10-May-15 Physics 1 (Garcia) SJSU Chapter 23 Electric Current

10-May-15 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.

10-May-15 Physics 1 (Garcia) SJSU Electric 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

10-May-15 Physics 1 (Garcia) SJSU Demo: Ammeter Ammeter measures electrical current. Power Supply CURRENT 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

10-May-15 Physics 1 (Garcia) SJSU Voltage Sources Charges 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 Battery Batteries separate positive and negative charges by using a chemical reaction. Chemical potential energy is converted into electrical energy.

Rechargeable Battery Eventually 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

10-May-15 Electrical Resistance Current 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. 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.

10-May-15 Physics 1 (Garcia) SJSU Jacob’s Ladder Electrical resistance through air is greater for a larger gap. Biblical Jacob’s Ladder Physics Jacob’s Ladder

10-May-15 Physics 1 (Garcia) SJSU Ohm’s Law Relation between current, voltage, and resistance is Ohm’s law, (Current) = Ampere is unit of current; symbol is A Volt is unit of voltage; symbol is V Ohm is unit of resistance; symbol is  (Voltage) (Resistance)

10-May-15 Physics 1 (Garcia) SJSU Lab: Ohm’s Law Measuring voltage, current, and resistance in simple circuits to verify Ohm’s law. Ammeter Galvanometer Resistor Battery

10-May-15 Physics 1 (Garcia) SJSU Demo: Resistance of Water Pure water has very high resistance; impurities, such as salt, lower resistance. Power Supply CURRENT Light Bulb Ammeter Salty Water When salt dissolves the sodium and chlorine atoms are charged (ions). These mobile charges carry the current in the water.

Nervous 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 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

10-May-15 Physics 1 (Garcia) SJSU Conduction in Human Heart The 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

10-May-15 Physics 1 (Garcia) SJSU Electric Shock The damaging effects of shock are the result of current passing through the body. Current (A)Effect 0.001Can be felt 0.005Is painful 0.010Causes involuntary muscle contractions (spasms) 0.015Causes loss of muscle control 0.070If 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.

10-May-15 Physics 1 (Garcia) SJSU Check Yourself If 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  ?

10-May-15 Physics 1 (Garcia) SJSU Direct & Alternating Current Direct 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. AC Easy 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.

10-May-15 Physics 1 (Garcia) SJSU Electrons in Conductors Electrons 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. Without Voltage With Voltage

10-May-15 Physics 1 (Garcia) SJSU 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. Without Voltage With Voltage With Voltage & Cold Liquid Nitrogen Current increases when wire cooled

10-May-15 Physics 1 (Garcia) SJSU Ohmic Heating Flowing electrons strike atoms in a conductor, heating the material. Toaster

10-May-15 Physics 1 (Garcia) SJSU Rear Window Defogger Ohmic heating evaporates fog on car window.

10-May-15 Physics 1 (Garcia) SJSU Demo: Ohmic Cooking An electric current running through a hot dog generates enough heat to cook it.

10-May-15 Physics 1 (Garcia) SJSU Electric Power Power 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.

10-May-15 Physics 1 (Garcia) SJSU Check Yourself A 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?

10-May-15 Physics 1 (Garcia) SJSU Lab: Simple Electric Circuits Combining 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

10-May-15 Physics 1 (Garcia) SJSU Series Circuit Same current passes through each element. Disconnect one of the bulbs and the circuit is broken (other bulbs go out).

10-May-15 Physics 1 (Garcia) SJSU 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.

10-May-15 Physics 1 (Garcia) SJSU Check Yourself How 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?

10-May-15 Physics 1 (Garcia) SJSU Overloading a Circuit More 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.

10-May-15 Physics 1 (Garcia) SJSU Fuses & 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 Circuit Breaker

10-May-15 Physics 1 (Garcia) SJSU Check Yourself If 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?