electric cuircit

BIG IDEA

Electric currents carry electrical energy that can be transformed into
other forms of energy.

CHAPTER
22
Electric Current

MAIN IDEA

Electric current is the flow of electric charges.

Essential Questions

•

What is electric current?

•

How can you think about energy in electric circuits?

•

What is Ohm’s law?

•

How are power, current, potential difference and resistance
mathematically related?

Review Vocabulary

• electric potential difference the work done moving

a positive test charge between two points in an
electric field divided by the magnitude of that test
charge

New Vocabulary

• Electric current
• Conventional current
• Battery
• Electric circuit
• Ampere

• Resistance
• Resistor
• Parallel connection
• Series connection

• Flowing water at the top of a waterfall has both potential

and kinetic energy.

• However, the large amount of natural potential and kinetic

energy available from resources such as Niagara Falls are
of little use to people or manufacturers who are 100 km
away, unless that energy can be transported efficiently.

• Electric energy provides the means to transfer large

quantities of energy over great distances with little loss.

Producing Electric Current

• This transfer is usually done at high potential differences through power lines.

• Once this energy reaches the consumer, it can easily be converted into another form or combination of forms, including sound, light, thermal energy, and motion.

• Because electric energy can so easily be changed into

other forms, it has become indispensable in our daily
lives.

Producing Electric Current (cont.)

Open Ended

what is meant by electric current?

Multiple Choice

Current is...

• moving protons
• moving electrons
• measured in volts
• measured in ohms

Multiple Choice

A complete, unbroken path that charges can flow through is called an __________________.

• conductor
• electric circuit
• electric current
• insulator

Multiple Choice

__________________ is the continuous flow of electric charges through a material.

• Electric current
• Resistance
• Voltage
• Static Discharge

• When two conducting spheres touch, charges flow from the sphere at a higher potential to the one at a lower potential.

• The flow continues until there is no potential difference between the two

spheres.

• A flow of charged particles is an electric current.

Producing Electric Current (cont.)

Open Ended

search for: what is convection current and what is the difference between convection current and electron flow?

• In the figure, two conductors, A and B, are connected by a

wire conductor, C.

• Charges flow from the higher potential difference of B to A

through C.

• The flow stops when the potential

difference between A, B, and C
is zero.

• The direction in which a positive test

charge moves is called conventional current.

Producing Electric Current (cont.)

• Usually, it is the negative charges (electrons) that flow. The

flow of electrons and the direction of the conventional current
are in opposite directions.

• You could maintain the electric potential difference between B

and A by pumping charged particles from A back to B, as
illustrated in the figure.

• Since the pump increases the

electric potential energy of the
charges, it requires an external
energy source to run.

• This energy could come from a

variety of sources.

Producing Electric Current (cont.)

• One familiar source, a voltaic or galvanic cell (a

common dry cell), converts chemical energy to electric
energy.

• A battery is made up of several galvanic cells

connected together.

• A second source of electric energy— a photovoltaic cell,

or solar cell—changes light energy into electric energy.

Producing Electric Current (cont.)

Multiple Choice

Batteries and power plants are examples of ______________. 

• energy sources
• energy conduction
• energy transformation

• The charges in the figure

move around a closed
loop, cycling from pump B,
through C to A, and back
to the pump.

• Any closed loop or

conducting path allowing electric
charges to flow is called an electric
circuit.

Electric Circuits

• A circuit includes a charge pump, which

increases the potential energy of the
charges flowing from A to B, and a device
that reduces the potential energy of the
charges flowing from B to A.

• The potential energy lost by the charges, qV, moving

through the device is usually converted into some other
form of energy.

• For example, electric energy is converted to kinetic energy

by a motor, to light energy by a lamp, and to thermal energy
by a heater.

• A charge pump creates the flow of charged particles that

make up a current.

Multiple Choice

Conventionally, current is described as flowing from the ................ terminal of a battery to the ................ terminal.

• Negative.....negative

• Negative....positive

• Positive......negative

• Positive....positive

ch22_1_movanim_linking

Electric Circuits (cont.)

• Charges cannot be created or destroyed, but they can be

separated.

