CURRENT ELECTRICITY

Chapters 18 - 19

CHAPTER 18 TOPICS:ELECTRIC CURRENTS The Electric Battery Electric Current Ohm’s Law: Resistance and Resistors Resistivity Electric Power

CHAPTER 19 TOPICS:DC CIRCUITS EMF and Terminal Voltage Resistors in Series and in Parallel Kirchhoff’s Rules EMFs in Series and in Parallel; Charging

a Battery Circuits Containing Capacitors in Series

and in Parallel RC Circuits – Resistor and Capacitor in

Series Ammeters and Voltmeters

LAUNCH LAB Question:

Given a light bulb, a wire, and a battery. Can you get the light bulb to light?

Procedure:1. Try to find as many ways as possible to

get the light bulb to light.2. Diagram two ways in which you are able

to get the bulb to light. Be sure to label the battery, the bulb, and the wire.

3. Diagram three ways in which the bulb does not light. Again, label!

LAUNCH LAB - ANALYSIS1. How did you know if electric current was

flowing?

2. What do your diagrams of the lit bulb have in common?

3. What do your diagrams of the unlit bulb have in common?

4. From your observations, what conditions seem necessary in order for the bulb to light?

5. Critical Thinking: What causes electricity to flow through the bulb?

6. Draw a bulb. (Show the inside of the metal base.)

Answ

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THE ELECTRIC BATTERY

Volta discovered that electricity could be created if dissimilar metals were connected by a conductive solution called an electrolyte.

This is a simple electric cell.

THE ELECTRIC BATTERY A battery

transforms chemical energy into electrical energy.

Chemical reactions within the cell create a potential difference between the terminals by slowly dissolving them.

This potential difference can be maintained even if a current is kept flowing, until one terminal is completely dissolved.

THE ELECTRIC BATTERY Several cells

connected together make a battery.

(Although we now refer to a single cell as a battery as a well.)

ELECTRIC CURRENT The purpose of a battery is to produce a

potential difference, which can then make charges move.

When a continuous conducting path is connected between the terminals of a battery, we have an electric circuit.

On a diagram of a circuit, the symbol for a battery is:

ELECTRIC CURRENT When such a circuit

is formed, charge can flow through the wires of the circuit from one terminal of the battery to the other, as long as the conducting path is continuous.

A flow of charged particles is an electric current.

ELECTRIC CURRENT

Current is measured in coulombs per second.

This unit is given a special name, the ampere (amp or A), after French physicist André Ampére.

A current can flow in a circuit only if there is a continuous conducting path. We then have a complete circuit.

If there is a break in

the circuit we call it an open circuit and no current flows.

ELECTRIC CURRENT The electric

current in a wire is defined as the net amount of charge that passes through the wire’s full cross section at any point per unit time.

Where: I = current (amps)ΔQ = the amount of

chargeΔt = time interval

ELECTRIC CURRENT In any single

circuit, with only a single path for current to follow, a steady current at any instant is the same at one point as at any other point.

This is because of the law of conservation of electric charge.A battery does

not create charge and a light bulb does not absorb or destroy charge.

EXAMPLE 1 - CIRCUITSWhat is wrong with each of the schemes for

lighting a bulb with a battery and a single wire as shown on this and the next two slides?

There is no closed path for a charge to

flow through.

This scheme will not light the bulb.

EXAMPLE 2 - CIRCUITS

There is a closed path passing to and from the light bulb, however, the wire is only connected to one terminal.

No potential difference in the circuit to make the charge move.

EXAMPLE 3 - CIRCUITS Nothing is wrong

here.

This is a complete circuit: charge can flow out from one terminal of the battery, through the wire and the bulb, and into the other terminal.

EXAMPLE 4 - CURRENT

A steady current of 2.5 A exists in a wire for 4.0 min. a) How much total charge passed by a

given point in the circuit during those 4 minutes?

b) How many electrons would this be?

Homework

:Practice Problems

p.515 #1-3

ELECTRIC CURRENT In many real

circuits, wires are connected to a common conductor to provide continuity.

This common conductor is called ground.

This is the symbol for ground in a circuit diagram:

ELECTRIC CURRENT By convention, current is defined as

flowing from positive to negative.

Electrons actually flow in the opposite direction, but not all currents consist of electrons.

ASSIGNMENT Conceptual Questions: p.514 #1-4

OHM’S LAW

George Ohm (1787-1854) studied the relationship between the potential difference and the current.

OHM’S LAW Experimentally,

it is found that the current in a wire is proportional to the potential difference between its ends:

The ratio of voltage to current is called the resistance:

OHM’S LAW Resistance is

the property that determines how much current will flow.

