electric circuits

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Electric CircuitsChapter 18

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18.1 Schematic Diagrams and Circuits

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What do you think?

• Scientists often use symbols to represent electrical components, such as batteries, bulbs, and wires. On the next slide, you will see the symbols for eight common electrical components that you have seen and discussed previously. • Predict the component shown by looking at

each symbol. • Briefly explain why you think each symbol

represents that particular electrical component.

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What do you think?

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3

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5

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7

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+Schematic Diagrams

Schematic diagrams use symbols to represent components.

They show how the parts in an electrical device are arranged.

+Electric Circuits

A path through which charges flowCan have one or more complete paths

LoadAn element or group of elements in a

circuit that dissipates energyEx: A simple circuit consists of a source of

potential differences and electrical energy, such as a battery, and a load, such as a bulb or a group of bulbs

+Electric Circuits

An electric circuit is a set of components providing a complete, closed-loop path for the movement of electrons. Called a closed circuit

If the path is broken, the electrons do not flow. Called an open circuit

+Inside a Light bulb

A complete conducting path is established inside the light bulb. The tip of the bulb (a) is

connected to one side of the filament (see the black line).

The threads on the side of the bulb (c) are connected to the other side of the filament (see the white line).

+Short Circuits

A short circuit bypasses the light bulb or other load. It is a closed circuit.Electrons flow directly from - to + without

passing through the bulb.The current is large and the wire becomes

hot.Short circuits in homes can cause fires.

Fuses or circuit breakers are designed to turn off the electron flow if short circuits occur.

+Potential Difference in Circuits

A device that increases the PE of the electrons, such as a battery, is a source of emf (electromotive force).Not really a force, but a PE difference

Energy is conserved in electric circuits.The potential difference (DV) for the battery

equals the energy converted into heat as the electrons move through the bulb.Electrons gain energy (battery) and lose

energy (bulb) as they make a complete trip.

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Now what do you think?

• Draw schematic diagrams showing each of the following circuits:• An open circuit including a battery, open

switch, and bulb• A closed circuit including a battery, closed

switch, and resistor• A short circuit including a battery, bulb,

and closed switch

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18.2 Resistors in Series or Parallel

+What do you think?

• Figure (a) shows a single bulb and battery as seen before. Figures (b) and (c) each show two bulbs connected to the battery. The batteries and bulbs are all identical. Answer the three questions on the next slide and explain your reasoning.

+What do you think?

• How will the brightness of (b) and (c) compare to each other and how does each compare to (a)? Explain.

• How will the brightness of (d) and (e) compare to each other and how does each compare to (a)? Explain.

• Compare the total current leaving the battery in each of the three circuits. Explain.

+Resistors in Series

Series describes components of a circuit that provide a single path for the current. The same electrons must pass through both

light bulbs so the current in each is the same.


Light bulb filaments are resistorsWhen many resistors are connected in

series, the current in each resistor are the same


Vbattery= V1 + V2

Conservation of energy

Vbattery= IR1 + IR2

Ohm’s law

Vbattery= I(R1 + R2)

Vbattery= IRequivalent

Requivalent = R1 + R2

+Equivalent Resistance

Solving problems with series resistors: Find the equivalent resistance. Use Req with Ohm’s law to find V or I.

Use I and R1, R2, etc. to find V1, V2, etc.


The potential difference across the batter, V, must equal the potential difference across the load.

+Classroom Practice Problems

A 6.00 V lantern battery is connected to each of the following bulb combinations. Find the equivalent resistance and current in each circuit.One bulb with a resistance of 7.50 Two bulbs in series, each with a

resistance of 7.50 Four bulbs in series, each with a

resistance of 7.50


1. Start by drawing a picture

2. Take your inventory

3. Decide which equations to use

4. Solve

Answers: 7.5 , 0.800 A15 , 0.400 A30 , 0.200 A

+Resistors in Parallel

Parallel describes components providing separate conducting paths with common connecting points. The potential difference is the same for

parallel components. Electrons lose the same amount of energy

with either path.

+Resistors in Parallel

Ibattery = I1 + I2 Conservation of charge

Ohm’s law

Vbattery= V1 = V2 Potential energy loss is the same across all parallel resistors.