• Thus, the total amount of charge—the number of negative

electrons and positive ions—in the circuit does not change.

• If one coulomb flows through the generator in 1 s, then one

coulomb also will flow through the motor in 1 s.

• Thus, charge is a conserved quantity.

• Energy is also conserved.

• The change in electric energy, ΔE, equals qV. Because q is

conserved, the net change in potential energy of the charges going completely around the circuit must be zero.

• The increase in potential difference produced by the

generator equals the decrease in potential difference across the motor.

Electric Circuits (cont.)

• Power, which is defined in watts, W, measures the rate at

which energy is transferred.

• If a generator transfers 1 J of kinetic energy to electric

energy each second, it is transferring energy at the rate of 1
J/s, or 1 W.

• The energy carried by an electric current depends on the

charge transferred, q, and the potential difference across
which it moves, V. Thus, E = qV.

Rates of Charge Flow and Energy
Transfer

• The unit for the quantity of electric charge is the

coulomb.

• The rate of flow of electric charge, q/t, called electric

current, is measured in coulombs per second.

• Electric current is represented by I, so I = q/t.

• A flow of 1 C/s is called an ampere,A.

Rates of Charge Flow and Energy
Transfer (cont.)

• The energy carried by an electric current is related to the voltage, E =

qV.

• Since current, I = q/t, is the rate of charge flow, the power, P = E/t, of

an electric device can be determined by multiplying voltage and
current.

Rates of Charge Flow and Energy
Transfer (cont.)

• To derive the familiar form of the equation for the power delivered to

an electric device, you can use P = E/t and substitute E = qV and q = It

PowerP = IV

• Power is equal to the current times the potential

difference.

Rates of Charge Flow and Energy
Transfer (cont.)

Multiple Choice

SI unit of current is

• ampere

• metre

• ohm

• volt

Multiple Choice

SI unit of voltage is

• ampere

• metre

• ohm

• volt

Multiple Choice

Current can be measured using ............

• ammeter

• light bulb

• resistor

• voltmeter

Multiple Choice

Voltage can be measured using ............

• ammeter

• light bulb

• resistor

• voltmeter

Multiple Choice

An ammeter is connected in ...........

• Parallel

• Series

Multiple Choice

A voltmeter is connected in ...........

• Parallel

• Series

Multiple Choice

The electrical energy used if a charge of 5 C is supplied by a 12 V cell is....

• 2.2 A

• 60 A

• 2.2 J

• 60 J

• An electric circuit is drawn using standard symbols for the circuit

elements.

Diagramming Circuits

• Such a diagram is called a circuit schematic. Some of the symbols

used in circuit schematics are shown below.

Diagramming Circuits (cont.)

• An artist’s drawing and a schematic of the same circuit are shown below.

Diagramming Circuits (cont.)

• An ammeter measures current and a voltmeter measures

potential differences.

• Each instrument has two terminals, usually labeled

+ and –. A voltmeter measures the potential difference
across any component of a circuit.

• When connecting the voltmeter in a circuit, always connect

the + terminal to the end of the circuit component that is
closer to the positive terminal of the battery, and connect
the – terminal to the other side of the component.

Diagramming Circuits (cont.)

• Suppose two conductors have a potential difference

between them.

• If they are connected with a copper rod, a large current is

created.

• On the other hand, putting a glass rod between them

creates almost no current.

• The property determining how much current will flow is

called resistance.

Resistance and Ohm’s Law

• Resistance is measured by placing a potential difference

across a conductor and dividing the voltage by the
current.

• The resistance, R, is defined as the ratio of electric

potential difference, V, to the current, I.

• Resistance is equal to voltage divided by current.

Resistance

Resistance and Ohm’s Law (cont.)

• The table lists some of the factors that impact resistance.

• The resistance of the conductor, R, is measured in ohms.

• One ohm (1 Ω) is the

resistance permitting an
electric charge of 1 A to
flow when a potential
difference of 1 V is applied
across the resistance.

• A simple circuit relating

resistance, current, and
voltage is shown in the figure.

Resistance and Ohm’s Law (cont.)

• A 12-V car battery is connected to one of the car’s 3-Ω

brake lights.

