1. Group Work 1. Light the bulb!

2. Electric Current Effects of moving charges

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

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

5. Light Bulb Anatomy

6. Electric Current is Charge Flow Current = charge flux / time = qq tt Unit: C / s = ampere = A current

7. What is the direction of the electric current? - - - - - - + + + + + + A. right B. left Poll Question

8. 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.

9. 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

10. Ohm’s Law I = V R I = current V = voltage = electric potential change R= resistance Unit: V / A = ohm (  )

11. Voltage Causes Current Potential drop is the cause. Current is the effect. Resistance reduces the effect of voltage.

12. 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.

13. 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

14. 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

15. 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?

16. 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

17. Group Work 3.What is the power dissipated by the car headlight in problem 2?

18. 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

19. 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

20. Verify the Formulas For the car headlight: –V I –I 2 R –V 2 /R

21. Circuit Diagrams resistor Voltage source

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

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

24. 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