Current Current ElectricityElectricity

ElectricityElectricity

Electricity is the flow of electrons Electricity is the flow of electrons through a conducting material.through a conducting material.

The flow of electricity is called The flow of electricity is called currentcurrent..

Current is measured by Current is measured by counting counting the number of electrons that flow the number of electrons that flow past a point in a given time.past a point in a given time.

CurrentCurrent

An electric current is moving charges An electric current is moving charges (electrons)(electrons)

The The brightnessbrightness of a bulb depends on of a bulb depends on the current through the bulb (qualitative the current through the bulb (qualitative measure)measure)

As the electrons move through the As the electrons move through the conductor, they collide with the fixed conductor, they collide with the fixed particles of the conductor and the kinetic particles of the conductor and the kinetic energy is transferred into heat and light energy is transferred into heat and light in the resistancein the resistance

CurrentCurrent

More current means more electrons More current means more electrons collide and therefore, more energy is collide and therefore, more energy is dissipated in the resistance dissipated in the resistance

Resistance should be considered an Resistance should be considered an obstacle to flow. Less resistance obstacle to flow. Less resistance implies more flow and vice versaimplies more flow and vice versa

Current is conserved. Current is conserved. What goes What goes in one end must come out the other in one end must come out the other endend

Electrical Charge and Electrical Charge and CurrentCurrent

Electrical charge is measured in Electrical charge is measured in Coulombs (C)Coulombs (C)

1 coulomb is equal to the amount of 1 coulomb is equal to the amount of charge in 6.25 x 10charge in 6.25 x 101818 electrons electrons

1 coulomb of charge moving past a 1 coulomb of charge moving past a point in 1 second is equal to point in 1 second is equal to 1 Ampere1 Ampere of currentof current

1 Ampere = 1 coulomb/1 second1 Ampere = 1 coulomb/1 second

Calculating CurrentCalculating Current

To calculate current we use the To calculate current we use the formula;formula;

I = I = Q Q or Q = I x t or Q = I x t tt

I = current and is measured in Amperes I = current and is measured in Amperes (A)(A)

Q = charge and is measured in Q = charge and is measured in coulombs (C)coulombs (C)

t = time and is measured in seconds (s)t = time and is measured in seconds (s)

ExamplesExamples

If a bulb has 0.25A, how many electrons If a bulb has 0.25A, how many electrons pass through the bulb in one second?pass through the bulb in one second?

I = Q/tI = Q/t and and Q = ItQ = It thereforetherefore

Q = (0.25A)(1s) or 0.25C, but 1C = 6.25 x Q = (0.25A)(1s) or 0.25C, but 1C = 6.25 x 10^1810^18

So… (0.25A)(6.25 x 10^18) = 1.56 x 10^18 So… (0.25A)(6.25 x 10^18) = 1.56 x 10^18 ee

That is a lot of electrons!That is a lot of electrons!

VoltageVoltage

Voltage (electric potential Voltage (electric potential difference) is measured in Volts (V).difference) is measured in Volts (V). Voltage is the energy per unit charge Voltage is the energy per unit charge

between two points along a conductorbetween two points along a conductor

V = E/QV = E/Q

Five Sources of Five Sources of ElectricityElectricity

1. Chemical – chemical sources 1. Chemical – chemical sources accumulate charge through chemical accumulate charge through chemical processes.processes.

Ex. batteriesEx. batteries

2. Photoelectric (light) – 2. Photoelectric (light) – photoelectric materials emit photoelectric materials emit electrons when they are struck by electrons when they are struck by light.light.

Ex. Solar powered calculatorsEx. Solar powered calculators

3. Electromagnetic – a wire moving 3. Electromagnetic – a wire moving in a magnetic field generates a in a magnetic field generates a current.current.

Ex. generatorEx. generator

4. Thermoelectric (heat) – the 4. Thermoelectric (heat) – the temperature difference between temperature difference between different materials can create a different materials can create a current.current.

Ex. A thermocouple in a furnaceEx. A thermocouple in a furnace

5. Piezoelectric (crystals) – some 5. Piezoelectric (crystals) – some crystals will create a current when crystals will create a current when put under mechanical stress (force).put under mechanical stress (force).

Ex crystals found in quartz watches.Ex crystals found in quartz watches.

The Electric CellThe Electric Cell

The electric cell is a device that has The electric cell is a device that has the ability to produce an electric the ability to produce an electric charge for a longer period of time charge for a longer period of time than an electrostatically charged than an electrostatically charged object.object.

The HistoryThe History

1794 – Luigi 1794 – Luigi Galvani researched Galvani researched the link between the link between muscle muscle contractions and contractions and electric electric ““sparkssparks””

1800 – Alessandro 1800 – Alessandro VoltaVolta

Discovered that if two Discovered that if two different metals are different metals are placed in a salt or placed in a salt or acid solution a acid solution a charge was created.charge was created.