• The circuit is completed by

a connection to an ammeter,
which is a device that
measures current.

• The current carrying the

energy to the lights will
measure 4 A.

• The unit for resistance is named for German scientist

Georg Simon Ohm, who found that the ratio of potential
difference to current is constant for a given conductor.

• The resistance for most conductors does not vary as the

magnitude or direction of the potential applied to it
changes.

• A device having constant resistance independent of the

potential difference obeys Ohm’s law.

Resistance and Ohm’s Law (cont.)

• Most metallic conductors obey Ohm’s law, at least over a

limited range of voltages.

• Many important devices, such as transistors and diodes in radios

and pocket calculators, and lightbulbs do not obey Ohm’s law.

• Wires used to connect electric devices have low resistance.

• A 1-m length of a typical wire used in physics labs has a

resistance of about 0.03 Ω.

Resistance and Ohm’s Law (cont.)

• Because wires have so little resistance, there is almost no

potential drop across them.

• To produce greater potential drops, a large resistance

concentrated into a small volume is necessary.

• A resistoris a device designed to have a specific

resistance.

• Resistors may be made of graphite, semiconductors, or

wires that are long and thin.

Resistance and Ohm’s Law (cont.)

Multiple Choice

If the voltage stays the same and resistance is increased, what will current do? 

• Increase
• Decrease
• Stay the same
• X

Multiple Choice

If there is a current of 2A and a resistance of 24 ohms, what voltage will there be?

• 26 V

• 48 V

• 12 V

• 22 V

Multiple Choice

If the voltage is increased and the resistance stays the same, what will current do?

• Increase
• Decrease
• Stay the same
• X

Multiple Choice

How do you calculate voltage?

• Current X Resistance
• Resistance ÷ Current
• Current ÷ Resistance
• Measure it with a ruler

Multiple Choice

What size resistor would produce a 3 Amp current flow with a 12 Volt battery

• 1 Ohm
• 3 Ohms
• 4 Ohms
• 12 Ohms

Multiple Choice

SI unit of resistance is

• ampere

• metre

• ohm

• volt

Multiple Choice

What is the voltage of a cell if it uses 4.2 J of energy as it pushes 6 C of charge round a circuit?

• 0.7 v

• 26 V

• 0.7 V

• 26 v

Multiple Choice

If a toaster produces 12 ohms of resistance in a 120-volt circuit, what is the amount of current in the circuit?

• 5A

• 10A

• 15A

• 20A

• There are two ways to control the

current in a circuit.

• Because I =V/R, I can be changed by

varying V, R, or both.

• The figure A shows a simple circuit.

• When V is 6 V and R is 30 Ω, the

current is 0.2 A.

Resistance and Ohm’s Law (cont.)

• How could the current be reduced to

0.1 A? According to Ohm’s law, the
greater the voltage placed across a
resistor, the larger the current passing
through it.

• If the current through a resistor is cut

in half, the potential difference also
is cut in half.

Resistance and Ohm’s Law (cont.)

• In the first figure,the voltage

applied across the resistor is
reduced from 6 V to 3 V to
reduce the current to 0.1 A.

• A second way to reduce the

current to 0.1 A is to replace the
30-Ω resistor with a 60-Ω
resistor, as shown in the second
figure.

Resistance and Ohm’s Law (cont.)

• Resistors often are used to control the current in circuits or parts

of circuits.

• Sometimes, a smooth, continuous variation of the current is

desired.

• For example, the speed control on some electric motors allows

continuous, rather than step-by-step, changes in the rotation of
the motor.

Resistance and Ohm’s Law (cont.)

• To achieve this kind of control, a variable resistor, called a

potentiometer, is used.

• A circuit containing a potentiometer is shown in the figure.

Resistance and Ohm’s Law (cont.)

• Some variable resistors consist of a coil of resistance

wire and a sliding contact point.

• Moving the contact point to various positions along the

coil varies the amount of wire in the circuit.

• As more wire is placed in the circuit, the resistance of

the circuit increases; thus, the current changes in
accordance with the equation I = V/R.

Resistance and Ohm’s Law (cont.)

