11-sep-15physics 1 (garcia) sjsu. current 11-sep-15physics 1 (garcia) sjsu water continues to flow...

Apr 19, 2023 Physics 1 (Garcia) SJSU

Current


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 CurrentJust 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

Demo: AmmeterAmmeter measures electrical current.


PowerSupply

CU

RR

EN

T

Ammeter

Ammeter

Current increases as the voltage increases.

Due to charge conservation, same current into and out of light bulb. (adjustable

voltage)

Light Bulb

Ammeter readings always the same.

Galvanometer is an ammeter with both positive & negative

Voltage SourcesCharges flow only when they are “pushed” or

“driven.” A sustained current requires a suitable pumping device to provide a difference in electric potential—a voltage.


Simple Chemical Battery Simple Mechanical Generator

Lemon

Copper

Aluminum

Chemical BatteryBatteries separate positive and negative charges by using a chemical reaction.

Chemical potential energy is converted into electrical energy.

Rechargeable 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

Electrical ResistanceCurrent depends not only on the voltage but also on the electrical resistance of the conductor.

The wider the wire is, the less the resistance.

A short wire offers less resistance than a long wire.

Apr 19, 2023

More water flows through a thick hose than through a thin one connected to a city's water system (same water pressure).

Jumper cables use thick wires so as to minimize the electrical resistance.

Jacob’s LadderElectrical resistance through air is greater for

a larger gap.


Biblical Jacob’s LadderPhysics Jacob’s Ladder

Ohm’s LawRelation between current, voltage, and

resistance is Ohm’s law,

(Current) =

Ampere is unit of current; symbol is AVolt is unit of voltage; symbol is VOhm is unit of resistance; symbol is


(Voltage)

(Resistance)

Lab: Ohm’s LawMeasuring voltage, current, and resistance in

simple circuits to verify Ohm’s law.


Ammeter Galvanometer

Resistor

Battery

Demo: Resistance of WaterPure water has very high resistance;

impurities, such as salt, lower resistance.


PowerSupply

CU

RR

EN

T

Light Bulb

Ammeter

SaltyWater

When salt dissolves the sodium and chlorine atoms are charged (ions). These mobile charges carry the current in the water.

Nervous SystemNervous 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 HeartThe 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 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

Electric ShockThe damaging effects of shock are the result of current passing through the body.


Current (A) Effect

0.001 Can be felt

0.005 Is painful

0.010 Causes involuntary muscle contractions (spasms)

0.015 Causes loss of muscle control

0.070 If through the heart, serious disruption; probably fatal if current lasts for more than 1 s

Effects of Electric Shock on Human Body

From Ohm's law, current depends on the voltage and on electrical resistance.

When dry, skin’s resistance around 100,000 .

Resistance drops as low as 100 when wet and salty.

Check YourselfIf your resistance is 100,000 , what is the

current passing through you when you touch an electric socket (120 volts)?

What if your resistance is only 100 ?


Direct & Alternating CurrentDirect current (DC) is

current that flows in only one direction.

Alternating current (AC) is current that flows back and forth with alternating direction.


DC vs. ACEasy to produce small

DC currents using batteries, which also have low voltages.

For major power lines, less ohmic heating if high voltage AC current is used instead of DC.

Electrons in ConductorsElectrons in a wire are in

constant, rapid, but random motion.

With direct current the electrons slowly drift down the wire.

With alternating current the electrons slosh back and forth.


WithoutVoltage With

Voltage

Demo: Resistance & Temperature


Battery

CURRENT

Ammeter

Resistance in a material goes down when the material is cooled because the electrons don’t bump into atoms often.

WithoutVoltage With

Voltage

WithVoltage& Cold

Liquid

Nitrogen

Current increases when wire cooled

Ohmic HeatingFlowing electrons strike

atoms in a conductor, heating the material.


Toaster

Rear Window DefoggerOhmic heating evaporates fog on car window.


Demo: Ohmic CookingAn electric current running through a hot dog generates enough heat to cook it.


Electric PowerPower is rate at which energy is delivered.Power = (Voltage) x (Current)

For example, (100 Watts) = (120 Volts) x ( 5/6

Ampere)


Line voltage in the US is 120 Volts.In Europe voltage is 240 Volts.

Check YourselfA kilowatt is 1000 watts, and a kilowatt-

hour is the amount of energy consumed in one hour at the rate of 1 kilowatt.

If electric energy costs 5 cents per kilowatt-hour, what does it cost to operate a 100-watt light bulb for 10 hours?

What does it cost to run a 1200 watt microwave for 10 minutes?


Lab: Simple Electric CircuitsCombining circuit

elements (battery, wires, bulbs, resistors, etc.) in different arrangements can give different results.

Can predict results by tracing current and voltage differences.


These three circuits are equivalent

Series CircuitSame current passes through each element.


Disconnect one of the bulbs and the circuit is broken (other bulbs go out).

Parallel Circuit


Disconnect one of the bulbs and the other bulbs stay light with same brightness.

Same voltage on each bulb; current splits through each branch.

Check YourselfHow do the brightnesses of the identical light bulbs compare?

Which bulb draws the most current? What happens if bulb A is

unscrewed? What happens if bulb C is

unscrewed?


Overloading a CircuitMore appliances added to a

parallel circuit, the more current flows.

A large current can cause significant ohmic heating in the wires, which is a fire hazard.

Protect against overloading a circuit by adding a fuse.


Fuses & Circuit BreakersFuse 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

Check YourselfIf a 1200 watt hair dryer is connected to a 120

Volt line, how much current will it draw? How many hairdryers can you operate before

blowing a 30 amp fuse?


11-sep-15physics 1 (garcia) sjsu. current 11-sep-15physics 1 (garcia) sjsu water continues to flow...

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