Lesson 6: Electricity

Technology 3º ESO

Index

What is electricity Electrical magnitudes Ohms law Measuring with a multimeter Electric circuits: Elements Electric symbols Types of circuits Electric machines

What is electricity? Electricity is a form of energy It has its origin in the electrical

charges existing in the atoms (electrons and protons), which strongly interact

Electricity can be natural, such as ligthning or artificial (electric circuits)

As a form of energy, it is easy to produce and transform.

Index

ELECTRIC MAGNITUDES

Magnitude Definition Units Expression

Voltage, (tension)

Energy given to the charges to move

them

V (volts) V=I·R

Resistance Difficulty to be passed by electricity

(ohm) R= ρ·l/s2

Intensity Charge passing through a conductor

in a second

A (amp) I=Q / t(s)

Power Energy produced (or lost) by the current in

a time unit

W (watt) P=V·I

PL=R·I2

Index

Ohm’s law

Ohm’s law says that the tension is equal to the product of the intensity and the resistance

V = R · I This is equivalent to:

I=V/R ↔R=V/I

Index

Electric circuits and their elements An electric circuit is a closed

path where the charges flow between to points with different electric tension

We use electric circuits to transport and transform energy

An electric circuits needs at least

1. A power supply2. A conductor3. A receiver It is also important to include

control elements

Index

Power suppliers

A power supply gives energy to the charges so that they flow trough the circuit

There are different types of suppliers

1. Chemical: batteries DC

2. Mechanical: dynamos DC and alternators AC

Conductors

A conductor is an element where the charges flow without a significant resistance

They are normally wires: several long and thin copper (or aluminum) conductors covered by an insulating plastic layer

Receivers Receivers transform electricity into a useful form of energy so that we

make the circuit profitable. They can convert electricity into:1. Ligth: lamps2. Movement: motors3. Heat: resistances4. Sound: speakers5. etc

Control elements

Control elements let us stop or allow the flow of charges (switch) or change the way of the electricity through the circuit (conmuter)

Index

Electric symbols

An electric symbol is a picture which represents a circuit or an element of it

They make it easier to represent and understand circuits

Index

Electric machines

Many essential devices work or help us to control electricity

Transformer Motor Dynamos or alternators Relay

Index

TransformersTransformers change (raise or lower the)

tension of the alternating currentThey are formed by a square core with two

coils (copper wires), primary (N1 turns , V1 and I1) and the secondary.

The alternating current induces a magnetic flux wich travels trough the iron core and induces a current I2 with a voltage V2 in the secondary coil of N2 turns. This equations are satisfied

P1=P2 ↔ V1·I1=V2·I2

V1/N1=V2/N2 ↔ V1·N2=V2·N1

I1·N1=I2·N2

Motors

Motors can be AC or DC. They are both based on magnetic fields. A current flows trough a coil in the rotor / stator. The stator / rotor is a magnet. Due to the variation of the magnetic field flow caused by the electric current

Dynamos / alternators

Dynamos (DC) or alternators (AC) are like motors which generate DC or AC current when the rotor is moved inside the stator. The variation of the magnetic field due to the rotation of a magnet induces an electric current in the stator.

Relays A Relay is a swith controlled by an

electromagnet. An electromagnet is a coiled

conductor wire where the current flows creating a magnetic field.

The magnetic field makes the swith change its position due to magnetic atraction

When current stops, the magnetic field dissapears and the switch returns to its original position due to a spring.

Relays have two independent circuits:

1. the control circuit: Activates the electromagnet

2. The power circuit, which is controlled by the switch

Index