1. SCI 340 L42 current
Group Work
1. Light the bulb!

2. Effects of moving charges
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 FlowDq Dt
Current = charge flux / time =
current
Unit: C / s = ampere = A

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

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 V V = voltage = electric potential change I = R I = current
SCI 340 L42 current
Ohm’s Law
I = V R
I = current
V = voltage = electric potential change
R = resistance
Good conductors have low resistance.
Unit: V / A = ohm (W)

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

12. Question
If you want to increase the current through a resistor, you need to
Increase the resistance or voltage.
Decrease the resistance or voltage.
Increase the resistance or decrease the voltage.
Decrease the resistance or increase the voltage.

13. Ohm’s Law Rearranged If you know two, you can find the third. I = V R
SCI 340 L42 current
Ohm’s Law Rearranged
If you know two, you can find the third.
I = V R
R = V I
V = IR
Good conductors have low resistance.
I = current
V = voltage
R = resistance

14. Calculate the Current
A 1.5-V battery powers a light bulb with a resistance of 9 W. What is the current through the bulb?
Ohm’s Law I = V / R
V = 1.5 V; R = 9 W
I = (1.5 V ) / (9 V/A) = 1/6 A

15. Board Work
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
SCI 340 L42 current
Electric Power Formula
Voltage is work done per charge: V = DE / q
Current is charge per time: I = q /Dt
So, (voltage times current) = (work per time) = power
Good conductors have low resistance.
Power = VI

17. Board Work
What is the power dissipated by the car headlight drawing 15 A when connected to a 12-V car battery?

18. Another Formula Power = VI Power = I2R
If you know current I and resistance R
Ohm’s law tells you V = I R
Substitute IR for V in power formula
VI = IRI = I2R
Power = I2R

19. Yet Another Formula Power = VI Power = V2/R
If you know voltage V and resistance R
Ohm’s law tells you I = V /R
Substitute V/R for I in power formula
VI = V(V/R) = V2/R
Power = V2/R

20. Verify the Formulas
For the car headlight: VI
I2R
V2/R

21. Circuit Diagrams
Potential source
resistor

22. Kirchoff’s Laws Current into any node equals current out of the node
I1 = I2 + I3 0 = I2 + I3 – I1
“Current law”
“Junction law”
Conservation of charge

23. Kirchoff’s Laws Potential drop around any closed loop is zeroV1
V – V1 – V2 = 0
V1 + V2 = V V
“Voltage law”
“Loop law” V2
Conservation of energy