Group Work 1. Light the bulb!

Electric Current Effects of moving charges

Objectives Relate current, potential and resistance using Ohm’s Law. Determine the power in a current flow.

Charges Move in Circuits Charges travel in a circuit to maintain charge balance

Light Bulb Anatomy

Electric Current is Charge Flow Current = charge flux / time = qq tt 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

Circuit Diagrams resistor Voltage source

Kirchoff’s Laws 1.Current into any node equals current out of the node I 1 = I 2 + I 3 I1I1 I2I2 I3I3

Kirchoff’s Laws 2.Potential drop around any closed loop is zero  – V 1 – V 2 = 0 V 1 + V 2 =  V1V1 V2V2 

Reading for Next Time Electric circuits Main ideas –Parallel and series pathways –Combining resistances Magnetic fields Main ideas –How magnetic forces act –How magnetic fields are created