Current Electricity
STD. 9
Direct current
Direct current (d.c.) is a current of constant
magnitude flowing in one direction.
A cell is a source of d.c.
In a cell, chemical energy is converted to
electrical energy.
Types of cells
Primary Cell Secondary Cell
• It is an irreversible chemical • It is a reversible chemical
reaction. reaction.
• These cannot be recharged. • These can be recharged.
• Capable of giving a weak • Capable of giving a strong
current. current.
• It is light and cheap • It is heavy and costly.
Simple voltaic cell, Lead (acid) accumulator,
Leclanche cell, Ni-Fe(alkali) cell,
Daniel cell, Li-H battery.
dry cell.
Electric Current
In metals, the current is due to flow of electrons.
In electrolytes, the current is due to movement of ions.
Charge (Q) = n x e
Current is the rate of flow of charge across a cross section
normal to the direction of flow of current.
Current (I) = Charge (Q) = ne
Time (t) t
Direction of current
Conventional → positive to negative
Flow of electrons → negative to positive
Unit of current
Coulomb per second → ampere (A)
The current is said to be 1 ampere when a
charge of one coulomb passes through the
conductor per second.
When 6.25 x 1018 electrons pass through in 1
second.
Smaller units → mA = 10-3 A
μA = 10-6 A
Symbols used in circuit diagrams
1. Source of current
2. Key
3. Resistance
4. Ammeter
5. Voltmeter
6. Galvanometer
7. Load
8. Connecting wires
1. Source of Current
a. Alternating current – a.c.
→mains in the house and a.c. generator
b. Direct current – d.c.
→ Cell and battery
Anode = + Cathode = -
2. Key
It is used to put the current on and off in a
circuit.
Three types → plug key, switch, tapping key.
It is represented by the symbol K.
When the key is open, the circuit is incomplete
and current does not pass through.
When the key is closed, the circuit is complete
and current passes through.
3. Resistance
A resistance wire is made from an alloy called
manganin (copper, manganese and nickel)
A rheostat is a device used to vary the resistance
continuously. It consists of a constantan wire
over an asbestos pipe in an iron frame provided
with a sliding jockey on a brass rod.
4. Ammeter
It is an instrument used to measure the
magnitude of current flowing in the circuit.
It is connected in series with the circuit.
An ammeter must have low resistance.
5. Voltmeter
A voltmeter is used to measure the potential
difference between two points of a circuit.
It is always connected in parallel to the circuit.
A voltmeter usually has very high resistance.
5. Galvanometer
It is used when we want to detect the presence
of very weak current in an electric circuit or to
know the direction of the current in the circuit.
It is connected in series with the circuit.
It does not measure the magnitude of current.
7. Load
An appliance which is connected in a circuit is
called load.
It may just be a resistance, bulb, heater.
8. Connecting wires
These are the wires used to connect various
electrical components.
They are made of copper or aluminium as they
have negligible resistance.
Simple electric circuit
Insulators vs. Conductors
Insulators Conductors
• Substances which do not • Substances which allow
allow current to flow current to flow through
through them. them.
• They have almost no free • They have a large number
electrons. of electrons.
• They offer high resistance. • They offer very small
resistance.
Cotton, rubber, plastic, wood, Copper, aluminium, silver, iron,
paper, glass, leather, pure brass, steel, impure water,
water. mercury.
Flow of electrons
When two charged conductors are joined by a
metallic wire or placed in contact, free electrons
flow from the conductor having a higher
concentration of electrons to the conductor
having lower concentration of electrons.
An electric cell works as a source of electrons.
Potential
A conductor having an excess of electrons is said to
be at a negative or lower potential.
A conductor having a deficit of electrons is said to
be at a positive or higher potential.
Electrons flow from low potential to high potential.
Electric current (conventional) flows from high
potential to low potential.
Potential difference
The potential difference between two conductors is equal
to the work done in transferring a unit positive charge
from one conductor to another.
V = W/Q
S.I. unit → volt (V) or joule / coulomb
The potential difference is said to be 1 volt if work done
in transferring 1 coulomb of charge from one point to
another is 1 joule.
Resistance
The obstruction offered to the flow of current by a
conductor is called resistance.
S.I. unit → ohm (Ω) or volt/ampere
The resistance is said to be one ohm if a current of 1
ampere flows through when the potential difference
across its ends is 1 volt.
Ohm’s Law
The current flowing through a conductor is
directly proportional to the potential difference
applied across its ends, provided that the
temperature remains constant.
V = IR
Factors affecting the resistance
Length
Area of cross section
Temperature
Material
Efficient use of energy
Reduce cost of energy
Reduce emission of green house gases
Reduce global warming
THE END
STD. 9
