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

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

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

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 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)

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

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.

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

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

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?

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

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

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

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

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

Circuit Diagrams Potential source resistor

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

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