electricity

so close yet so far

Like charges

repel

Opposite Charges

attract

Neutral objects have…

the same number of positive and negative charges

Neutral objects do not…

repel and attract other neutral objects

charge

number of protons - number of electrons

+ve charge

more protons than electrons

-ve charge

more electrons than protons

neutral charge

equal number of protons and electrons

an ion is a…

an atom with a +ve or -ve charge

Charge (Q)

is the rate of flow of electric charge through a conductor

measured in Couloumbs

proton charge

1.60 × 10^-19 C (positive charge version of electron)

Electrostatic induction

Charged objects attract uncharged (neutral) objects due to induced electric charge

Induction is a

temporary separation of some electrons from their atoms when an electrically charge object is placed near them

this charge disappears when the electrically charged object is removed

Insulators

do not conduct electricity as they have no free electrons or ions

can maintain an induced charge

includes non-metals, plastics, paper and ceramics

Conductors

conduct electricity because they have delocalised electrons

quickly dissipate their induced charge

moisture in the air can also dissipate induced charges

Why is water attracted to charge?

The oxygen atom is more electronegative than hydrogen

Electrons are pulled towards the oxygen atom

The oxygen atom is left with a small negative charge and hydrogen atom with a small positive charge

The charges are attracted and repelled by the charged ends of other water molecules dissolved ions and charged objects

A positively charged conductor is moved near a neutral conductor

Do they attract or repel?

They attract

Electrostatic induction occurs

The electrons from the neutral object are attracted to and thus move towards the positive charged conductor

A positively charged conductor is moved near a neutral conductor

If the positive charged conductor touches the neutral conductor, explain what happens to the charges in each object

electrons move from the neutral conductor to the positively charged conductor

This reduces the positive charge on the first conductor and leaves the second conductor with a positive charge.

Therefore they are both positive

Explain how a negatively charged balloon can attract neutrally charged hair

The negatively charged balloon induces a positive charge in the hair which is closest to the balloon

The negatively charged balloon and the positive part of the hair are then attracted to each other

Voltage Source

a voltage source forces -ve charge (electrons) to the negative terminal

it also strips electrons from the +ve terminal

therefore the negative terminal builds up an excess of -ve charge

and the positive terminal builds up an excess of positive charge

Connecting the terminals allows the the electrons to flow towards the +ve terminal

IN SHORT: electrons flow from negative to positive

Electrical Work

work is done when moving the negative charge to the negative terminal

repulsive force is overcome and the charge is moved through a distance

work done is stored as electrical potential energy (EPE)

Voltage

Voltage is the energy per unit charge that pushes charges through a circuit

1V gives 1C of charge 1J of EPE

Potential Difference

As an electron flows through a circuit element, it loses potential energy

The PD is the difference in potential energy per unit charge between two points on a circuit

Electric Current

DELETE

Conventional Current

current flows in the opposite direction to the flow of electrons

Alternating Current

flows in both directions

it does this because the voltage which pushes the current alternates direction

direct current

flows only in one direction

produced by solar cells and batteries

Resistance

 is a material property that reduces current for a given voltage

the larger resistance the harder it is to push electrons through the circuit

as the flow of electrons is resisted, energy converts to other forms such as heat and light

Factors that affect resistance

material type, length of conductor (resistance increases with length), cross-sectional area (thicker wire has lower resistance), temp (increases resistance) and resistivity

Ohmic conductors

have constant resistance

(they have a linear V vs I graph)

Non-Ohmic Conductors

have a resistance that is dependent on current

non linear V vs I graph

Voltmeter

measures the potential difference across a resistor

high resistance device connected in parallel to the circuit element

does not draw current

Ammeter

measures the current that flows through a resistor

low resistance device that is placed in series with a resistor in a circuit

Electromotive Force

is the voltage supplied by the battery or power source

Kirchhoff’s Current law

the sum of the currents entering a junction equals the sum of the currents leaving a junction

IT= I1+I2+I3

Kirchhoff’s Voltage Law

the sum of the potential difference from sources is equal to the sum of the potential difference used in a circuit

VT=Vd1+Vd2+Vd3

Thevenin’s Theorem

All circuits can be reduced to one source of emf that is equivalent to all the sources, and one load that is equivalent to all the loads

Series Circuits

each coulomb of electrons in the circuit are given a certain amount of electrical by the voltage source

each circuit component uses some of this PE

when the electrons return to the power source they have none left

(electrons have negligible KE)

What is the water analogy for an electric circuit?

Voltage = pressure/push
Current = flow rate
Resistance = restriction to flow
Battery = pump
Load = water wheel using the energy

Memory trick: Voltage PUSHES, current FLOWS, resistance SLOWS.

Electrical Distribution

Most of the electrical energy used in Perth is generated in the Collie and Muja power stations, 200km south of Perth

Once the electricity is generated by the power station, the AC voltage is stepped up (increased by transformers)

The high voltage electrical power is then transmitted to Perth along the high voltage power lines

Once in Perth the electrical power is stepped down to lower voltages by transformers

Transformers found on the street power poles reduce the voltage further to 240V

Transformer

is an electrical device that changes the voltage of an alternating current (AC) supply.

Explain why low currents are used to transmit electricity over long distances rather than high currents

Power = VI

Output V is high, current is lower

Power is lost in cable of resistance

P(loss)= I²R, so if I is low, P(loss) is low

Street Circuit

Three live wires at 11kV

One neutral wire at 0V

Household Circuit

In Aus, the electricity supplied to houses is 240V, 50Hz single phase

The electrical cable comes from the power lines to the main connection box which contains a fuse

The cable goes to the fuse box or switchboard

From there its distributed to the home

Active Connections

called live/hot

brings 240V AC at 50Hz to the home

has a main switch and seperate fuses for light/power and other main circuits

Neutral Connections

The neutral wire completes the circuit from the power station and is held at 0V

The neutral wire is connected to a bar called the neutral bar, which is connected back to the Earth via a metal stake

The neutral wire is also earthed at the substation

Earth Connection

safety device

connected to the metal casing of electrical appliances and to the Earth to produce a short circuit in case of faulty equipment

Double Insulation - no Earth

Some equipment and appliances are not connected to the Earth wire; their plugs have only active and neutral connections

They have 2 layers of double insulation

• the insulation around the live wires

• the non conductive plastic covering the device

Fuses and Circuit breakers

both break a circuit when the current reaches a limit (typically 8A for light circuits and 16A for power circuits

Fuse=melts, and circuit breaker flicks a magnetic switch

Fuse no reuse and circuit breakers switch can be reset

Both fuses and circuit breaker do not protect life but protect electrical equipment

Earth Leakage Systems

disconnect a circuit whenever the electric current in the active and neutral are different

an imbalance may indicate current leakage through the body of a person who is grounded accidentally

RCD’s disconnect quickly enough to prevent injury and death