Electric Current and Electric Current and Direct-Current CircuitsDirect-Current Circuits

Pre AP Pre AP

Mrs. MartinMrs. Martin

The Electric BatteryThe Electric Battery

Converts chemical energy into Converts chemical energy into electrical energyelectrical energy

Made of two dissimilar metalsMade of two dissimilar metals One metal becomes positively One metal becomes positively

charged and the other becomes charged and the other becomes negatively chargednegatively charged

The Electric BatteryThe Electric Battery

Electric CurrentElectric Current

Flow of electric charge from one Flow of electric charge from one place to anotherplace to another

FormulaFormula I = Q/tI = Q/t I = Current (Ampere, A)I = Current (Ampere, A) Q = Charge (C)Q = Charge (C) t = time (s)t = time (s)

Example ProblemExample Problem

The disk drive in a portable CD The disk drive in a portable CD player is connected to a battery player is connected to a battery that supplies it with a current of that supplies it with a current of 0.22 A. How many electrons 0.22 A. How many electrons pass through the drive in 4.5s?pass through the drive in 4.5s?

Direction of Current FlowDirection of Current Flow

When speaking of current we When speaking of current we are referring to the direction of are referring to the direction of positive flow.positive flow.

Sometimes called conventional Sometimes called conventional currentcurrent

Two types of CurrentTwo types of Current

Direct Current (DC)Direct Current (DC) Current always flows in one Current always flows in one

directiondirection Alternating Current (AC)Alternating Current (AC)

Current is periodically reversedCurrent is periodically reversed

Batteries and Electromotive Batteries and Electromotive ForceForce Electromotive Force Electromotive Force

The potential between terminals of The potential between terminals of batteriesbatteries

Called EMFCalled EMF Battery has a little internal Battery has a little internal

resistanceresistance Also called terminal voltageAlso called terminal voltage

Schematic DiagramsSchematic Diagrams

Diagram of a circuitDiagram of a circuit

Electromotive ForceElectromotive Force

Difference in electric potential Difference in electric potential between the terminalsbetween the terminals

Called emfCalled emf

Electron flow begins instantly, Electron flow begins instantly, but is very slowbut is very slow

Resistance and Ohm’s LawResistance and Ohm’s Law

ResistanceResistance Opposition to the flow of electronsOpposition to the flow of electrons

Like Friction for electricityLike Friction for electricity

Ohm’s LawOhm’s Law V = IRV = IR

V = Potential Difference (V)V = Potential Difference (V) I = Current (A)I = Current (A) R = Resistance (R = Resistance (ΩΩ))

ExampleExample

A potential difference of 24 V is A potential difference of 24 V is applied to a 150 applied to a 150 ΩΩ resistor. resistor. How much current flows through How much current flows through the resistor?the resistor?

ResistivityResistivity

The quality of that characterizes The quality of that characterizes the resistance of a given the resistance of a given material.material.

Represented as Represented as ρρ The greater the resistivity, the The greater the resistivity, the

greater the resistancegreater the resistance

ResistanceResistance

FormulaFormula R = R = ρρ (L/A) (L/A)

R = Resistance (R = Resistance (ΩΩ)) ρρ = Resistivity ( = Resistivity (ΩΩ • m) • m) L = Length of wire (m)L = Length of wire (m) A = Cross-sectional area of wire (mA = Cross-sectional area of wire (m22))

Example:Example: A current of 1.82 A flows through a A current of 1.82 A flows through a

copper wire 1.75 m long and 1.10 mm in copper wire 1.75 m long and 1.10 mm in diameter. Find the potential difference diameter. Find the potential difference between the ends of the wire. The between the ends of the wire. The resistivity of copper is 1.68 x 10resistivity of copper is 1.68 x 10-8-8 Ω•Ω•m m

Temperature Dependence Temperature Dependence and Superconductivityand Superconductivity

As electrons move, the As electrons move, the conductor become hotconductor become hot

The hotter the conductor, the The hotter the conductor, the more resistivity due to increased more resistivity due to increased Kinetic EnergyKinetic Energy

SuperconductorSuperconductor Conducts with little or no resistivityConducts with little or no resistivity Must be at very low temperaturesMust be at very low temperatures

Electric PowerElectric Power

Rate of change of energyRate of change of energy P = W/tP = W/t

FormulaFormula P = IVP = IV

P = Power (W)P = Power (W) I = Current (A)I = Current (A) V = Potential Difference (V)V = Potential Difference (V)

ExampleExample A handheld electric fan operates on a A handheld electric fan operates on a

3.00 V battery. If the power generated 3.00 V battery. If the power generated by the fan is 2.24 W, what is the current by the fan is 2.24 W, what is the current supplied by the battery?supplied by the battery?

Power dissipated in a resistorPower dissipated in a resistor

P = VP = V22/R/R P = Power (W)P = Power (W) V = Potential Difference (V)V = Potential Difference (V) R = Resistance (R = Resistance (ΩΩ))

ExampleExample A battery with an emf of 12 V is A battery with an emf of 12 V is

connected to a 545 connected to a 545 ΩΩ resistor. resistor. How much energy is dissipated in How much energy is dissipated in the resistor in 65s?the resistor in 65s?

Energy UsageEnergy Usage

Kilowatt hours are used by electric Kilowatt hours are used by electric companies to bill for energy usage.companies to bill for energy usage.

