Trafalgar High School
Unit 4 Electrickery 2008
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8.1 ELECTRIC CHARGE
Electrostatics is the study of stationary charges, as opposed to electrodynamics, which is the study of moving
charges (currents). On the basis of many observations, that after rubbing, that objects can be picked up, a
theory was developed that identified two types of charge, the negative and positive. Like charges repel each
other, eg. two positives would repel each other, but a positive and negative would attract each other. Neutral
atoms can be considered electrically as groups of positive and negative charges that cancel each other. Thus if
an electron is removed from an atom, an unbalanced proton remains.
Charge, like energy, can not be created or destroyed, but it can be transferred from one object to another.
So, to explain what happens when you rub, matter is made up from atoms, which in turn are made up of protons
and neutrons and electrons. Each proton has a positive charge, and each electron has a negative charge equal
but opposite to the proton charge. Usually in a piece of matter there are equal numbers of electrons and protons,
so that all the charges balance out to zero. However, when two materials are brought into close contact, eg. by
vigorous rubbing, electrons can transfer from one material to the other. The material that loses the electrons is
now positively charged, the material that gains the electrons is now negatively charged.
ELECTROSTATIC INDUCTION
 


-----
    
-----
+++++++
When a charged object is brought near one end of a conductor, electrons in the conductor are attracted or
repelled. This induces a movement of charge.
CHARGE
The number of electrons in an electrically neutral body is equal to the number of positive charges. This infers
that the size of the charge on the electron is the same as the size of the charge on the proton. The elementary
charge, e, is the magnitude of the charge on a proton or electron. It is the smallest charge found in nature.
Because it is so small, we have a measure of charge, called COULOMB (C), which is a standard number of
elementary charges.
1 coulomb = 6.25 × 1018 elementary charges, or
e = 1.6 × 10-19 coulomb.
CONDUCTORS AND INSULATORS
A conductor allows the flow of electrons through the material. Metal is a good conductor of electricity, because
the structure of metals is such that the outmost electrons are free to move around in the fixed crystal lattice
made up of the atoms.
Insulators don't allow the free movement of electrons, materials such as plastic and glass are good insulators.
Semiconductors, used in electronics, can be doped to allow them to significantly improve their conduction
qualities.
MILLIKANS EXPERIMENT
In 1907 Robert Millikan set out to show that electric charge came in fundamental units (called the elementary
unit). He set up two plates X and Y, which were charged by battery (B) to 8000V. He used small identical oil
drops and observed their motion under a microscope.
The small spheres fell due to the force of gravity. An opposing upward force due to air resistance increases as
the velocity of the spheres increases. In a short time the two forces equalised, this allowed him to calculate the
weight from the measurements of their speed.
When the plates were charged by the battery, the speed changed as a result of the added electric force on the
drops. Some drops fell even faster, others almost stopped or even rose. As the speed at which the drops fell was
directly related to the total force on them, he was able to calculate the strength of the electric force. From this he
was able to calculate the electric charge on the drop.
Trafalgar High School
Unit 4 Electrickery 2008
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X
Air resistance
sphere
weight
Y
++++++++++++++++++++++++++++++++++++++++++++++++++++
X
Air resistance
Fe
sphere
V
weight
Y
-----------------------------------------------------------The direction of Fe depends on the charge on the sphere. Millikan found that the charge on the oil drop was
always a multiple of a particular value. I.e. the charge on the oil drop is given by: q = ne, where n is a whole
number, and e was 1.6 × 10-19C. Millikan suggested that 'e' is the charge on an electron. Millikan argued that an
oil drop got its charge by gaining or losing electrons, hence the 'e' charge on the drop had to be a whole number
times the charge on one electron.
In 1909 Millikan determined that
Mass of electron = 9.1 × 10-31 kg
Charge of electron e = -1.6 × 10-19 C
LIGHTNING
A thunder cloud has several regions inside where there are enormous numbers of charged particles. The
particles that get charged are tiny ice crystals formed by the rapid cooling of the air as it rises and the hailstones
that are falling from the cloud. This separation of charge produces large electric fields and forces. The field
accelerates the free electrons in the cloud and they collide with other molecules, creating an avalanche of
charge. This lightning flash mainly occurs within the cloud, only a small number actually strike the ground. The
potential difference in a cloud can be hundreds of millions of volts and can cause a brief current of up to 10 000
amp. A moderate thunder cloud with a few flashes per minute generates several hundred megawatts of electrical
power, the equivalent of a small power station.
The thunder associated with the lightning is the air particles being accelerated to a speed greater than the speed
of sound, hence we get a sonic boom. The light is actually the electrons being put into excited states and giving
off energy as they return to the ground state.
8.2
ELECTRIC FORCES AND FIELDS
COULOMBS LAW
The force of attraction or repulsion between two charges Q 1 and Q2 a distance 'r' metres apart is proportional to
the product of the charges and inversely proportional to the square of the distance between the charges.
kQ1Q2
F=
where F is the force in newtons, Q1 & Q2 are measured in coulomb, and 'r' is measured
r2
1
in metres then k =
where 0 is called the permittivity of free space. k = 9 × 109 N m2 C-2. This is only used
4  0
when the two charges are in the air.
ELECTRIC FIELDS
An electric field is the space around a charge, or a group of charges. We know if a field is present at a particular
space because if we place a point charge there, it will experience a force. (Much the same as a point mass
experiences a force in a gravitational field). The stronger the force, the stronger the field.
The magnitude of the field is the size of the force it causes to act on a charge of one coulomb. The direction of
the field is defined as the direction of the force it causes to act on a positive charge.
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Unit 4 Electrickery 2008
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An electric field E is the region around a charged body where another charged body would experience electric
forces of attraction or repulsion. The direction of the electric field is defined as the direction of the force on a
positive charge placed in the field.


