Direct Current Direct Current (DC) Electric (DC) Electric

CircuitsCircuits

Physics

Montwood High School

R. Casao

Electric Current IElectric Current I

An electric current is the movement of positive and/or negative charges Q through a conductor.

Current I is the rate of charge movement through a cross-sectional area of the conductor.

tQ

I

Electric Current IElectric Current I

The charge carriers in metallic conductors are electrons.

Electric Current IElectric Current I

Charge is measured in coulombs C.The charge on an electron and a

proton is: Q = 1.602 x 10-19 CCurrent is measured in amperes A;

sC1

A1

Two Types of CurrentTwo Types of Current

DC current (direct current) is a steady flow of current in one direction.

AC current (alternating current) - direction of current flow changes many times a second. In the US, the frequency of change is 60 Hz. Therefore, the current changes direction 60 times per second.

Electric CircuitsElectric Circuits

A simple electric circuit will consist of:– A source of energy (in

this case a battery).– Conducting wires.– A resistor R that uses

the energy.– A switch to open/close

circuit. The source of energy

has an internal resistance r.

Electromotive Force (EMF)Electromotive Force (EMF)

Batteries, generators, and solar cells, transform chemical, mechanical, and radiant energy, respectively, into electric energy. These are examples of sources of EMF.

EMF is measured in Volts V;

The source of EMF provides the energy the charge carriers will conduct through the electric circuit to the resistor.

C1J1

V1

Potential Difference or Voltage VPotential Difference or Voltage V

Current in a circuit moves from an area of high electric potential energy to an area of low potential energy. This difference in electric potential energy is necessary for current to move through a conductor.

The positive terminal of a battery is the high electric potential energy terminal and the negative terminal is the low electric potential energy terminal.

Potential difference V is also measured in volts.

Chemical BatteryChemical Battery

Batteries separate positive and negative charges by using a chemical reaction.

Chemical potential energy is converted into electrical energy.

Rechargeable BatteryRechargeable BatteryEventually the battery’s chemicals are consumed unless

the reaction can be reversed by passing a current into the battery. Automobile

battery is recharged while the gasoline engine is running since the engine powers a generator that produces a recharging current.

Starting the car

Engine running

Potential Difference or Voltage VPotential Difference or Voltage V

Within the battery, a chemical reaction occurs that transfers electrons from one terminal to another.

Because of the positive and negative charges existing on the battery terminals, a potential difference (voltage) exists between them.

Potential Difference or Voltage VPotential Difference or Voltage V

The battery creates an electric field within and parallel to the wire, directed from the positive toward the negative terminal.

This field exerts a force on the free electrons, causing them to move. This movement of charge is known as an electric current.

The current in the circuit is shown to flow from the positive terminal to the negative terminal.

Potential Difference or Voltage VPotential Difference or Voltage V

EMF is the maximum amount of energy per charge the battery can provide to the charge carriers.

Voltage is the energy per charge the charge carriers have after moving through the internal resistance r of the battery.– Some of the energy added to the charge carriers

has to be used to travel through the battery.– The remaining energy is carried to the resistors

outside the battery.

Batteries in Series and Parallel:

How can birds perch or squirrels run along high voltage (1000’s of volts) wires and not be fried??

To receive a current (shock) there must be a difference in potential between one foot and the other, but every part of the bird or squirrel is at the same potential as the wire. IF they landed with one foot on one wire and the other foot on a neighboring wire at a different voltage, ZAP!!!!!

Resistance RResistance R Resistance is the

opposition to the flow of charge through the conductors.

Resistance of a solid conductor depends upon: 1. nature of the material 2. length of the conductor 3. cross-sectional area of

the conductor 4. temperature

Resistance The resistance of a conductor is proportional to the

length.– Resistance increases with increased length.

The resistance of a conductor is inversely proportional to the cross-sectional area of the conductor.– Resistance decreases with increased cross-

sectional area. Resistance is also dependent upon the temperature

of the conductor. Collisions of electrons with other electrons and with atoms raises the temperature of a material as the added heat energy causes the electrons to move faster and hence collide more often. This increases the resistance of the conductor.

Resistance Resistance is measured in ohms .

Resistivity is related to the nature of the material. Good conductors have low resistivity (or high conductivity). Poor conductors have high resistivity (or low conductivity).

Unit for resistivity is ·m. Resistance:

Resistivity: = o + o ··(T – To)

A1V1

Ω1

Alρ

R

Factors Affecting Resistance

CurrentCurrent

Water continues to flow because a difference in pressure is maintained with the pump.

Water flows from the reservoir of higher pressure to the reservoir of lower pressure; flow stops when the pressure difference ceases.

Electric CurrentElectric Current

Just as water current is flow of water molecules, electric current is the flow of electric charge.

In circuits, electrons make up the flow of charge.

ON

OFF

Current and its Effect on the Human Body

Current in Amps Effect on human body

0.001 (1 mA) Can be felt

0.005 (5 mA) Painful

0.010 (10 mA) Involuntary Muscle Spasms

0.015 (15 mA) Loss of Muscle Control

0.070 (70 mA) If through heart, serious disruption; probably fatal if > 1 second

0.1-0.2 (100-200 mA) Uncontrolled “twitching” of heart

> 0.2 (> 200 mA) Heart stops, but may be able to be revived easier than 0.1 – 0.2 A

Nervous SystemNervous System

Nervous systems in animals use electrical currents to signal the contraction and relaxation of muscles.

Frog leg jumps when electrical current passes through it.

Conduction in Human HeartConduction in Human Heart

The most important electrical signal in our body is the periodic signal that contracts and relaxes our heart muscle to pump blood.

Without a constant flow of blood the brain can suffer permanent damage.