Unit of resistance: the ohm, Ω. 1 Ω = 1 V/A.

Ex. Suppose two conductors have a potential difference between them. When connected by

copper rod, a large current is created.

However, connecting them with a glass rod creates almost no current.

RESISTORS A resistor is a

device designed to have a specific resistance.

Resistors may be made of graphite, semiconductors, or wires that are long and thin.

This is the symbol for a resistor in a circuit diagram.

RESISTORS AND RESISTANCE Standard

resistors are manufactured for use in electric circuits; they are color-coded to indicate their value and precision.

RESISTORS AND RESISTANCE

RESISTORS AND RESISTANCE Some clarifications:

Batteries maintain a (nearly) constant potential difference; the current varies. (Details in the next chapter.)

Resistance is a property of a material or device.

Current is not a vector but it does have a direction. In a wire, the current is always parallel to the wire

and the direction of conventional (positive) current is from high potential (+) toward lower potential (-).

Current and charge do not get used up. Whatever charge goes in one end of a circuit comes out the other end.

RESISTIVITY The resistance of

a wire is directly proportional to its length and inversely proportional to its cross-sectional area:

The constant ρ, the resistivity, is characteristic of the material.

RESISTIVITY For any given material, the resistivity

increases with temperature:

Semiconductors are complex materials, and may have resistivities that decrease with temperature.

The Greek letter alpha represents the temperature coefficient.

RESISTIVITY

FACTORS THAT IMPACT RESISTANCE

Factor How Resistance Changes

Length Resistance increases as length increases

Cross-sectional Area

Resistance increases as cross-sectional area decreases

Temperature Resistance increases as temperature increases

MaterialKeeping length, cross-sectional area, and temperature constant, resistance

varies with the material used.

PlatinumAluminumIronGoldCopperSilverR

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ASSIGNMENT Questions

p.514, #5+6

Practice Problems p.515-516#4-6, 9, 11-13

OHM’S LAW LAB ACTIVITY

ELECTRIC POWER

Power, as in kinematics, is the energy transformed by a device per unit time:

So:

ELECTRIC POWER The unit of power is the watt, W.

For ohmic devices, we can make the substitutions (Using V = IR):

POWER IN HOUSEHOLD CIRCUITS What you pay

for on your electric bill is not power, but energy – the power consumption multiplied by the time.

We have been measuring energy in joules, but the electric company measures it in kilowatt-hours, kWh.

POWER IN HOUSEHOLD CIRCUITS The wires used in

homes to carry electricity have very low resistance.

However, if the current is high enough, the power will increase and the wires can become hot enough to start a fire.

To avoid this, we use fuses or circuit breakers, which disconnect when the current goes above a predetermined value.

POWER IN HOUSEHOLD CIRCUITS

Fuses are one-use items.

If they blow, the fuse is destroyed and must be replaced.

POWER IN HOUSEHOLD CIRCUITS Circuit breakers, which are now much

more common in homes than they once were, are switches that will open if the current is too high; they can then be reset.

ASSIGNMENT

Questions:p.515

#9, 10, 14

Problems:p.516-517

#26, 27, 29, 30, 32, 33-37

SERIES AND PARALLEL LAB

Be sure to draw labeled circuit diagrams for both series and parallel circuits.

Answer the following:1. Compare and contrast the current flow in

a series circuit with that in a parallel circuit.

2. Compare and contrast the voltage in a series circuit with that in a parallel circuit.

Both of the above responses need to be backed up with data.

CHAPTER 19 TOPICS:DC CIRCUITS EMF and Terminal Voltage Resistors in Series and in Parallel Kirchhoff’s Rules EMFs in Series and in Parallel; Charging

a Battery Circuits Containing Capacitors in Series

and in Parallel RC Circuits – Resistor and Capacitor in

Series Ammeters and Voltmeters

EMF AND TERMINAL VOLTAGE To have current in

an electric circuit, we need a device (such as a battery or an electric generator) that transforms one type of energy into electric energy.

Such a device is called a source of emf.

The potential difference given to the charges by a battery is called emf. (Not an actual force, it

is a potential difference measured in volts.)

The emf is the influence that makes current flow from a lower potential to a higher potential.

EMF AND TERMINAL VOLTAGE

Remember: A battery is not a source of constant current.The current

out of a battery varies according to the resistance in the circuit.

A battery is, however, a nearly constant voltage source. It does have a small

internal resistance, r, which reduces the terminal (actual) voltage from the ideal emf.

