group work 1. light the bulb!. electric current effects of moving charges
TRANSCRIPT
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