Electric field around
positive point charge
Electric field around
negative point charge.
The strength of the field is indicated by the closeness of the field lines.
ELECTRIC FORCES
Electric forces are given by the product of the electric field and the quantity of charge.
FE = qE
Where FE = electric force (N)
q = charge (C)
E = electric field (N/C)
ELECTRIC FIELDS BETWEEN CHARGED PLATES
In the region between parallel charged plates, the electric field E is uniform. The strength of the field depends on
the potential difference between the plates and the distance between the plates.
E =
Where E = electric field strength (V/m)
V
d
 V = potential difference (V)
d = distance between plates (m)
+ + + + + + + + + +
V
E
d
- - - - - - - - - - -
8.3 ELECTRIC CURRENT, EMF, and ELECTRIC POTENTIAL
Electric currents are moving charges; they can flow in solids, liquids and gases.
 In gases an electric current can flow if the potential difference is large enough, eg. spark plug, lightning.
Electrons have been stripped from the gas molecules, forming free electrons and positive ions. The
movement of these particles makes up the electric current.
 In liquids a current can flow if there are ions present. This may happen in solutions of electrolytes.
 In solids electric current is due to the movement of electrons. Only substances made up of atoms with a
weak hold on some of their electrons allow electrons to move through them. Such substances are called
conductors. Substances that are made up of atoms that have a very tight hold on all their electrons are
called insulators.
The convention is that electric current always flows from the positive to the negative. In other words, the direction
of the current is the opposite direction to the movement of the electrons in the solid. So when you see an arrow
giving the direction of the current in a wire, remember that the electrons are going in the opposite direction. The
direction of electrical current (conventional current) is the direction of flow of positive charge. Positive charge can
be transferred either by positive charges moving in the direction of the current or by negative charges moving in
the opposite direction.
The unit of electric current is the ampere, A. (after André Marie Ampère, 1775 - 1836). A current of one ampere
is flowing if one coulomb of charge passes a particular point every second. The total charge passing a given
point when a current 'I' flows for a time 't' is given by
q = It.
Trafalgar High School
Unit 4 Electrickery 2008
Electric current is the rate of transfer of charge:
I=
Page |4
q
.
t
1mA = 10-3 A, 1  A = 10-6 A.
AC or DC
In a direct current, or DC circuit, the polarity of the voltage stays constant. In other words, the positive terminal is
always positive and the negative terminal is always negative, as is the case in a battery. When a battery is
connected in a circuit, the current always flows along the wire in the one direction.
In an alternating current, or AC, circuit, the polarity of the voltage can change in a regular way, so that the
charge first flows in one direction, then in the other. The Australian mains electricity supply is AC, changing
polarity first one way and then back again 50 times per second. I.e. 50 Hz.
There is no difference in the charge carried in AC or DC.
EMF and electric potential.
If charges are moving, they have energy, a battery or generator is the usual source of this energy. When
electrons flow through a battery or other power supply, they gain electrical potential energy. As the electrons flow
around a circuit, they lose this energy when passing through components that have resistance, eg. globes.
The energy transferred to one coulomb of charge within the
battery is called the electromotive force of the battery.
The unit of electromotive force (EMF) is the volt (V). EMF can be considered as a 'concentration of charge'. The
work done in pushing the charges together is stored as electrical potential energy. A battery with an emf of 6V
transfers 6J of energy to each coulomb of charge, when charge moves through the battery.
The energy transferred to a circuit component per unit charge is called
the potential difference between the two terminals of the component.
The unit for potential difference (P.D.) is also the volt. One volt equals one joule per coulomb.
1 V = 1 J C-1
The term 'emf' and 'p.d.' are often simply referred to as voltage. The SI unit for (electrical) potential is joules per
coulomb, this is given the name volt after Alessandro Volta, the inventor of the first chemical battery.
8.4 RESISTANCE, OHMIC AND NON-OHMIC CONDUCTORS
If a light globe is connected to a power supply, via a switch, then it is possible change the voltage of the power
supply and to measure both the voltage across the globe and the current flowing through the globe.
A
V
Plotting values of voltage against current we get the following graph.
Voltage
The ratio of V to I for a component is called its
resistance. It is constant for some components
like a resistor, and varies with current for others
like the light globe.
The relationship between voltage and current is
V
R=
which is often written as
I
V = IR, this is Ohm's Law.
The unit of resistance is the ohm (  ).
Current
Trafalgar High School
Unit 4 Electrickery 2008
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If the light globe in the circuit above was replaced with a resistor.
Then the voltage v current graph is like:
Voltage
When the ratio of V to I is constant, the
graph is a straight line of gradient R.
This device is said to be Ohmic.
(after G. S. Ohm, 1789-1854).
If the voltage across the device is 1 volt
and the current is 1 amp, then the resistance
is 1 ohm.
Current
The size of the current in a conductor when a particular voltage is applied depends on the resistance of the
conductor. Substances with a high resistance allow a small current to flow when a particular voltage is applied, a
large current is produced by the same voltage in a conductor of low resistance. Conductors that do not give a
straight line when V is plotted with I, are called non-ohmic. Eg. transistors, diodes, thermistors, light globes etc.
RESISTIVITY
The resistivity of a material is the resistance across opposite faces of a one metre cube of the material. If this
l
quantity is known, the resistance of a piece of this material is given by
R=
A
where  is resistivity, l is the length of the conductor and A is the cross-sectional area of the conductor.
8.5 ELECTRICAL ENERGY AND POWER
ELECTRICAL ENERGY
When a current passes through a resistor, the resistor gets hot. Electrical energy is being converted to thermal
energy. Electrical energy can also be converted into mechanical energy. If 'Q' of charge passes through a
potential difference of 'V', the work done by the electrical force is
W = QV.
So if it takes 'V' joules of energy to get '1' coulomb of charge form one place to another, then it takes 'VQ' joules
to get 'Q' coulomb from one place to another
As the current is the rate at which charge is moving, the total charge Q = It.

