Electric Current is Charge Flow Current = charge flux / time = qq tt Unit: C / s = ampere = A current
What is the direction of the electric current? - - - - - - + + + + + + A. right B. left Poll Question
Conductors and Insulators A conductor is a material through which current flows easily. An insulator is a material through which current does not flow. No material is a perfect insulator or conductor! –OK, superconductors do exist.
Resistance Hinders Current Current does not flow unhindered Electrical resistance is akin to friction or drag Expressed as voltage needed to maintain a current, V/A
Ohm’s Law I = V R I = current V = voltage = electric potential change R= resistance Unit: V / A = ohm ( )
Voltage Causes Current Potential drop is the cause. Current is the effect. Resistance reduces the effect of voltage.
Poll Question If you want to increase the current through a resistor, you need to A.Increase the resistance or voltage. B.Decrease the resistance or voltage. C.Increase the resistance or decrease the voltage. D.Decrease the resistance or increase the voltage.
Ohm’s Law Rearranged I = V R I = current V = voltage R= resistance If you know two, you can find the third. R = V I V = I R
Calculate the Current A 1.5-V battery powers a light bulb with a resistance of 9 . What is the current through the bulb? Ohm’s Law I = V / R V = 1.5 V; R = 9 I = (1.5 V ) / (9 V/A) = 1/6 A
Group Work 2.A car headlight draws a current of 15 A when connected to a 12-V car battery. What is the resistance of the headlight?
Electric Power Formula Voltage is work done per charge: V = E / q Current is charge per time: I = q / t So, (voltage times current) = (work per time) = power Power = VI
Group Work 3.What is the power dissipated by the car headlight in problem 2?
Another Formula If you know current I and resistance R Ohm’s law tells you V = I R Substitute I R for V in power formula V I = I R I = I 2 R Power = V I Power = I 2 R
Yet Another Formula If you know voltage V and resistance R Ohm’s law tells you I = V /R Substitute V/R for I in power formula V I = V(V/R) = V 2 /R Power = V I Power = V 2 / R
Verify the Formulas For the car headlight: –V I –I 2 R –V 2 /R