Current Electricity

http://www.physicsclassroom.com/class/circuits

Electric energy provides the means to transfer large quantities of energy over great distances with little loss.

Producing Electric Energy

Because electric energy can so easily be changed into other forms, it has become indispensable in our daily lives.

Producing Electric Energy

Potential Energy

If you do work against gravity, the gravitational field stores that energy as Gravitational Potential Energy, GPE

If you do work against an electrostatic force, the electric field stores that energy as electric Potential Energy, EPE

Low PE

High PE

Electric Potential Energy Electrical potential energy is

the energy contained in a configuration of charges.

Like all potential energies, when it goes up the configuration is less stable; when it goes down, the configuration is more stable.

The unit is the Joule.

Electric Potential EnergyElectrical potential energy increases when charges are brought into more unstable configurations.

+ + +

Lower PE Higher PE

dFe

+

Moving q1 closer to q2 requires work and that will increase the PE of the charge. Work against electric force increases electric PE

Stable Unstable

Electric Potential EnergyElectrical potential energy decreases when charges are brought into more stable configurations.

+ -

Higher PE Lower PE

d

Fe+ -

q1 will naturally move or fall towards q2 in the direction of E. No work is required and the PE of the charge will decrease. Work with the electric force decreases electric PE

StableUnstable

Electric Potential EnergyElectrical potential energy, EPE, is stored or lost as charges, q, move in an electric field, E. EPE is dependent on both the location in the E field and the amount of charge, q moved.

E is not the same for every situation and it changes in space. It depends on the configuration of the source charges

Electric Potential Difference

+

++

+

+

+

-

--

-

-

-

Lower V

Higher V

+ +

Moving a charge in an electric field requires work or energy input

VqW A t every location, a charge has a position-dependent potentialPotential difference is simply the difference in potential at any 2 points

Electric Potential greatest at?

A B

Fe

Circuit

Lower V

High V

Charges flow from high to low V through conducting wireThis flow of positive charge is called conventional current

The flow stops when the potential difference between A and B is zero.

A

B

Potential Difference and Current

Lower V

High V

To be a circuit, charges must flow continuously thru a loop, returning to their original position and cycling thru again. To do so requires energy input, a charge pump that raises the electric potential of the charge

Electric Circuits

A circuit is simply a closed loop through which charges can continuously move

Flow of charge is CURRENT

1. Voltaic or galvanic cell converts chemical E to electric E.

A battery is made up of several galvanic cells connected together.

2. Photovoltaic cell, or solar cell—changes light energy into electric energy.

Producing Electric Current (Charge Pump)

Requirements of a Circuit

Requirements of a Circuit1. Closed conducting loop that extends from the positive to negative terminal

What do the 4 successful

arrangements have in

common?

Requirements of a Circuit1. Closed conducting loop that extends from the + to - terminal

What do the 4 successful

arrangements have in

common?

2. There must be an energy source that maintains an electric potential difference across the ends of the circuit.

Electric Circuits A circuit is

simply a closed loop through which charges can continuously move

CurrentOnce the two requirements of a

circuit are met, charge will flow.The flow or movement of charge is called CURRENT

Current

• The flow or movement of charge is called CURRENT.

• Electric current is represented by I

• It is the overall rate of flow of electric charge, q/t.

I = q/tUnit of current:

Ampere (A)1 A = 1 C/s

Current A 2 mm long cross section of wire is isolated and 20 C of charge is determined to pass through it in 40 s.

I = _____________ A20C/40s = 0.5

Conventional Current Direction The particles that carry charge

through a wire are mobile electrons which move in a direction opposite the electric field.

Ben Franklin, who conducted extensive scientific studies in both static and current electricity, envisioned positive charges as the carriers of charge. The convention has stuck and is still used today. The direction of an electric current is by convention the direction in which a positive charge would move. Electrons would actually move through the wires in the opposite direction.

ProblemA long wire is connected to the terminals of a battery.  In 5.0 sec, 5.8 x 1020 electrons pass a cross section along the wire.a) Determine the current in the wire (if you need any extra information, ask your classmates).

b) If the electrons flow from left to right, in which direction is the current?

