Transcript Electricity
Electricity • Electricity did not become an integral part of our lives until scientists learned to control the movement of electric charge, known as current. • Luigi Galvani (1737-1798) was conducting electrical experiments near a recently dissected frog. Sparks caused the frogs legs to twitch. • We now know that electric currents transmit signals from the brain to our muscles to cause movement. • Current is a rate of charge movement. Q C I t s C 1 1 Ampere s • Conventional current is defined in terms of positive charge movement. • In metal conductors, negative charges move. • In particle accelerators, protons may be set in motion. • In gases and dissolved salts current is the result of (+) charges moving in one direction and (-) charges moving in the other. • Charge carriers are either positive or negative charges that are in motion. • Electric fields may set charges in motion. • When a circuit is closed, the electric field travels at nearly the speed of light. The actual charges travel more slowly. Drift Velocity • Drift velocity is the net velocity of charge carriers. • When a conductor is in electrostatic equilibrium, the electrons move randomly. • When a potential difference is applied across the conductor, an electric field is set up inside the conductor. • The force due to that field sets the electrons in motion creating a current. • Temperatures increase in the conductor due to collisions. • The electrons move slowly in a direction opposite the electric field. Strong E Field The E Field weakens with distance. • In a copper wire with a current of 10 Amps, the drift velocity of electrons is 2.46 EE -4 m/s. • It would take an electron 68 minutes to travel one meter. • Differences in potential maintain the current in a circuit. • There are two different types of current: direct (DC) and alternating (AC). Alternating Current • In alternating current, the terminals of the source of potential difference are constantly changing sign. • There is no net motion of charge carriers in AC. • In the US, AC alternates 60 times each second. f = 60 Hz. • The current in a circuit is dependent upon potential difference (V) and the resistance that a conductor has. • Some conductors allow charges to move through them more easily than others. Resistance • Resistance is defined as the ratio of potential difference to current. V R I R () Volt 1 1 Amp • Resistance is constant over a range of potential differences. • This was discovered by Georg Simon Ohm. V constant I Ohm’s Law • The law is usually written as V IR • R is understood to be independent of • NASA Link . V • Ohm’s Law is not a fundamental law. Materials that have a constant resistance over a wide range of potential differences are said to be ohmic. • Walter Fendt’s website • Walter Fendt’s website • A graph of current versus potential difference for an ohmic material is linear. I R V • Materials that do not function according to Ohm’s Law are said to be nonohmic. Diodes are nonohmic. The resistance is small for currents in one direction and large for currents in the reverse direction. I R V • Resistance depends on length, crosssectional area, material, and temperature. • The smaller the length of wire the less the resistance. • The greater the cross-sectional area of wire the less the resistance. • Copper is less resistant than aluminum. • The lower the temperature the less the resistance. • When V remains constant, current decreases when resistance increases. • V = IR • When potential difference increases, current increases but resistance is not affected. Changes in potential difference do not affect the resistance of a resistor. The Wheatstone Bridge • A Wheatstone bridge is a measuring instrument invented by Samuel Hunter Christie in 1833 and improved and popularized by Sir Charles Wheatstone in 1843. It is used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component. Its operation is similar to the original potentiometer except that in potentiometer circuits the meter used is a sensitive galvanometer. • Salt water and perspiration lower the body’s resistance. • When the skin is dry, the human body’s resistance to current is around 500,000 . • Resistance decreases when skin is wet. • When the body is soaked with salt water or lots of ions, the resistance can be as low as 100. • Low resistance can be dangerous because it’s the flow of electrons that is lethal. • Perspiration contains ions that conduct electric charge. In a galvonic skin response test (GSR), a small potential difference is set up across the body. Perspiration increases when a person is nervous or stressed. This decreases the resistance of the body.