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.

CurrentCurrentCurrent: 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/sb. Equation symbol is ‘I’

Factors Affecting CurrentFactors 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. Ag3. Temperature of conductor

Increase temperature decreases current.

Factors Affecting CurrentFactors 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 CurrentFactors Affecting Current

NOTE: Increase in drift causes an increase in collision and increase in temperature resulting in a decrease in current.

QuestionsQuestions

1. How does the current change if the number of charge carries increases.

ans: current increases2. 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

EquationEquation

I = ΔQ/Δt

I = charge passing = C/s = Ampere

change in time

Potential DifferencePotential DifferenceThe 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.

PowerPower

Power is the rate of flow.P = IV

I = electric current measured in c/s = A

V = potential difference, V = E/qI = 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 SamplerPower SamplerAn 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.

ResistanceResistance

1. 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: = v / I

Note Table 22-1 page 595

Effects on ResistanceEffects on ResistanceIncrease length; Decrease current; Increase

resistance.Increase cross section; Increases current;

Decrease resistance.More less conductive; Decreases current;

Increase resistance.Increase temperature; Increases current;

Decreases resistanceIncrease collision; Decrease current; Increase

resistance

ProblemProblem

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 I = ?

= v/I ; I = v/ = 1.5 v/ 3.5 = ans: 0.43 A

TLS: Problem Set Ohm’s Law

Sources for Electric Sources for Electric CurrentCurrent1. 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 Types of Electric Current: AC v. DCv. 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 pointa. household current = 60 Hz means every one second the current changes direction 60 times

PowerPowerMechanical 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 differenceIf current, I = V/R then P = ΔV ΔV = ΔV2

R R

Thermal EnergyThermal Energy

Conversion of power to energy with respect to time.

E = PtEnergy with respect to flow: Sub P= I2R

E = I2RtEnergy with respect to voltage: Sub P =

V2/RE = (V2/R)t

NOTE: 5000J / 1oC change

Power SamplerPower 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 / PR = 120 V2 / 3200 WR = 4.5 Ω

Power in TimePower in TimePower 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/hTLS: Problems 23-31 textbook, pp 603-605

Electric CircuitElectric Circuit

A pathway of flow of electric current through a conductor.

Defined by Ohm’s LawR = 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 DiagramCircuit Diagram

Shows or represents the pathway with symbols. Flow is from the (+) terminal to the (-) terminal.NOTE: figure 22-6 page 597.

Series CircuitSeries 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 SamplerSeries Circuit SamplerYou 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 SamplerSeries Circuit SamplerYou 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 circuitsParallel 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 SamplerParallel 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?