Semiconductor (cont’d)
Silicon is a semiconductor and its ability to conduct can be altered by adding atoms of other
semiconductors s u c h a s g e r m a n i u m . So m e s e m i c o n d u c t o r s a r e m a d e t o
c o n d u c t i n one direction and only if the potential difference across it is high enough. This
effect is used in diodes and transistors.
Electrolytes
When sodium chloride is dissolved in water an ionic solution is formed. This contains two
different types of ions: Na+ and CI-.
If electrodes are placed in the solution and connected to a battery, both of these charged
particles can flow because of the potential difference between the two electrodes.
In this case, current is both due to the flow of positive Na+ ions and negative Cl- ion.
• The sodium ions are positively charged and so are attracted to the negative electrode. Upon
reaching the electrode they obtain electrons and form sodium. This rapidly reacts with the
water to form sodium hydroxide and hydrogen gas and so bubbles can be seen on the
electrode.
• The chlorine ions are negatively charged and are attracted to the positive electrode. Upon
reaching the electrode they lose an electron and form chlorine gas.
This process is called electrolysis and can be carried out on an industrial scale to produce
useful chlorine gas and sodium hydroxide solution. Electrolysis of other ionic solutions can
produce other useful chemicals.
CURRENT AND CHARGE
Current is the flow of electrical charges. This flow of charge is caused by a potential
difference between two points.
Note:
As seen before, these charges don’t necessarily have to be electrons like in metals, they could
be holes like in semiconductors or even negative and positive ions like in electrolytes.
More specifically, Current is the rate of flow of electrical charges. It is represented by the
symbol, I, and its unit is the Ampere, A.
Mathematically,
Q= I x t
Where,
Q – quantity of charge transferred, measured in Coulombs, C.
1 C = 1 As (Ampere-second)
I – amount of current flow in Amperes, A
t- time taken in seconds, s
Example:
1. What is the current in a wire if a charge of 360 C is transferred in one minute?
Solution
1 min = 60 s
Q= It
I= Q / t
I=360C / 60-s= 6.0A
Quantity of charge
Given the number of charge carriers, N, flowing in a circuit. The quantity of charge can be
found using the formula:
Q = Ne
Where,
Q- quantity of charge in Coulombs, C
N- number of charge carriers
e- charge of an electron, 1.6 x 10-19 Coulumbs
Example:
2. What is the total number of electrons necessary to produce 1 C of charge?
Solution
Q= Ne
N= Q / e
I= 1C / 1.6 x 10-19 C= 6.25 x 1018 electrons
Therefore 1C of charge contains 6.25 x1018 electrons.
Practice Questions
1. a) The current in a wire is 0.72 A. Calculate the charge that passes through
the wire in:
I.
4s
II. 7 min
b) Calculate the average current in a wire if a charge of 3500 C passes in:
I. 9 s
II. 200 s
2. What is the average electric current in a wire when a charge of 150 C passes
in 30 s?
3. Calculate how long it takes for the following charges to pass a point in the
circuit:
I. 6.0 C, when the current is 0.5 A
II. 48 C, when the current is 36 mA
4. In a lightning flash a typical amount of charge which reaches the Earth is 10
C. If the flash lasts for 0.50 ms, what is the average current?
5. In a television tube the picture is formed by streams of electrons hitting the
screen. The current is 20 mA. How much electric charge hits the screen in 30
min?
6. The current in a small torch bulb is 0.20 A:
I. What is the total electric charge which passes a point in the circuit in 12
min?
II. How many electrons pass this point in this time?
7. Calculate the number of electrons passing a point in the wire in 1 min when
the current 1S:
I. 1A
II. 5.0 MA
8. Calculate the number of electrons passing a point in a circuit when there is a
current of
a. 4.0 A for 60 s
b. 12 mA for 25 s
9. In a certain semiconducting material the charge carriers each have a charge of
1.6 x 10-19. How many charge carriers are entering the semiconductor per second
when the current is 2.0 nA?
10.A certain type of rechargeable battery is capable of delivering a current of 0.2 A
for 4000 s, before its voltage drops and it needs to be recharged. Calculate:
a. the total charge the battery can deliver before it needs to be recharged,
b. the maximum time it could be used for without being recharged if the current
through it were 0.5 A
Current in circuits
An Electric circuit is a continuous and closed path of electric current.
Electrical charges move around circuits in a current. In an electrical circuit a current is used
to transfer energy to electrical devices.
In a simple circuit the current is a flow of electrons through metal wires and components .
The current is not used up as it travels around the circuit. All of the electrons complete the
whole journey releasing energy in the components as they pass through them.
Conventional current
Conventional current is described in terms of a flow of positive charge. In circuits the energy
is actually carried by a flow of negative charges. This means that the particles carrying charge
are actually moving in the opposite direction to the conventional current.
Electric Potential and Potential Difference
Electric Potential refers to the amount of electric potential energy at a point in a circuit.
The difference in the amount of electric potential energy between two points in an electric
circuit is called electric Potential difference.
A charged particle will move to a position where it will have less potential energy and so will
move to a 'lower' point in an electric field. (Like a ball rolling down a hill)
Electric potential difference is known as Voltage, which is equal to the amount of work
done to move the unit charge between two points against static electric field.
Therefore, Voltage =
𝑾𝒐𝒓𝒌 𝒅𝒐𝒏𝒆
𝑪𝒉𝒂𝒓𝒈𝒆
Voltage or electric potential difference is denoted by ‘V’.
Therefore, V =
𝑾
𝑸
or W = QV
Where,
W = Work done
Q = Charge
S.I. Unit of Electric Potential Difference (Voltage) is Volts denoted by ‘V’ This is named in
honour of Italian Physicist Alessandro Volta.
Since Joule is the unit of work and Coulomb is the unit of charge, 1 Volt of electric
potential difference is equal to the 1 Joule of work to be done to move a charge of 1
coulomb from one point to another in an electric circuit.
Therefore
1V = 1Joule/1Coulomb = 1J/1C
1V = 1JC-1
Example:
How much energy is transferred when a charge of 3.2 C passes through a potential difference
of 5.0V?
Solution
V=
𝑊
𝑄
W= QV = 3.2 C x5.0 V = 16 J
Voltage, potential difference and electromotive force
The voltage produced by an electrical power source is called the electromotive force (e.m .f
.) and the voltage across an electrical component is usually called the potential difference
(p.d.). Both are measured in Volts and are often referred to simply as voltages.
Usually batteries are used as electrical power supplies in circuits. In circuits the positive
terminal of a battery or power supply is at higher potential than the negative terminal.