Terminal Voltage: VAB = Va - Vb

Ideal emf: VAB = E

EMF AND TERMINAL VOLTAGE

VAB = terminal voltage E = emf I = current r = internal resistance

Internal resistance increases as the batteries get older and the electrolyte dries out.

The internal resistance behaves as though it were in series with the emf.

VAB = E - Ir

EXAMPLE:BATTERY WITH INTERNAL RESISTANCE

A 65.0-Ω resistor is connected to the terminals of a battery whose emf is 12.0 V and whose internal resistance is 0.500 Ω. Calculate:a) the current in the circuitb) the terminal voltage of the battery, Vab

c) the power dissipated in the resistor R and in the battery’s internal resistance r.

PART A – FIND CURRENT IN CIRCUIT Start with Ohm’s law: V = IR Voltage of emf with internal resistance: Vab = E – Ir

V = Vab for this circuit, Ohm’s Law again: Vab = IR

Substitute E – Ir for Vab: E – Ir = IR Rearrange to solve for I:

E = IR + Ir E = I(R + r) I = E /(R + r)

I = E /(R + r) = 12.0 V/(65.0 Ω + 0.500 Ω ) = 0.183 A

PART B – FIND TERMINAL VOLTAGE

Terminal Voltage = Vab = E – Ir

Vab = E – Ir

= 12.0 V – (0.183 A)(0.500 Ω) = 11.9 V

PART C – FIND POWER DISSIPATED

Power Dissipated in R

Power Dissipated in r

PR= I2R

PR= (0.183 A)2(65 Ω)

= 2.18 W

Pr= I2r

PR= (0.183 A)2(0.5 Ω)

= 0.0167 W

Power dissipated = P = I2R

INTERNAL RESISTANCE

In much of what follows, unless otherwise stated, we assume that the internal resistance of a battery is negligible.

Therefore, the battery voltage that is given is its terminal voltage, which we will write as V instead of Vab.

RESISTORS IN SERIES A series connection has:

two or more resistors connected end to end.a single path from the battery, through

each circuit element in turn, then back to the battery.

RESISTORS IN SERIES The current through each resistor is the

same. The voltage depends on the resistance. The sum of the voltage drops across the

resistors equals the battery voltage. Note that when you add more resistance

to a circuit, the current through the circuit will decrease.

RESISTORS IN SERIES From this we get the equivalent

resistance (that single resistance that gives the same current in the circuit).

RESISTORS IN PARALLEL A parallel connection splits the current into

different paths. The voltage across each resistor is the same.

The devices in houses are wired in parallel. If one device is disconnected, the current to

the other devices is not interrupted.

RESISTORS IN PARALLEL The total current is the sum of the

currents across each resistor:

This gives the reciprocal of the equivalent resistance:

Since the resistors are all

equal, the voltage will drop

evenly across the 3

resistors, with 1/3 of 9 V

across each one. So we get

a 3 V drop across each.

CONCEPTEST : SERIES RESISTORS I

9 V

Assume that the voltage of the

battery is 9 V and that the three

resistors are identical. What is

the potential difference across

each resistor?

1) 12 V

2) zero

3) 3 V

4) 4 V

5) you need to know the actual value of R

CONCEPTEST: SERIES RESISTORS II

12 V

R1= 4 WR2= 2 W

In the circuit below, what

is the voltage across R1?

1) 12 V

2) zero

3) 6 V

4) 8 V

5) 4 V

The voltage drop across R1

has to be twice as big as

the drop across R2. This

means that

V1 = 8 V and V2 = 4 V. Or

you could find the current

I = V/R = (12 V)/(6 )W = 2

A, then use Ohm’s Law to

get the voltages.

The voltage is the same (10

V) across each resistor

because they are in

parallel. Thus, we can use

Ohm’s Law, V1 = I1 R1 to

find the current I1 = 2 A.

CONCEPTEST: PARALLEL RESISTORS I

In the circuit below, what

is the current through R1?

10 V

R1= 5 W

R2= 2 W

1) 10 A

2) zero

3) 5 A

4) 2 A

5) 7 A

CONCEPTEST: PARALLEL RESISTORS II

1) increases

2) remains the same

3) decreases

4) drops to zero

As we add parallel

resistors, the overall

resistance of the circuit

drops. Since V = IR, and

V is held constant by the

battery, when resistance

decreases, the current

must increase.

Points P and Q are connected to

a battery of fixed voltage. As

more resistors R are added to

the parallel circuit, what

happens to the total current in

the circuit?

ASSIGNMENT

Terminal Voltage and emf

Resistors in Series and Parallel

Practice Problems: p.547

#1-3

Practice Problems: p.547

#5-7, 9, 11-13, 17, 18