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Ohm’s Law &

Resistors

Single Bulb Circuit

How does voltage affect current flow?

Qualitatively: Increasing voltage

(adding more batteries) causes a larger current flow

Quantitatively:

?

Single Bulb Circuit

How does voltage affect current flow?

Voltage (V) Current (A)

0.0 0.0

1.5 0.100

3.0 0.200

4.5 0.300

6.0 0.400

Single Bulb Circuit

How does voltage affect current flow?

Linear Relationship – Known as Ohm’s Law

Ohm’s Law (Conceptually) •  The current through a conducting material

is directly proportional to the voltage across the material

5V 200mA

10V 400mA

Current through Bulb 1

150mA

300mA

Current through Bulb 2

Current flows less easily through Bulb 1 than Bulb 2 Bulb 2 has more resistance than Bulb 1

Single Bulb Circuit

How does voltage affect current flow?

I = 0.6667A1.0V

⎛

⎝⎜

⎞

⎠⎟⋅ΔV

Single Bulb Circuit

How does voltage affect current flow?

I = 0.667A1.0V

⎛

⎝⎜

⎞

⎠⎟⋅ΔV

Light Bulb’s Electrical Conductance

For every additional 1.0V increase in voltage of the batteries, there will be

0.667A more current flowing through the bulb

Electrical Conductance

I =G ⋅ ΔVG stands for Electrical Conductance •  how easily current flows through a circuit element

(like a bulb) •  Larger Conductance = Easier for Charges to Flow •  SI Units: A / V

The inverse of Electrical Conductance is called Electrical Resistance (R) •  the difficulty with which current flows through a

circuit element •  Larger Resistance = Harder for Charges to Flow

Electrical Conductance vs. Resistance

I =G ⋅ ΔV I = 1R⋅ ΔV

ΔVI= R

ΔV = I ⋅R

This is the mathematical

representation of Ohm’s Law

Ohm’s Law

•  The current through a conducting material is directly proportional to the voltage across the material

ΔVI= R

•  R stands for Electrical Resistance •  SI Unit: Ohm (Ω) 1Ω = 1 V/A

Conceptual Example

•  A certain conductor has a resistance of 150Ω. What does this mean?

A potential difference (voltage) of 150V is required to cause 1A of current to flow.

A potential difference (voltage) of 300V is required to cause 2A of current to flow.

Quantitative Example

•  A conductor is known to have a resistance of 400Ω. It’s plugged into a wall (120V outlet). What will be the current flowing through the conductor?

ΔV = I ⋅R120V = I ⋅ (400Ω)I = 0.30A = 300mA

ΔV=120V R = 400Ω

I = ?

KLuES:

Your Turn

•  ER WS 1, #1

Ohm’s Law Limitations

•  Ohm’s Law can only be applied to ohmic materials (materials in which we see the linear pattern between ΔV and I)

•  Some ohmic materials may become non-ohmic at very hot or low temperatures

Resistors

•  Circuit elements that reduce/impede current flow

•  Purpose: To protect devices that can be damaged by large currents

•  Generally Ohmic under normal conditions

R R Br O B V Br Bl R

22 x 101 Ω 220 Ω

36 x 107 Ω 360,000,000 Ω

10 x 102 Ω 1000 Ω

Your Turn

•  ER WS 1, #2