AS Physics 9702 unit 3: Electric Charge 1 UNIT 3 – ELECTRIC CHARGE ELECTRIC CIRCUITS: For the current to exist it must have a complete path of conductors. This complete path of conductors is called electric circuit. For example copper wires, which are conductors, are used to connect lamps in a circuit to complete the path for the flow of charges. The battery is also attached to the circuit to push these charges around. To draw on a paper the diagram of complete path of charges is called circuit diagram. It contains symbols for every component of the circuit. Following are some common circuit symbols: There are two different ways of connecting circuit components in a circuit to the same battery. They are called series and parallel circuits. We will discuss about these circuits later in this section. ELECTRIC CHARGE: All matter is made up of atoms. Atoms have three elementary particles that are electrons, protons and neutrons. Since the atom and its particles are very small for us to observe we can understand the presence of these atomic particles by rubbing two polythene rods (an insulator) with woolen cloth. These rods when brought close to each other they repel. It means that rods have acquired an electric charge. There are two types of charges - the positive charge and negative charge. The positive charge is carried by protons or positive ions and negative charge is carried by electrons or negative ions. Following table will describe how different substances behave when rubbed with different materials. Charging an object by rubbing is called electrostatic induction by rubbing or by friction. Material Rubbed with Charge acquired Behaviour Polythene rod Woollen cloth Negative Attract each other Perspex Woollen cloth Positive Ebonite Fur Negative Attract each other Glass Silk positive It must be noted that the charges are not created by rubbing action. When polythene rod is rubbed with a woolen cloth, some of the electrons of the surface atoms of the cloth transferred to the rod and therefore the polythene rod become negatively charged and cloth becomes positively charge. That means that the charges are transferred from cloth to polythene and total charge is always conserved or same. The unit of charge is coulomb or C (capital C). The most common letters used to express charges are Q, q or e. Small ‘e’ is specifically used for expressing the charge of elementary particle electron (e - ) or proton (e + ).
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AS Physics 9702 unit 3: Electric Charge
1
UNIT 3 – ELECTRIC CHARGE
ELECTRIC CIRCUITS:
For the current to exist it must have a complete path of conductors. This complete path of conductors
is called electric circuit. For example copper wires, which are conductors, are used to connect lamps
in a circuit to complete the path for the flow of charges. The battery is also attached to the circuit to
push these charges around. To draw on a paper the diagram of complete path of charges is called
circuit diagram. It contains symbols for every component of the circuit. Following are some common
circuit symbols:
There are two different ways of connecting circuit components in a circuit to the same battery. They
are called series and parallel circuits. We will discuss about these circuits later in this section.
ELECTRIC CHARGE:
All matter is made up of atoms. Atoms have three elementary particles that are electrons, protons and
neutrons. Since the atom and its particles are very small for us to observe we can understand the
presence of these atomic particles by rubbing two polythene rods (an insulator) with woolen cloth.
These rods when brought close to each other they repel. It means that rods have acquired an electric
charge. There are two types of charges - the positive charge and negative charge. The positive charge
is carried by protons or positive ions and negative charge is carried by electrons or negative ions.
Following table will describe how different substances behave when rubbed with different materials.
Charging an object by rubbing is called electrostatic induction by rubbing or by friction.
Material Rubbed with Charge acquired Behaviour
Polythene rod Woollen cloth Negative Attract each other
Perspex Woollen cloth Positive
Ebonite Fur Negative Attract each other
Glass Silk positive
It must be noted that the charges are not created by rubbing action. When polythene rod is rubbed
with a woolen cloth, some of the electrons of the surface atoms of the cloth transferred to the rod and
therefore the polythene rod become negatively charged and cloth becomes positively charge. That
means that the charges are transferred from cloth to polythene and total charge is always conserved
or same.
The unit of charge is coulomb or C (capital C). The most common letters used to express charges are
Q, q or e. Small ‘e’ is specifically used for expressing the charge of elementary particle electron (e-)
or proton (e+).