1kWh = 3.6 x 101kWh = 3.6 x 1066 J J ExampleExample

A holiday goose is cooked in the kitchen A holiday goose is cooked in the kitchen oven for 4.00 hr. Assume that the stove oven for 4.00 hr. Assume that the stove draws a current of 20.0 A, operates at a draws a current of 20.0 A, operates at a voltage of 220.0 V, and uses electrical voltage of 220.0 V, and uses electrical energy that costs $0.048 per kWh. How energy that costs $0.048 per kWh. How much does it cost to cook your goose?much does it cost to cook your goose?

Household CircuitsHousehold Circuits

A household circuit can become A household circuit can become overloaded if too much current overloaded if too much current flows through the circuit than is flows through the circuit than is considered safeconsidered safe

Circuit breakers and fuses are Circuit breakers and fuses are installedinstalled

They act as switches and break They act as switches and break the current when the current the current when the current becomes too large.becomes too large.

Resistors in SeriesResistors in Series

Equivalent ResistanceEquivalent Resistance Total Resistance for a circuitTotal Resistance for a circuit

Series CircuitSeries Circuit Resistors connected one after anotherResistors connected one after another All resistors have the same currentAll resistors have the same current Potential Difference across the resistors Potential Difference across the resistors

must sum to the emf of the batterymust sum to the emf of the battery RReqeq = R = R11 + R + R22 + R + R33 … …

Example ProblemExample Problem

A circuit consists of three A circuit consists of three resistors connected in series to resistors connected in series to a 24.0 V battery. The current in a 24.0 V battery. The current in the circuit is 0.0320 A. Given the circuit is 0.0320 A. Given that R1 = 250.0 that R1 = 250.0 ΩΩ and R2 = and R2 = 150.0 150.0 ΩΩ, find (a) the value of R3 , find (a) the value of R3 and (b) the potential difference and (b) the potential difference across each resistor.across each resistor.

Resistors in ParallelResistors in Parallel

Parallel CircuitParallel Circuit Connected across the same Connected across the same

potential difference.potential difference. The total current is the sum of all The total current is the sum of all

the individual currentsthe individual currents Potential difference is the same Potential difference is the same

across each resistoracross each resistor 1/R1/Reqeq = 1/R = 1/R11 + 1/R + 1/R22 + 1/R + 1/R33 … …

ExampleExample

Consider a circuit with three Consider a circuit with three resistors , R1 = 250.0 resistors , R1 = 250.0 ΩΩ, R2 = , R2 = 150.0 150.0 ΩΩ, and R3 = 350.0 , and R3 = 350.0 ΩΩ, , connected in parallel with a 24.0 connected in parallel with a 24.0 V battery. Find (a) the total V battery. Find (a) the total current supplied by the battery current supplied by the battery and (b) the current through each and (b) the current through each resistor.resistor.

Combination CircuitsCombination Circuits

In the circuit shown in the In the circuit shown in the diagram, the emf of the battery diagram, the emf of the battery is 12.0 V, and all the resistors is 12.0 V, and all the resistors have a resistance of 200 have a resistance of 200 ΩΩ. . Find the current supplied by the Find the current supplied by the battery to this circuit.battery to this circuit.

Kirchhoff’s RulesKirchhoff’s Rules

The Junction RuleThe Junction Rule Charge conservationCharge conservation The current entering any point in a The current entering any point in a

circuit must equal the current circuit must equal the current leaving that pointleaving that point

The algebraic sum of the currents The algebraic sum of the currents should equal zeroshould equal zero

+ current going into the point, - + current going into the point, - current going out of the pointcurrent going out of the point

The Loop RuleThe Loop Rule

Energy ConservationEnergy Conservation The algebraic sum of all The algebraic sum of all

potential differences around a potential differences around a closed loop in a circuit is zero.closed loop in a circuit is zero.

Example ProblemExample Problem

Capacitors in ParallelCapacitors in Parallel

Equivalent Capacitance is the sum of Equivalent Capacitance is the sum of all the capacitors.all the capacitors.

ΣΣC = C1 + C2 + C3 …C = C1 + C2 + C3 … ExampleExample

Two capacitors, one 12.0Two capacitors, one 12.0μμF and the F and the other of unknown capacitance are other of unknown capacitance are connected in parallel across a battery connected in parallel across a battery with an emf of 9.00 V. The total energy with an emf of 9.00 V. The total energy stored in the two capacitors is 0.0115 J. stored in the two capacitors is 0.0115 J. What is the value of the capacitance C?What is the value of the capacitance C?

Capacitors in SeriesCapacitors in Series

ΣΣ1/C1/Ceqeq = 1/C = 1/C11 + 1/C + 1/C22 + 1/C + 1/C3 …3 …

ExampleExample Consider the electrical circuit Consider the electrical circuit

drawn, consisting of a 12 V battery drawn, consisting of a 12 V battery and three capacitors connected and three capacitors connected partly in series and partly in partly in series and partly in parallel. Find (a) the equivalent parallel. Find (a) the equivalent capacitance of this circuit and (b) capacitance of this circuit and (b) the total energy stored in each the total energy stored in each capacitor.capacitor.

AmmeterAmmeter

Designed to measure the Designed to measure the current in a particular part of a current in a particular part of a circuit.circuit.

Must be hooked up in series Must be hooked up in series

VoltmeterVoltmeter

Measures the potential Measures the potential difference across two pointsdifference across two points

Must be in parallel to the circuit.Must be in parallel to the circuit.