• In this way, the speed of a motor can be adjusted from fast, with little

wire in the circuit, to slow, with a lot of wire in the circuit.

• Other examples of using variable resistors to adjust the levels of

electrical energy can be found on the front of a TV: the volume,
brightness, contrast, tone, and hue controls are all variable resistors.

Resistance and Ohm’s Law (cont.)

• The human body acts as a variable resistor.

• When dry, skin’s resistance is high enough to keep currents

that are produced by small and moderate voltages low.

• If skin becomes wet, however, its resistance is lower, and the

electric current can rise to dangerous levels.

• A current as low as 1 mA can be felt as a mild shock, while

currents of 15 mA can cause loss of muscle control, and
currents of 100 mA can cause death.

Resistance and Ohm’s Law (cont.)

Current Through a Resistor
A 30.0-V battery is connected to a 10.0-Ω resistor.
What is the current in the circuit?

Step 1: Analyze and Sketch the Problem

• Draw a circuit

containing a battery,
an ammeter, and a
resistor.

• Show the direction

of the conventional
current.

Identify the known and unknown variables.

Unknown:

I = ?

Known:

V = 30.0 V

R = 10 Ω

Step 2: Solve for the Unknown

Use I = V/R to determine the current.

Substitute V = 30.0 V, R = 10.0 Ω

• When a voltmeter is connected

across another component, it is
called a parallel connection
because the circuit component and
the voltmeter are aligned parallel to
each other in the circuit, as
diagrammed in the figure.

Parallel and Series Connections

• Any time the current has two

or more paths to follow, the
connection is labeled parallel.

• The potential difference across

the voltmeter is equal to the
potential difference across the
circuit element.

• Always associate the words voltage across with a

parallel connection.

• An ammeter measures the current through a circuit

component.

• The same current going through the component must go

through the ammeter, so there
can be only one current path.

• A connection with only

one current path is called
a series connection.

• To add an ammeter to a circuit, the wire connected to the

circuit component must be removed and connected to the
ammeter instead.

• Then, another wire is connected from the second terminal

of the ammeter to the circuit component.

• In a series connection, there can be only a single path

through the connection.

• Always associate the words current through with a series

connection.

What is an electric current?

Section Check

An electric current is a flow of charged particles. It is measured in C/s,
which is called an ampere, A.

Answer

In a simple circuit, a potential difference of 12 V is applied across
a resistor of 60 Ω and a current of 0.2 A is passed through the
circuit. Which of the following statements is true if you want to
reduce the current to 0.1A?

A.Replace the 60-Ω resistor with a 30-Ω resistor.

B.Replace the 60-Ω resistor with a 120-Ω resistor.

C.Replace the potential difference of 12 V by a potential

difference of 24 V.

D.Replace the 60-Ω resistor with a 15-Ω resistor.

Reason:There are two ways to control the current in a

circuit. Because I = V/R, I can be changed by
varying V, R, or both.

According to Ohm’s law, the greater the
resistance of the resistor, the smaller the
current passing through it. In order to halve
the current passing through a resistor, the
resistance of the resistor must be doubled.
Hence, to reduce the current to 0.1 A, the 60-
 resistor must be replaced with a 120-
resistor.

Answer

A 12-V battery delivers a 2.0-A current to an electric motor. If the
motor is switched on for 30 s, how much electric energy will the motor
deliver?

A.

B.

C.

D

Reason:Energy is equal to the product of power and

time.

That is, E = Pt.

Also, power is equal to the product of current
and potential difference.

That is, P = IV.

Therefore, E = IVt = (2.0 A) (12 V) (30 s).

Energy is measured is Joules (J).

Answer

• Electric current is a flow of charged particles. By convention, current

direction is the direction in which positive test charge moves.

• A circuit transforms electrical energy to thermal energy, radiant energy or some other form of energy.

• Ohm’s law states that the ratio of potential difference to current is a

constant for a given conductor. Any resistance that does not change with
potential difference or the direction of charge flow obeys Ohm’s law.

Current and Circuits

Study Guide

• The following equations show how power, current, potential

difference and resistance are mathematically related.

P = IV

Thermal Energy, A By-product