Prepared by Faisal Jaffer, revised on Jan 2012
ELECTRIC FIELD:
1. It is a field of electric force.
2. An electric field is a space or region around a charged object Q where a
stationary positive charge qo experience electric force.
3. It is a vector quantity and its direction is along the direction where the
positive charge would move. This means that the electric field is always
out from positive charge and in to negative charge.
4. The electric field is represented by the radial straight lines around the
charge object. Stronger the field more the number of lines.
Electric field intensity or electric field strength
of an ‘E’ electric charge
1. The electric field intensity is defined as force per unit charge. In
equation form this is represented as:
2. The unit of electric field intensity is newton per coulomb or N/C.
3. The electric field around charge Q, is considered to be a uniform
radial field. This means that a charge qo experiences same force
around a charge object Q if it is at equal distance from the centre of
the charge at any position.
4. We can plot a graph of electric field intensity E against the distance r
from its centre. We can see that the graph shows inverse square law
curve that is E inversely proportional to r2.
5. When two charged plates are placed together, the radial fields of the
charges combine to make a uniform electric field. Notice that the
field bulges at the ends; generally we ignore this. In this case we can
show that the electric field intensity is given by a simpler
relationship:
6. E – electric field intensity; V – potential difference between
the two plates and; d - distance between the two plates in
meters. In this case the unit of electric field intensity is
volts/metre V/m which is same as the other unit that is
newton/coulomb N/C.
7. When a charged particle is moving in between the two
parallel plates that are carrying opposite charges and
have uniform electric field between the plates then the
charge particle experience a constant force centripetal
perpendicular to the motion of the particle. The
deflection of the particle will be towards the opposite
charge plate as show in the diagram.
AS Physics 9702 unit 3: Electric Charge
3
Exercise no 3.1: Solve the following questions from past paper.
1. Oct/Nov 2010, Paper 12, questions 28, 29
2. May/Jun 2010, Paper 12, questions 26, 27, 28
3. Oct/Nov 2009, Paper 12, questions 26, 27, 28
4. May/Jun 2009, Paper 1, questions 27, 28, 29
5. May/Jun 2008, Paper 1, questions 30, 31,
6. Oct/Nov 2007, Paper 1, questions 26, 27
Prepared by Faisal Jaffer, revised on Jan 2012
ELELCTIC CURRENT (I):
Current (I) is defined as the rate of flow of electric charges (Q) in
an electric circuit.
The unit of current is ampere (A). Multiple units of ampere are:
milli-ampere (mA) = 10-3
A and micro-ampere (μA) = 10-6
A
Current is measured by a device called ammeter or multimeter. There are two
types of ammeters; analogue and digital.
Electric charge (Q) in a conductor is carried by atomic particles – electrons or
negative ions. Unit of electric charge is coulomb (C). The quantity of electric
charge of electron or proton is 1.6×10-19
C.
One coulomb is defined as:
a charge passing through any point in a circuit when a steady current of
1 ampere maintained for 1 second, that is:
1coulomb (C) = 1ampere (A)×1second (t).
Conventional current:
The electric current is really a flow of electrons from negative to positive terminal of the battery.
However when it was first discovered, scientists wrongly guessed that something that carries charges
flows from positive to negative terminal and therefore they describe it as conventional current.
Whenever we study electric current and flow of charges we always consider conventional current that
is from positive to negative.
Direct and alternating currents (d.c. and a.c.):
The electrons constantly flowing around the circuit, from the negative
terminal of the battery to the positive terminal, produce direct current
(d.c). All batteries produce direct current.
In mains electricity at homes, the electrons in the circuit move
backwards and forwards 50 to 60 times in one second. This kind of
current is called alternating current (a.c.). The main advantage of using
alternating current over direct current is it can be transmitted from
power stations to our homes at very high voltage which reduces the
amount energy that is lost during the transmission.