W = Vit.
ELECTRIC POWER
The rate of energy transferred or the rate of work done is called power. In a circuit component the power can be
calculated in terms of potential difference and current
P=
W QV

 IV
t
t
The unit for power is the watt (W). One watt is equal to one joule per second.
In terms of the volt and ampere
1W=1VA
A larger unit for power is the kilowatt (kW).
1 kW = 1000 W.
1 W = 1 J s-1
Since P = IV and V = IR

P = IV = I2R =

W = I2Rt =
V2
R
The energy supplied in time 't' is
V2
t = VIt = Pt
R
UNITS FOR ELECTRICAL ENERGY
The unit for electrical energy is the joule. In industry and in household consumption of electricity the unit kilowatt
hour (kWh) is used. One kWh is the energy delivered in one hour at a rate of 1000 W.

1 kWh = 1000 × 3600 s

= 3.6 × 106J
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Unit 4 Electrickery 2008
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ELECTRIC POWER PRODUCTION AND TRANSMISSION
Electrical energy available in a household circuit originates from a power station. The current is in the form of
alternating current or AC. This type of current changes direction in the circuit 50 times per second, i.e. it
alternates. An electric current from a battery is known as direct current or DC. In Australia, household electricity
is supplied at 240 V AC at 50 Hz.
In order to transmit large blocks of power over transmission lines, it is necessary to use high voltage. The current
can be kept down to a reasonable value so that conductors of practical size can be used as the size of the
cables (conductors) needed depends on the current.

P = VI, so large V leads to small I.
Typically, power is generated at 20 kV (20 000 volts) but is transformed up to 400 kV, for transmission to
Melbourne. This increase in the voltage leads to a decrease in the current by a factor of 20.
Power losses in cables have a large effect if the cables are long or have very high resistance. We decrease the
resistance of cables by increasing the cross-sectional area, but there is a limit to this, from costs and weight
considerations.

Ploss = I2R
so minimising I if of utmost importance.
This is achieved by transforming the voltage up at the power station and then down when it reaches Melbourne.
AC voltage can be transformed but DC cannot.