SA

AV

Conduction in Human HeartConduction in Human HeartThe normal electrical conduction in the

heart allows the impulse that is generated by the sinoatrial (SA) node of the heart to be propagated to (and stimulate) the myocardium (muscle of the heart).

When the myocardium is stimulated, it contracts, pumping blood in the body.

As the electrical activity is spreading throughout the atria, it travels via specialized pathways, known as internodal tracts, from the SA node to the Atrioventricular (AV) node.

The AV node functions as a critical delay in the conduction system. Without this delay, the atria and ventricles will contract at the same time, and blood won't flow effectively from the atria to the ventricles.

SA

AV

Ohm’s LawOhm’s Law

Ohm's Law deals with the relationship between the voltage and current in a conductor with resistance R.

Mathematically:

RV

I

RIV

types of circuittypes of circuit

There are two types of electrical circuits;

SERIES CIRCUITS PARALLEL CIRCUITS

Series CircuitSeries Circuit Resistors can be

connected in series; that is, the current flows through them one after another. The circuit in Figure 1 shows three resistors connected in series, and the direction of current is indicated by the arrow.

Figure 1: Resistors connected in series.

Series CircuitSeries Circuit

Since there is only one path for the current to travel, the current through each of the resistors is the same. All the charge carriers that come out of the battery must pass through each resistor.

I = I1 = I2 = I3

Series CircuitSeries Circuit Also, the voltage drops

across the resistors must add up to the total voltage supplied by the battery.

The charge carrier will supply energy to each resistor in the circuit; the amount of energy each resistor receives depends upon the resistance itself.

The greater the resistance, the more energy it uses.

E = V1 + V2 + V3

Series CircuitSeries Circuit When working with circuits, it is

customary to simplify the circuit by combining resistances in series into a single equivalent resistor Req.

The equivalent resistance is the single resistance that could be used in the circuit to replace the three separate resistances.

Req = R1 + R2 + R3 If R1 = 2 , R2 = 4 , and

R3 = 8 , the equivalent resistance Req = 2 + 4 + 8 = 14

Parallel CircuitsParallel Circuits Resistors can be

connected such that they branch out from a single point and join up again somewhere else in the circuit. This is known as a parallel circuit.

Each of the three resistors is another path for current to travel between points A and B.

Parallel CircuitsParallel Circuits Resistors in parallel have the

same voltage drop across them. Voltage is constant in parallel.

E = V1 = V2 = V3

The charge carriers come out of the battery carrying the same joules of energy per coulomb of charge and when they reach the junction point, some charge carriers (i1) will go through R1, some will go through R2 as i2, and the rest go through R3 as i3.

Parallel CircuitsParallel Circuits

Each charge carrier has the the same joules/coulomb no matter which resistor it passes through, which is why the voltage V is constant in parallel.

The sum of the currents in each parallel branch equals the total current entering the parallel branch of resistors.

In this example: i = i1 + i2 + i3

Usually: IT = I1 + I2 + I3

Parallel CircuitsParallel Circuits

At every junction point in a parallel circuit, the current that enters the junction point must equal the current that exits the junction point.

Parallel CircuitsParallel Circuits Resistances in parallel are

also simplified into an equivalent resistance Req.

My suggestion involves the reciprocal (x-1) calculator key:

321eq R1

R1

R1

R1

113

12

11

RRRReq

Parallel CircuitsParallel Circuits

If R1 = 2 , R2 = 4 , and R3 = 8 , the equivalent resistance Req = [(2 )-1 + (4 )-

1 + (8 )-1 ] -1 = 1.14286

Toll Road—Circuit Analogy

Toll Booth Explanation

Adding toll booths in series increases resistance and slows the current flow.

Adding toll booths in parallel lowers resistance and increases the current flow.

Electric PowerElectric Power Electric power P is the rate of doing

electrical work. Power is the product of current and voltage. P = V·I Unit: Watt, W 1 W = 1 Joule/sec = 1 Volt·Amp The total power in a series combination of

light bulbs and in a parallel combination of light bulbs is the sum of the individual wattages. Ex.: two 60 W light bulbs will dissipate 120 W in a series combination as well as in a parallel combination.

RIP 2 RV

P2

Electric EnergyElectric EnergyElectric companies sell

you electrical energy. Your energy consumption is computed by expressing power in kilowatts and time in hours. Energy is sold to you in units of kW-hr.

E = P·t E = V·I·t hrkW

amount$ECost

Ammeters and VoltmetersAmmeters and Voltmeters Ammeters are used to

measure current and must be placed in series with the circuit component you want to measure the current through.

Voltmeters are used to measure the voltage drop across the resistor or the circuit and must be placed in parallel with the component you want to measure the voltage drop across.

Fuses & Circuit BreakersFuses & Circuit Breakers

Fuse is designed to melt (due to ohmic heating) when current is too large.

Circuit breaker does same job without needing replacement; flip the switch to reconnect.

Fuse

CircuitBreaker

FusesFuses

•A fuse is a ribbon of wire with a low melting point

•If the current gets too large, the wire melts or blows out

•When fuses blow out, they open the circuit

•Once the circuit is open, electricity cannot flow through it

BreakersBreakers•A circuit breaker works like a fuse, but doesn’t need to be replaced after it opens the circuit

•Circuit breakers are made of 2 different metals

•If the current gets too large, the metal moves and acts like a switch to open the circuit

•You can reset the circuit breaker which closes the circuit so electricity can flow

TAKS Warning - Parts of a Light BulbTAKS Warning - Parts of a Light Bulb

•In order to light the bulb, electricity travels through one contact, up the support wires, through the tungsten filament and back down the other support wire to complete the circuit.

•What do you think happens in a light bulb when it “burns out”?