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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 =
current
Unit: C / s = ampere = A
Dq
Dt
Poll 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
I=
V
R
I = current
V = voltage = electric potential change
R = resistance
Unit: V / A = ohm (W)
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
If you know two, you can find the third.
V
I=
R
V = IR
I = current
V = voltage
R = resistance
R=
V
I
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
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 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
Power = VI
Group Work
3. What is the power dissipated by the car
headlight in problem 2?
Another Formula
Power = VI
•
•
•
•
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
•
•
•
•
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
Voltage source
resistor
Kirchoff’s Laws
1. Current into any node equals current out
of the node
I1
I1 = I2 + I3
I2
I3
Kirchoff’s Laws
2. Potential drop around any closed loop is
zero
V1
e – V1 – V2 = 0
V1 + V2 = e
e
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