Ce

CxxexexQ 8.92

1

106.1108.5108.5

192020

At

QI 86.1

5

8.92

Opposite, right to left

A typical flashlight battery will produce a 0.5-A current for about 3 h before losing its charge. Determine the total number of electrons that have moved past a cross section of wire connecting the battery and light bulb.

exCx

eCxCQ

sQtQI

22

19

1038.3106.1

154005400

)10800/(5.0/

Circuit Components

Cell+ -

+ -Battery

Wire

Light bulb

Switch

Resistor

Sample Problem – Draw a single loop circuit that contains a cell, a light bulb and a switch. Name the components

+ -

bulb

switchcell

Series Circuit–How do the brightnesses compare?

Which circuit has the greater current flow?

Does the charge get used up?

+ - + -

1 2 3brighterdimmer

I + - + -

Parallel ConnectionsAs the number of light bulbs increases, what happens to the current through

the circuit?

3I

More I

less I

Increases, There are more pathways so less

resistance

Circuit Connections

Series ConnectionsIf one resistor is turned off (a light bulb goes out), what happens to the other resistors in the circuit?

If one resistor goes out, there is no longer a closed loop for current flow and all other devices in series will go out. There is an OPEN CIRCUIT

Parallel ConnectionsIf on resistor is turned off (a light bulb goes out), what happens to the other resistors in the circuit?

If one resistor goes out, there is still a closed loop for current flow and so the other devices in series will stay on

Series or Parallel? The light bulbs are identical and have identical resistance, R. Which configuration produces more light? Which way do you think the headlights of a car are wired?

More pathways,

Less resistance,

More current

More light

Circuit Components

Cell+ -

+ -Battery

Wire

Light bulb

Switch

Resistor

Circuit Components

Voltmeter

Ammeter

V

A

W Ohmmeter

Measuring Current

Current to be measured must pass through the ammeter, so it must be placed in SERIES mode in the circuit.

Ideally ammeters have ZERO resistance so that they do not affect the energy of the circuit

Ammeter – measures current

Measuring VoltageVoltmeter – measures

voltage

Does NOT require the current to pass through it. It must be placed in parallel to the circuit element.

Ideally voltmeters have INFINITE resistance so that they do not draw current away from circuit.

The amount of current in a circuit depends on BOTH the potential difference across the circuit, DV, AND the total resistance in the circuit, R.

Resistance

LOAD

Energy Source

II

An electron traveling through the wires and loads of a circuit encounters resistance, R. Resistance is a hindrance to the flow of charge.

DV

The table lists some of the factors that impact resistance.

Resistance

Resistors are devices designed to have a specific resistance.

Resistors are devices put in circuits to reduce the current flow

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

Resistors

R

VI

Current flow does NOT depend only on voltage. Charge traveling through the wires and loads of a circuit encounters resistance, R. Resistance is a hindrance to the current. The higher the resistance, the smaller the current.

VI

To produce electric current, I, a potential difference, DV, is required. Simon Ohm established experimentally that the current in a metal wire is proportional to the potential difference applied to its ends.

Ohm’s Law

IRV ResistanceUnits: Ohms ( )W

CurrentUnits: Amperes ( )A

Electric potentialUnits: Volts (v)

Every element in a circuit obeys Ohm’s Law

In which circuit does the light bulb have highest resistance?

Which of the following will cause the current through an electrical circuit to decrease? Choose all that apply.

a. decrease the voltageb. decrease the resistancec. increase the voltaged. increase the resistance

A certain electrical circuit contains a battery with three cells, wires and a light bulb. Which of the following would cause the bulb to shine less brightly? Choose all that apply.

a. increase the voltage of the battery (add another cell)

b. decrease the voltage of the battery (remove a cell)

c. decrease the resistance of the circuitd. increase the resistance of the circuit

If the resistance of a circuit were tripled, then the current through the circuit would be ____.

a. one-third as muchb. three times as muchc. unchangedd. ... nonsense! There would be no way to make such a prediction.

+ -

I

+ -

1/3 I

If the voltage across a circuit is quadrupled, then the current through the circuit would be ____.

a. one-fourth as muchb. four times as muchc. unchangedd. ... nonsense! There would be no way to make such a prediction.