Movement of charges in liquids:
Electrolysis:
Electrolysis is the process in which chemical changes are occur in a conducting
liquid when electric charges are passing through it.
The conducting liquid is called electrolyte. The word electrolysis means the
process of breaking molecules of conducting liquid into its parts (ions) by using
electric current. Positive and negative poles of an electric source, such as a
battery, can absorb opposite ions of an electrolyte, causing separation of ions and
creation of a new substance. Liquid metals are not electrolyte since they can pass
current without there being any associated chemical change or making of ions
for example in mercury which is liquid. However solution of sodium chloride
(NaCl) in water is a good example of electrolyte. Some substances that do not
conduct electricity is called non-electrolyte for example sugar-solution.
AS Physics 9702 unit 3: Electric Charge
5
The figure illustrates a simple arrangement of producing electrolysis. The plates by which the current
enters and leaves are called electrodes. Electrode connected to the positive terminal of the battery is
called anode and an electrode connected to the negative terminal is called cathode. Consider the
example of sodium chloride (NaCl) in water as an electrolyte. It contains Na+ and Cl
- ions which are
free to move in water and it conduct electric current. Solid sodium chloride cannot conduct electricity
because ions are not free to move but when the current exists through the solution, NaCl splits into
Na+ and Cl
- ions. Sodium ion (Na
+) gains an electron at the cathode and deposit on the surface of the
plate and similarly Chlorine ion (Cl-) losses an electron at anode and deposit on the surface of anode.
Movement of charges in solids:
Metals:
The atomic structures of metals are such that each atom on
average has one outer electron which is not required for
bonding and which need not to remain attached with its atom.
This electron is called free electron or de-localized electron.
When the current does not exist these free electrons move
randomly in all direction throughout the conductor. When the
battery is attached and potential difference is put across the
conductor, it produces an electric field and affects the flow of
free electrons. It pushes the free electrons towards the positive
end of the battery. Thus this creates the flow of charges across
the conductor which means electric current.
Exercise no 3.2: Solve the following questions from past papers.
1. Oct/Nov 2009, Paper 12, question 33
2. Oct/Nov 2008, Paper 1, question 34
3. Oct/Nov 2007, Paper 1, question 30
Prepared by Faisal Jaffer, revised on Jan 2012
SERIES AND PARALLEL CIRCUITS:
Series Circuit:
A series circuit is a circuit in which components (eg
resistors) are arranged in a chain, so the charges have only
one path to follow.
The current (rate of flow of charges) is same through each
component in series circuit.
The total resistance of the circuit is found by simply adding
up the resistance values of the individual resistors. Equivalent resistance (R) in series circuit
can be expressed by.
where R1 and R2 are the resistances of each resistor.
In series circuit the total potential difference (battery voltage) is the sum of individual
potential differences (p.d.) across each resistor. That is
V = V1 + V2 + ... where V1 and V2 are p.d. across the component R1 and R2.
The current (I) is same in each resistance therefore the ammeter is connected in series with
the other resistances.
In series circuit if one component breaks down then the whole circuit will stop working.
More the resistance of the component, the higher the potential difference across it.
In series circuit the voltage across each resistor divides according to the ratio of resistance
value of each resistor.
Parallel Circuit:
A parallel circuit is a circuit in which the components are
arranged such that each component is directly connected to
the battery. The parallel circuit makes branches for the
current.
The current in a parallel circuit breaks up with some current
flowing along each parallel branch and re-combining when
the branches meet again.
The voltage across each resistor in parallel circuit is same.
Lesser the resistance in the branch more the current in that
branch.
Equivalent resistance of each component or resistors R1 and
R2 in parallel circuit can be expressed by:
or
In parallel circuit the total current is the sum of individual currents in each branch.
I = I1 + I2 + …
Voltage remains same across each resistor that is it has same value as the voltage of the
battery.
The combine resistance of all resistors in parallel circuit is less than the least resistor in the