The charge produced The charge produced seemed like it was seemed like it was continuous. This type continuous. This type of cell is called a of cell is called a Volta Cell or Wet CellVolta Cell or Wet Cell

The Wet CellThe Wet Cell

Two different metals called Two different metals called electrodeselectrodes are placed in a salt or are placed in a salt or acid solution called an acid solution called an electrolyte.electrolyte.

Able to give off sparks without being Able to give off sparks without being rechargedrecharged

Produces an electric current – a Produces an electric current – a continuous supply or continuous supply or ““flowflow”” of of electrons.electrons.

ExampleExample

A zinc electrode A zinc electrode and a copper and a copper electrode are electrode are placed in an placed in an electrolyte solution electrolyte solution of NaCl. This of NaCl. This solution contains solution contains (Na+) sodium ions (Na+) sodium ions and (Cl-) chloride and (Cl-) chloride ions.ions.

How this worksHow this works

The copper electrode gives electrons to the The copper electrode gives electrons to the positive sodium ions – because it has given positive sodium ions – because it has given away electrons it becomes positively away electrons it becomes positively charges.charges.

The zinc electrode attracts the negatively The zinc electrode attracts the negatively charged chloride ions and becomes charged chloride ions and becomes negatively charged.negatively charged.

The conducting loop (usually a metal) The conducting loop (usually a metal) provides a pathway for the negative charges provides a pathway for the negative charges (electrons) to flow from the negative Zn (electrons) to flow from the negative Zn electrode towards the positive Cu electrode.electrode towards the positive Cu electrode.

ExampleExample

The Dry CellThe Dry Cell

Works the same way as the wet cell Works the same way as the wet cell but the electrolyte solution used is in but the electrolyte solution used is in a paste not a liquid form.a paste not a liquid form.

Current Flow in Metal Current Flow in Metal ConductorsConductors

Electric current is the movement of Electric current is the movement of electrons through a conductor at a certain electrons through a conductor at a certain rate when something is rate when something is ““pushingpushing”” them. them.

When an electric cell is attached to a When an electric cell is attached to a conductor it moves electrons into the conductor it moves electrons into the conductorconductor

The electric cell pushes electrons from The electric cell pushes electrons from the –ve elecrode (by repulsion) into the the –ve elecrode (by repulsion) into the conductor which pushes the rest of the conductor which pushes the rest of the electrons towards the +ve electrode (by electrons towards the +ve electrode (by attraction)attraction)

CircuitsCircuits

The conducting loop in a cell is often The conducting loop in a cell is often referred to as a circuit.referred to as a circuit.

The circuit (conducting loop) can be The circuit (conducting loop) can be opened or closed using a opened or closed using a switch.switch.

If the switch is OPEN the conducting If the switch is OPEN the conducting loop is loop is ““brokenbroken”” so electrons cannot flow so electrons cannot flow

If the switch is CLOSED the conducting If the switch is CLOSED the conducting loop is complete so electrons can flowloop is complete so electrons can flow

In an electric circuit, electrons are In an electric circuit, electrons are present in ALL PARTS of the circuit present in ALL PARTS of the circuit all the timeall the time

In order to move electrons in a wire, In order to move electrons in a wire, we need a potential difference we need a potential difference between the ends of the wire, which between the ends of the wire, which is created through an accumulation of is created through an accumulation of charge at the ends of the wirecharge at the ends of the wire

As one electron is pushed into one As one electron is pushed into one end of the wire, ALL electrons in the end of the wire, ALL electrons in the wire move simultaneously and one wire move simultaneously and one electron moves off the other endelectron moves off the other end

A circuit can be defined as a pathway A circuit can be defined as a pathway used to direct an electric current in used to direct an electric current in order to perform a order to perform a ““tasktask””..

Every circuit must have 3 componentsEvery circuit must have 3 components A Source of energy – electric cell or A Source of energy – electric cell or

batterybattery A Conducting loop – usually a metal wireA Conducting loop – usually a metal wire A Load – something that uses potential A Load – something that uses potential

energy to do work such as a light bulb, energy to do work such as a light bulb, resistor or motor.resistor or motor.

Measuring Current and Measuring Current and VoltsVolts

In order to measure current, an In order to measure current, an ammeter must be placed in series such ammeter must be placed in series such that the current flows through the that the current flows through the meter. meter. If placed incorrectly (placed in parallel If placed incorrectly (placed in parallel

means it behaves as a low resistance wire – means it behaves as a low resistance wire – shorts the circuit)shorts the circuit)

Voltmeters are placed in a circuit in Voltmeters are placed in a circuit in parallel and have a very high resistanceparallel and have a very high resistance If placed in series, the large resistance If placed in series, the large resistance

decreases the current (effectively to zero) decreases the current (effectively to zero) and the circuit will not workand the circuit will not work

V

A

ResistanceResistance

Resistance is the opposition to the flow Resistance is the opposition to the flow of electrons when a current passes of electrons when a current passes through the circuit.through the circuit. Electron movement is slowed downElectron movement is slowed down Resistance converts electrical energy into Resistance converts electrical energy into

other forms of energy such as light or heatother forms of energy such as light or heat Devices called resistors are designed Devices called resistors are designed

specifically to provide resistance in a circuit.specifically to provide resistance in a circuit. The unit used to measure resistance is the The unit used to measure resistance is the