+ -

I

+ -

4I

If the voltage across a circuit is quadrupled, then the current through the circuit would be ____.

a. one-fourth as muchb. four times as muchc. unchangedd. ... nonsense! There would be no way to make such a prediction.

+ -

I

+ -

4I

Use the Ohm's law equation to provide numerical answers to the following questions:

a. An electrical device with a resistance of 3.0 Ω will allow a current of 4.0 amps to flow through it if a voltage drop of ________ Volts is impressed across the device.

b. When a voltage of 120 V is impressed across an electric heater, a current of 10.0 amps will flow through the heater if the resistance is ________ Ω.

c. A flashlight that is powered by 3 Volts and uses a bulb with a resistance of 60 Ω will have a current of ________ Amps.

Use the Ohm's law equation to determine the missing values in the following circuits.

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.

Charge is a conserved quantity.

Conservation of Charge in a Circuit

ConnectionsWhich circuit draws more current (how are I1 and I2 related)?What is the order of bulb brightness?How does charge flow in these circuits (how are I2 and I3 related)?Does the charge get used up?dimm

er

More I

less I

+ - + -

brighter

brighter

I1

I2

I__

I3

same

I3I2

At circuit nodes (junctions), the current divides, and each path gets a fraction of it. No charge is lost.

Kirchoff’s Junction RuleCurrent into node= Current out of

node

I I2

I3

I1I1

I2

I3

I = I1 + I2 + I3 I1

=V/R1

I2

=V/R2

I3

=V/R3

I The lower resistance the path, the greater the current.

Kirchoff’s Loop RuleIn a closed circuit, sum of all the voltage boosts = sum of all the voltage drops

0 loopV

dropbat VV

Energy is conserved as charge flows around a closed loop

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.

Conservation of Energy in a Circuit

Kirchoff’s Rules1. Junction Rule At any junction

point in a circuit, the sum of all the currents entering the junction must equal the sum of all currents leaving the junctionCurrent into node= Current out of node

2. Loop Rule The sum of the changes in potential around any closed path of a circuit must be zero.

(Conservation of charge)

(Conservation of energy)

Two Types of ConnectionsWhen there are 2 or more electrical devices in a circuit with an energy source, there are a couple of ways to connect them.

+ -

12 V

4 W

8 W

Rank the currents at points A-F from greatest to least

AB

CD

EF

If the resistors were light bulbs, which would be brighter?

Energy Transfer and Power Movement of charge from terminal to

terminal is of little use if the energy possessed by the flowing charge is not transformed into another useful form.

High current and high rate of energy consumption. Would heat wire and drain battery

Energy Transfer and Power When a LOAD is put on the circuit

(light bulb, beeper, motor…), electrical energy is transformed to other, useful forms of energy.

An electrical circuit is simply an energy transformation tool. Rate of energy transformation/transfer is POWER

LOAD

Energy Source

Energy Transfer and PowerPOWER, P, is the rate

that energy is supplied to the load or the rate of work done on the charge.

LOAD

Energy Source

RVRIIVP

t

qV

t

W

timeP

dtransformeEnergy

/22

Unit of Power: Watt (W)

1 W = 1 J/s

Energy transfer and PowerPOWER, P, is the rate that energy is

supplied to the load or the rate of work done on the charge.

LOAD

Energy Source

60 Watt light bulb means 60 J of

energy delivered to bulb every second

OR 60 J of energy used

by the bulb per second

Electric heater. An electric heater draws 15.0 A on a 120 V line. How much power does it use and how much does it cost per month (30 days) if it operates 3.0 h per day and the electric company charges 10.5 cents per kW-h?

To operate it for 30 days, 3 hr/day would total 90hrs and would use

Will a fuse blow? Determine the total current drawn by all the devices used at once. Will they blow a 20-A fuse?

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.

The Human Body

Example: Lightning is a spectacular, natural example of electric current. There is much variability to lightning bolts, but a typical event can transfer 109 J of energy across a potential difference of 5 x 107 V during a time interval of 0.2 s. Estimate the total charge transferred, the current, and the average power over the 0.2 s.