Because Vbattery= V1 = V2, the equation above reduces as follows:

1 2

11 1

eqR R R

1 2

1 2

battery

eq

V V V

R R R

1 2eq

V VV

VR VR VR


Solving problems with parallel resistors: Find the equivalent resistance. Use Req with Ohm’s law to find V or Itotal.

Use V to find I1, I2, etc.


The sum of currents in parallel resistors equals the total current

The Req for a parallel arrangement of resistors must always be the smallest resistance in the group of resistors


A 9.0V battery is connected to 4 resistors as shown below. Find the equivalent resistance for the circuit and the total current in the circuit.

24

57

9V


Given:

V= 9V R1= 2 R2= 4

R3= 5 R4= 7 Req= ?? I= ??

Equations:

V=IReqe 1 2 3

1 1 1 1....

R q R R R


1 1 1 1 1

2 4 5 7eqR

0.5 0.25 0.2 0.14 1.09

1 1 1 1 1

1 10.915

1.09eqR

99.84

0.915eq

V VI A

R


Find the equivalent resistance, the total current drawn by the circuit, and the current in each resistor for a 9.00 V battery connected to:One 30.0 resistorThree 30.0 resistors connected in

parallel

Answers:30.0 , 0.300 A, 0.300 A10.0 , 0.900 A, 0.300 A

+Summary

+Wiring Lights

The series circuit shows a bulb burned out. What will happen to the other bulbs? Would this also happen in the parallel

circuit?

Assuming the bulbs are identical: Which circuit will draw more current? In which circuit are the bulbs brighter?

+Now what do you think?

• How will the brightness of (b) and (c) compare to each other and how does each compare to (a)? Explain.

• How will the brightness of (d) and (e) compare to each other and how does each compare to (a)? Explain.

• Compare the total current leaving the battery in each of the three circuits. Explain.

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18.3 Complex Resistor Combinations

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What do you think?

• Household circuits typically have many outlets and permanent fixtures such as hanging light fixtures on each circuit. • Are these wired in series or in parallel?• Why do you believe one of these methods

has an advantage over the other method?• What disadvantages would the other

method of wiring have for household circuits?

+Complex Resistor Calculations

For complex resistors, you need to follow a few steps to be successful:1. Combine series in parallel2. Combine parallel sets3. Combine series set 4. Finish problem from there


To find the equivalent resistance for the circuit shown above, follow the steps shown to the right:

1. 6+2= 8

2.

3.

4. 3 + 9 + 2.70 + 1= 12.7

1 10.12 0.25 0.37

8 4

12.70

0.37


To find the equivalent resistance for the circuit shown above, follow the steps shown to the right:

Req for 6.0 and 2.0 Answer: 8.0

Req for 8.0 and 4.0 Answer: 2.7

Req for 3.0 and 6.0 and 2.7 and 1.0 Answer: 12.7

So, the resistance of all 6 resistors is equivalent to a single 12.7 resistor.


For the 2.0 resistor, find the current and the potential difference. To solve this problem, use

the step-by-step approach shown.

Find the total current in the equivalent circuit. Answer: 0.71 A This is the current through

the 1.0 , 6.0 (on the left), and 3.0 loads

Find the total potential drop across the parallel combination of three resistors. Answer: 1.9 V Continued on the next slide


Find the current through the combined 6.0 and 2.0 resistor. Answer: 0.24 A

Find the potential difference across the 2.0 resistor. Answer: 0.48 V


For the circuit shown, find the: Equivalent resistance Current through the 3.0 resistor Potential difference across the 6.0 resistor

Answers: 6.6 , 1.8 A, 6.5 V

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Now what do you think?

• Household circuits typically have many outlets and permanent fixtures such as hanging light fixtures on each circuit. • Are these wired in series or in parallel?• Why do you believe one of these methods

has an advantage over the other method?• What disadvantages would the other

method of wiring have for household circuits?

electric circuits

Technology

energy bulb

energy battery

series resistors

equivalent resistance

parallel resistors

closed circuit

single bulb

series vbattery