Current Voltage and Resistance Electricity Current Electricity n

Current, Voltage and Resistance Electricity

Current Electricity n n n What do turning on a light, turning on a radio, and turning on your television have in common? In each case, you are completing an electric circuit. In the last section, we studied static (stationary) charges. we are going to learn about charges that are on the move and the paths they follow!

n Alessandro Volta’s work led to the invention of the electric battery (voltaic cell), which produced the first steady flow of charged particles.

Conversion of Energy n n n Generator – coverts KE to electrical energy Battery – converts chemical energy to electrical energy Photovoltaic cell – converts solar energy to electrical energy

Electric Circuit n n a closed loop of a device (battery, generator, photovoltaic cell), that will increase the PE of electric charges, connected to a device that reduces the PE of the charges (radio, lamp, hair dryer) while converting electrical energy into a form of "useful" energy (sound, light, heat). In order to keep charges flowing, a potential difference must be maintained.

Electric Current n n n The rate of flow of electric charge is called electric current. Electric current in a wire can be defined as the net amount of charge that passes through it per unit time at any point. Symbol “I” unit “amperes”, “amps , “A”

Calculating Electric Current n I = ΔQ Δt I - electric current ( C/s or A) Q – amount if charge passing through conductor (C ) t – time interval (s)

n Conversion tips n 1 A = 1 C/s n Often measured in m. A and μA

Sample Problem n A steady current of 3. 5 A flowed in a wire for 2. 0 minutes. How much charge passed through the circuit?

Give it a go… n n A battery was charged using a current of 5. 7 A. How long did it take to charge the battery if 1. 2 x 105 C of charge passed through it? ( answer =5. 8 h) What is the current in amperes if 2000 Na+ ions were to flow across a cell membrane in 9. 8 µs? The charge on a sodium ion is the same as on an electron, but positive. (answer = 3. 3 x 10 -11 A)

Resistance and Ohm’s Law n n n Players so far: current (I) and potential difference (V) Georg Ohm experimentally determined that I was proportional to V. (I V) For example: n If you connect a wire to a 9 V battery, the current will be three times what it would be if the wire were connected to a 3 V battery.

n n The amount of current that flows in a circuit is also dependent on the resistance offered by the circuit. hallway example Current (I) is inversely proportional to resistance (R) I 1/R n as resistance increases, current decreases

Ohm’s Law n I = V/R n Usually written and V = IR n n n I = current (A) V = potential difference (V) R = resistance (Ω) (Greek letter omagha)

Resistors n n Connecting wires generally have very low resistance compared to the coils or wire filaments in some electrical devices like heaters and light bulbs. Resistors are devices designed to have a specific resistance and are often used in electronic devices to control the amount of current that flows. .

Rresistor n n n n The first two colors represent the first two digits in the value of resistance, the third represents the power of ten that it must be multiplied by the fourth is the manufactured tolerance. Red is 2, green is 5, orange is 103 silver indicates a tolerance of 10%. This resistor, then, has a resistance of 25 x 103 , or 2. 5 k , give or take 10%.

Sample Problems n What is the resistance of a toaster if 110 V produces a current of 3. 1 A? n A 4. 5 V battery is connected to a bulb whose resistance is 2. 5 Ω. What is the current? n A hair dryer draws 11. 0 A when plugged into a 120 V line. How much charge passes through it in 10 min?

Resistance in a Metal Wire n resistance of a metal wire is directly proportional to its length, and inversely proportional to its crosssectional area, A: n n R – resistance (Ω) ρ - resistivity(Ω m) L – length (m) A – cross sectional area (m 2)

How to Find Cross-Sectional Area of a Wire n n The cross-sectional area of a circular wire can be calculated using where d is the diameter of the wire or where r is the radius of the wire.

Sample Problem n What is the resistance of a 3. 5 m length of aluminum wire 1. 5 mm in diameter?

Give these a go…. . n n What is the length of a copper wire that has a cross-sectional area of 3. 4 x 10 -6 m 2 and a resistance of 7. 1 x 10 -2 Ω ? (answer – 14 m) What is the radius of a 1. 00 m length of tungsten wire whose resistance is 0. 25 Ω ? (answer – 2. 7 x 10 -4 m)