Ohm (Ohm (ΩΩ))

BatteryBattery

A combination of two or more dry A combination of two or more dry cells that act as an energy source for cells that act as an energy source for a circuita circuit

2 types of batteries may be used in a 2 types of batteries may be used in a circuitcircuit1.1. Cells in seriesCells in series

2.2. Cells in parallelCells in parallel

Cells in SeriesCells in Series Two or more dry cells connected end to Two or more dry cells connected end to

endend Connecting dry cells in series Connecting dry cells in series

INCREASES the voltage but DOES NOT INCREASES the voltage but DOES NOT CHANGE the current produced by the CHANGE the current produced by the batterybattery

Ex. If two 1.5V cells are connected in Ex. If two 1.5V cells are connected in series we would have a 3.0V batteryseries we would have a 3.0V battery

Cells in Parallel (last longer)Cells in Parallel (last longer) 2 or more dry cells connected side by side so 2 or more dry cells connected side by side so

that their + electrodes are joined and their – that their + electrodes are joined and their – electrodes are joinedelectrodes are joined

Connecting dry cells in parallel DOES NOT Connecting dry cells in parallel DOES NOT CHANGE the voltage produced by the battery CHANGE the voltage produced by the battery but it INCREASES the current produced by the but it INCREASES the current produced by the batterybattery

Each cell only has to contribute a portion of the Each cell only has to contribute a portion of the current available to the circuit. This current available to the circuit. This ““sharedshared”” load allows the current to be delivered for a load allows the current to be delivered for a longer period of time.longer period of time.

Series CircuitsSeries Circuits Circuits constructed in a way that all Circuits constructed in a way that all

of the current must pass through of the current must pass through each of the loads in the circuit each of the loads in the circuit

Resistors in SeriesResistors in Series

Two resistors placed in series have a Two resistors placed in series have a greater resistance than a single greater resistance than a single resistorresistor

The total resistance of resistors in The total resistance of resistors in series is the sum of the resistors:series is the sum of the resistors: RRtotaltotal = R = R11 + R + R22 + R+ R3 3 ……

Parallel CircuitsParallel Circuits

Circuits constructed in a way in Circuits constructed in a way in which the loads are parallel to each which the loads are parallel to each other creating BRANCHES in the other creating BRANCHES in the circuitcircuit Current has a choice of branches to Current has a choice of branches to

travel through. travel through. The branch with the lower resistance The branch with the lower resistance

will have the higher current passing will have the higher current passing through itthrough it

The current before the junction and The current before the junction and after the junction have the same after the junction have the same valuevalue

The sum of the current running The sum of the current running through each branch is equal to the through each branch is equal to the total current running through the total current running through the entire circuitentire circuit

Resistors in ParallelResistors in Parallel

When resistors are connected in When resistors are connected in parallel the total resistance is less parallel the total resistance is less than that of a single resistor.than that of a single resistor.

The electrons have an extra path to The electrons have an extra path to follow (like opening another door)follow (like opening another door)

The equivalent resistance of resistors The equivalent resistance of resistors in parallel is given by the formula:in parallel is given by the formula:

1/R1/Reqeq = 1/R = 1/R11 + 1/R + 1/R22 + 1/R + 1/R33 … …

Power, Energy and TimePower, Energy and Time

In physics the word Power is defined as In physics the word Power is defined as the energy per unit timethe energy per unit time

Electrical power describes the amount of Electrical power describes the amount of electrical energy that is converted into electrical energy that is converted into light, heat, sound or motion every second.light, heat, sound or motion every second.

The symbol for power is P. The equation The symbol for power is P. The equation that defines power in mathematical form that defines power in mathematical form is is

P =E/tP =E/t

Units of PowerUnits of Power Since the unit for energy is joule ( J ) Since the unit for energy is joule ( J )

and the unit for time is the second ( s ), and the unit for time is the second ( s ), power can be expressed in joules/secondpower can be expressed in joules/second

Joules/second has been named Joules/second has been named Watt Watt ( W )( W ) in honour of James Watt. in honour of James Watt.

When one joule of electrical energy is When one joule of electrical energy is converted into heat and light by a light converted into heat and light by a light bulb every second, the power of the bulb bulb every second, the power of the bulb is said to be one watt.is said to be one watt. Note that the energy delivered to a circuit Note that the energy delivered to a circuit

must be used in the circuit, or the wires will must be used in the circuit, or the wires will burnburn

Nothing is perfectNothing is perfect

If an electrical device were perfect, all the If an electrical device were perfect, all the electrical energy that it uses would be electrical energy that it uses would be converted into the desired form of energy.converted into the desired form of energy.

However some energy is always converted into However some energy is always converted into heat.heat.

You can determine the efficiency of an You can determine the efficiency of an electrical device by using the following electrical device by using the following relationship:relationship:

Percent efficiency = Percent efficiency = Useful energy output Useful energy output X X 100%100%

Total energy inputTotal energy input