Electricity by 2E1 Chen Meng

What is electricity?
It is a controllable and convenient form of energy,
evident from the fact that it runs machinery and can
be transformed into other types of energy such as
light and heat.

It is invisible. During an electrical
storm, we do not see electricity.
We observe the air being ionized when
the electricity travels through it.

Sources of electricity
Electrical energy comes from two main sources. There are mains electricity and electric cells.

Main electricity is generated in power stations. The plugs of electrical appliances are inserted into mains sockets to obtain this electricity. Mains electricity supplies a lot of energy. It can give us an electric shock which can kill us. Never touch bare wires connected to mains electricity.
Electric cells give out only a little energy. They are used in many portable electrical devices and are safe to use.

Types of electricity
Static electricity is electricity that says still on objects.
Current electricity is electricity that flows in an electric circuit.

Electric circuits
To make an electrical appliance work, electricity must flow through it. The flow of electricity is called an electric current.
The path along which the electric current moves is called the electric circuit.

In the laboratory, experiments with electricity are often done on a circuit board.
On the circuit board, we join up electrical components to make a circuit.

Closed circuit & open circuits
For the bulb to light up, there has to be a complete path without any gap from one end of the cell to the light bulb and back to the other end of the cell. This complete path is called a closed circuit.
Each circuit has a gap in it and the bulbs do not light up. Incomplete circuits are called open circuits.
An electric current flow only when there is a source of electrical energy and a closed circuit.

Cell
Supplies electrical energy.The larger terminal (on the left) is positive (+). A single cell is often called a battery, but strictly a battery is two or more cells joined together.

Battery
Supplies electrical energy. A battery is more than one cell.The larger terminal (on the left) is positive (+).

Output Devices: Lamps, Heater, Motor, etc.

Component
Circuit Symbol
Function of Component

Lamp (lighting)
A transducer which converts electrical energy to light. This symbol is used for a lamp providing illumination, for example a car headlamp or torch bulb.

Lamp (indicator)
A transducer which converts electrical energy to light. This symbol is used for a lamp which is an indicator.

Heater
A transducer which converts electrical energy to heat.

Motor
A transducer which converts electrical energy to kinetic energy (motion).

Bell
A transducer which converts electrical energy to sound.

Buzzer
A transducer which converts electrical energy to sound.

Push Switch(push-to-make)
A push switch allows current to flow only when the button is pressed. This is the switch used to operate a doorbell.

Push-to-Break Switch
This type of push switch is normally closed (on), it is open (off) only when the button is pressed.

On-Off Switch(SPST)
SPST = Single Pole, Single Throw.An on-off switch allows current to flow only when it is in the closed (on) position.

Resistor
A resistor restricts the flow of current, for example to limit the current passing through an LED. A resistor is used with a capacitor in a timing circuit. Some publications still use the old resistor symbol:

Variable Resistor(Rheostat)
This type of variable resistor with 2 contacts (a rheostat) is usually used to control current. Examples include: adjusting lamp brightness, adjusting motor speed, and adjusting the rate of flow of charge into a capacitor in a timing circuit.

Voltmeter
A voltmeter is used to measure voltage. The proper name for voltage is 'potential difference', but most people prefer to say voltage!

Ammeter
An ammeter is used to measure current.

Ohmmeter
An ohmmeter is used to measure resistance. Most multimeters have an ohmmeter setting.

Switches
A switch is used to open or close a circuit.

Conductors & Insulators
Allows electricity to pass through it is called an electrical conductor.
A material which does not allow electricity to pass through it is called an electrical insulator.

What is voltage?
Voltage is a measure of how much energy the electrons receive. To measure voltage we use a voltmeter.
The SI unit for voltage is the volt (V).

Series circuits & parallel circuits
Electric circuits can be classified into two main types----series circuits and parallel circuits.

In a series circuits, each component is joined to the next to form a single path. The current that flows through each of the components is the same. However, a break in any part of a series circuit stops the flow of current in the whole circuit.
☆ the current at different points of the electric circuit is the same.
☆ the voltage of the energy source is the sum of the voltage of all the batteries in series.
☆ the total voltage across the whole circuit is the sum of the voltage of all the batteries in series.

A parallel circuit divides into two or more branches with electrical components in each branch. The current divides and flows through each of the three branches. An advantage of this circuit is that if one bulb breaks or is removed, the other bulbs remain lit.
☆ the current is the sum of all the current in the braches of a parallel circuit.
☆ the voltage across each branch is the same.
☆ the total voltage of cells/batteries connected on parallel is the voltage of one cell/battery.

What is an Electric Current?
An electric current is the flow of electrons in one direction in a circuit.

How does electricity travel in circuits?
Electricity travels in closed circuits. It must have a complete path before the electrons can move. If a circuit is open, the electrons cannot flow. When we flip on a light switch, we close a circuit. The electricity flows from the electric wire through the light and back into the wire. When we flip the switch off, we open the circuit. No electricity flows to the light. Electricity flows through a tiny wire in the bulb when we turn a light switch on. The wire gets very hot. It makes the gas in the bulb glow. When the bulb burns out, the tiny wire has broken. The path through the bulb is gone.

Measuring electric current
An ammeter is used to measure the electric current.

The SI unit for electric current is the ampere (A). Smaller currents can be measured in milliamperes (mA)
1A=1000mA
1mA=1/1000A

What is resistor?
An electricity component that is specially made to have a certain resistance is called a resistor.
Resistors that have one fixed resistance are called fixed resistors.

We used rheostat to change the current in a circuit. (rheostat which is also called a variable resistor.)
Resistance = voltage across component /
Current though component

Effects of an electric current
The flow of an electric current produces three effects.
☆ HEATING effect
☆ CHEMICAL effect
☆ MAGNETIC effect

The above sources are taken from website and sec. 2 textbook.
http://www.eia.doe.gov/kids/energyfacts/sources/electricity.html
http://www.sciencetech.technomuses.ca/english/schoolzone/Info_Electricity.cfm#whatis
http://people.virginia.edu/~bwk7j/basic.htm
http://www.kpsec.freeuk.com/symbol.htm