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