Download Lecture Notes Chapter 19

Current
Current: the rate of motion per unit of
time.
Electric Current: the rate of flow of a
charge through a cross-sectional
area of a conductor.
a. SI unit = ampere (A) referred
to amp.
*1A = 1C/s
b. Equation symbol is ‘I’
Factors Affecting Current
NOTE: The more charge that moves the
greater the current.
1. Cross sectional surface area.
Increase the area increases the
current.
2. Type of conductor. Al v. Cu v. Ag
3. Temperature of conductor Increase
temperature decreases current.
Factors Affecting Current
4. Distance to move. The greater the
distance the less the flow.
5. Amount of electric force. Potential
difference. Increase potential
difference means to have greater
force therefore greater flow. High to
Low
**NOTE: charge is defined in terms of
(+) charge movement.
Factors Affecting Current
NOTE: Increase in drift causes an
increase in collision and increase in
temperature resulting in a decrease in
current.
Questions
1. How does the current change if the
number of charge carries
increases.
ans: current increases
2. How does the current change if the
time interval during which a given
number of charge carries pass the
cross sectional area increases
ans: current decreases
Equation
I = ΔQ/Δt
I = charge passing = C/s = Ampere
change in time
Potential Difference
The potential difference in a circuit is the amount
of work done to move a charge in an electric
field.
The standard metric unit on electric potential
difference is the volt, V after Alessandro Volta.
One Volt is equivalent to one Joule per Coulomb.
Power
Power is the rate of flow.
P = IV
I = electric current measured in c/s = A
V = potential difference,V = E/q
I = q/t and V = E/q
P = (q/t) (E/q) = E/t = rate flow of an
electric current, POWER
SI unit for power is Watt, W = J/c
Power Sampler
An electric vac is connected to a 120 V
outlet is rated at 3 A. What is the power
requirements of the motor?
P = IV = (3 c/s)( 120 J/c) = 360 j/c = 360 W
You have two cordless power drills both
connected to a 12V battery. One is rated
at 6 A and the other at 18 A. Which has a
greater requirement on the motor?
P = IV = (12V)(18A) = 216 W (72W)
HW: page 594 1-5.
Resistance
Impedes the rate of flow (current)
2. Resistance in mostly constant
3. Ohm’s Law: at a constant resistance, the
potential difference (voltage) is direct to
the current. Increase the voltage (the
force) increases the current.
Ohm’s Law: W = Dv / I
Note Table 22-1 page 595
1.
Effects on Resistance
Increase length; Decrease current; Increase resistance.
Increase cross section; Increases current; Decrease
resistance.
More  less conductive; Decreases current; Increase
resistance.
Increase temperature; Increases current; Decreases
resistance
Increase collision; Decrease current; Increase resistance
Problem
What is the rate of flow of an electric
current moving in a conductor that has
a resistance of 3.5 Ω connected to a
1.5 V battery?
V = 1.5 v
R = 3.5 W
I=?
W = v/I ; I = v/W = 1.5 v/ 3.5 W =
ans: 0.43 A
TLS: Problem Set Ohm’s Law
Sources for Electric Current
1. The #1 source of all energy in the world
is the:
battery
a. wet cell (car battery)
b. dry cell (flashlight)
Types of Electric Current: AC v. DC
1.
Direct current (dc): the charge moves in one
direction only.
a. current flows from high to low in
the battery
b. flow (+)  (-) in a battery b/c of the
potentail difference at the
terminal.
c. current flow (–) to (+) in the circuit
**(+) end is high potential and the (-) is low
potential so the flow is high to low.
++water does not run up hill++
2. Alternative current (ac): the source of the
potential is changing sign. There is NO net
motion of a charge. The charge vibrates at a
point
a. household current = 60 Hz means every
one second the current changes direction 60
times
Power
Mechanical power is the work done in a period of
time:
Electric power is the rate of flow determined by
the force pushing. SI unit is watt (W)
P = IΔV
Power = current x potential difference
If current, I = V/R then P = ΔV ΔV = ΔV2
R
R
Thermal Energy
Conversion of power to energy with respect
to time.
E = Pt
Energy with respect to flow: Sub P= I2R
E = I2Rt
Energy with respect to voltage: Sub P = V2/R
E = (V2/R)t
NOTE: 5000J / 1oC change
Power Sampler
The PowerMate 3200 is a hair dryer that
when connect across a 120 V outlet in
your home pulls 3200 W of power. What
is the resistance of this dryer?
V = 120 V
P = 3200 W
R=?
P = V2/R then R = V2 / P
R = 120 V2 / 3200 W
R = 4.5 Ω
Power in Time
Power measures the rate in which energy is
used in J/s or Watts.
At a power of 10 J/s, how much energy is
used in 10 s?
10 J/s x 10 s = 100 J
If it cost 10¢ per joule, how much did it cost?
100 J @ 0.1 = $10
OR
Energy Cost = PΔt express in kW/h
TLS: Problems 23-31 textbook, pp 603-605
Electric Circuit
A pathway of flow of electric current
through a conductor.
Defined by Ohm’s Law
R = V/I
Resistance is inverse to current
Resistance is direct to force (voltage)
Two type of general circuits based on the
arrangement of resistors.
Series and Parallel
Circuit Diagram
Shows or represents the pathway with
symbols.
Flow is from the (+) terminal to the (-)
terminal.
NOTE: figure 22-6 page 597.
Series Circuit
When the resistors are arranged in series.
***Total current = individual current across the
resistor***
IT = I1 = I2 = I3 …….
***Total potential difference (V) = SUM of each
volt across each resistor***
VT = V1 + V2 + V3 ……
***Total resistance = SUM of each resistance
across each resistor***
RT = R1 + R2 + R3 ……
Series Circuit Sampler
You are given a 3 cell battery at 2 V per cell
connected to an ammeter and a series of 3
resistor. R1 = 6 Ω , R2 = 3Ω and R3 = 3Ω
1. Draw and label the circuit diagram.
2. Since each cell is 2V, what is the total voltage
of this battery?
3. What is the voltage drop across each resistor?
4. What is the total current ?
5. What is the total voltage?
Series Circuit Sampler
You have a 12V battery connected to 3 resistors
set in series. R1 = 10 Ω , R2 = 25Ω, and R3 = 5 Ω.
1. Draw a circuit diagram.
2. What is the total resistance through the
circuit?
3. What is the voltage of each cell in a 4 cell
battery?
4. What is the total current through this circuit?
5. What is the voltage drop across each resistor?
Parallel circuits
When the resistors are arranged side by side .
***Total current = SUM of the individual
current***
IT = I1 + I2 + I3 ……
***Total potential difference is the same over
each resistor***
VT = V1 = V2 = V3 ……..
***Total resistance = SUM of the recepical of
each resistor
1/RT = 1/R1 + 1/R2 + 1/R3 ……
Parallel Sampler
A parallel series of three resistors is connected to
a 6 V battery. R1 = 3Ω, R2 = 2Ω, and R3 = 10Ω.
The circuit contains an ammeter and a
voltmeter.
1. Draw the circuit diagram.
2. What is the voltage drop over the 2nd resistor?
3. What is the total resistance?
4. What is the current over the 3rd resistor?
5. What is the total current?
6. If there are 4 cells in the battery set in series,
what is the voltage of each cell?