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

t

Unit: C / s = ampere = A

current

What is the direction of the electric current?

-

-

-

--

-+

++

+

+

+

A. rightB. 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

IV = IR

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 FormulaVoltage 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 IR for V in power formula

• VI = IRI = I2R

Power = VI

Power = I2R

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

• VI = V(V/R) = V2/R

Power = VI

Power = V2/R

Verify the Formulas

• For the car headlight:– VI– I2R– V2/R

Circuit Diagrams

resistor

Voltage source

Kirchoff’s Laws

1. Current into any node equals current out of the node

I1 = I2 + I3

I1

I2

I3

Kirchoff’s Laws

2. Potential drop around any closed loop is zero

– V1 – V2 = 0

V1 + V2 =

V1

V2

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