

When they detect too much current being drawn, they break the electric circuit to that part of your house.

Electrical circuits in homes are parallel circuits.

1. Each outlet has its own path. One can have something connected and be on, while another has nothing connected, or have something connected while turned off.

2 Every outlet sees the same voltage, because each outlet is connected to the same wire.

In a series circuit, current ( I ) is the same at all points.What goes in one end, must come out the other end !Analyze this circuit using Ohm’s law. 2 batteries, 3 bulbs.

Every charge starts with 3 volts of electrical “pressure”. (At the positive part of the top battery)

As charge moves through the circuit, some energy istransformed into light.

Thus after every bulb, the energy must be lower.

We see a drop in voltage from 3 v, to 2 v, to 1 V, to 0 v.

How much current is in this circuit? V = IR I = V / R

= 1.5 v / 3

= 0. 5 amps

Every part of a circuit has resistance, even wires and batteries. However, light bulbs, resistors, motors and heaters have much greater resistance than wires and batteries.

When doing problems, we usually can treat resistance of wires and batteries as negligible.

Find voltage drops in a series circuit: Conservation of Energy

What is the voltage drop across each resistor?

V = IR = ( 0.5 amps )( 1 ) = 0.5 volts

Energy is not created or destroyed. Energy may be transformed from one form to another.

As current flows along the circuit, each resistor uses some of the energy, transforming it into waste heat.

Thus, the voltage gets lower after each resistor.

Water pressure law:Around the path of any closed water system, water pressure changes must add up to zero.

If water pressure changes didn’t add up to zero, then somewhere we’d mysteriously be gaining water pressure, as the water loops around.

(Eventually the water pressure would rise to infinity?!)

Kirchhoff’s voltage law – Same as above, but for electricity Around any electric circuit, voltage changes must add up to zero

If voltage changes didn’t add up to zero, then somewhere we’d mysteriously be gaining voltage, as the charge loops around .(Eventually the voltage would rise to infinity?!)

Kirchhoff’s voltage law example

Batteries raise the voltage

Resistors, light bulbs, diodes,

lower the voltage

Voltage changes = + 1.5 v – 0.5 v - 0.5 v – 0.5 v = 0 volts

Current ( I ) always goes somewhere:

If I flows into a branching point, then the same I must flow out again.

Kirchhoff’s current law.

3 light bulbs in parallel

Each has a current of 1 amp.

Battery must supply 3 amps since each bulb draws 1 amp, and there are 3 bulbs.

At the first branch, 3 amps flow in

1 amp flows down to the first bulb, so 2 amps flow on to the remaining 2 bulbs.

V and R in parallel circuitsEach branch sees the same voltage

Imagine several series circuits connected to the same battery.

Each branch has a path back to the battery, without any other resistance in the way.

Branches may have different currents

Current in each branch depends on the branch’s Resistance.

When you plug a desk lamp and a power saw into an outlet, they each use different amounts of current.

Open circuits, closed circuits, and short circuits

Open circuit

Closed circuit

Short circuit = path with zero, or very low, resistance. Ex: Connect a wire directly between two ends of a battery.

Often accidentally made

Creates a parallel path with very low resistance.

In parallel circuits, the branch with the lowest resistance draws the most current

Why short circuits are dangerous – Danger, Will Robinson!

In the above example, current through your wire could be as high as 1,500 amps!

This could melt the wire in an instant and probably burn you as well.

They should always be a concern when working around electricity.

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