1) An electric current is a flow of electric charge through a conductor when a voltage is present. The rate of charge flow is called current, measured in amperes. 2) Ohm's law states that current through a conductor is directly proportional to the potential difference across it. The resistance of the conductor is the constant of proportionality. 3) Resistance depends on the material and resists the flow of electrons. It is measured in ohms using an ohmmeter. Conductors have low resistance while insulators have high resistance.

1. Chapter 12
*

2. *
*An electric current is a flow of electric
charge. Electric charge flows when there
is voltage present across a conductor.
*The rate at which charges move in a
conductor is called electric current i.e.
quantity of charges crossing a point in
unit time.
I = q/t
where I is current in amperes (A)
q is total charge in Coulombs (C)
t - denotes time in seconds (s).
∴ 1 A = 1 C/s

3. *
*The direction of conventional
current is in the direction of the
flow of positive charges.
*The flow of electrons is opposite
to conventional current.

4. *
*The SI unit of current is ampere (A)
*Smaller currents are measured in milli amperes
(mA).
*Different electrical components and appliances
require different sizes of current to turn them
on.
1 A = 1,000 mA
1 mA = 0.001 A

5. *
*A continuous and closed path of an
electric current is called an electric
circuit.
Electric circuits are made up of
electrical components
*These components must be joined
together without any gap in between to
form a closed circuit.
∴ a circuit should be in a closed path.

6. *
++

9. *
*In electricity potential is like
height, level, temperature.
*Some work has to be done by
charging a body.
*This work done is stored in a
body as its electrical potential
energy.

10. *
*Electrical potential difference
between 2 points A & B on a
conductor through which the current
is flowing is defined as amount of
work done to move a unit charge
from point A to point B in a closed
circuit.
*If work is done to move a charge
from one a point A to point b on a
conductor then the potential
difference between A & B is V=w/q

11. *Where V= potential difference
W=Work done
q=Charge
*Unit of potential is volt
*Potential difference is said to be one
volt of one joule of work is done to
move a charge(q) of one coulomb
from one point to another in a given
circuit.
*1 v =1j/1c
*

12. *Ohm's law states that
the current through a conductor between
two points is directly proportional to
the potential difference across the two
points. Introducing the constant of
proportionality, the resistance, i=v/r
*where I is the current through the
conductor in units of amperes, V is the
potential difference measured across the
conductor in units of volts, and R is
the resistance of the conductor in units
of ohms. More specifically, Ohm's law
states that the R in this relation is
constant, independent of the current.
*

13. *
*Resistance is a property that
resists the flow of electrons in a
conductor. It controls the
magnitude of the current. The SI
unit of resistance is ohm (Ω).
*Resistance is measured in ohms
(Ω) using an ohmmeter.
*An ohmmeter is a device for
measuring resistance.

14. *
*Conductors, such as
metals, allow electrons to flow
freely through them and have
low resistance values.
*Insulators resist electron flow
greatly and have high resistance
values.

15. *
*Both resistance and resistivity of a material vary with
temperature.

16. *

17. *
*Resistors can be used to control
current or potential difference in
a circuit.
*In a circuit, electrons have a
higher potential difference as
they enter a resistor compared to
when they leave the resistor
because they use up some energy
in passing through the resistor.

18. *
*There are two methods of joining the
resistors together.
(a) Resistors in series
(b)
Resistors in parallel

19. *
*The current through each of the resistors is
the same. As ….
*V = V1+ V2 + V3
*Applying Ohm’s law to the entire circuit we
have.. V = I R, then
*On applying ohm’s law to the three resistors
seperately,we further have
*V=IR1
*V=IR2
*V=IR3 from that we have I R = IR1 +
IR2 + IR3
Or Rs = R1 + R2 + R3

20. *
*The reciprocal of the equivalent resistance of
a group of resistance joined in parallel is equal
to the sum of the reciprocals of the individual
resistances. As….
*I = I1 + I2 + I3
*I = V/Rp (by ohm’s formula to parallel combination of resistors we have)
*I1 = V/R1 ; I2 = V/R2 ; and I3 = V/R3 (by ohm’s law
to each resistor)
*V/Rp = V/R1 + V/R2 + V/R3

21. *As some power is converted into heat
energy, this phenomenon can be
described by Joules law, which states
that H = i2.r.t, where H is the
generated heat in calories,
*When an electric current flows through a
wire, the wire heats up. Electrical
energy has been converted into heat
energy.
*The greater the resistance of the
wire, the greater the amount of heat
produced. This heating effect is used in
common electrical appliances.

22. *
*Electric power is the rate at which electric energy is
transferred by an electric circuit.
*The power (p) is given by
*P=VI Or P= I2R= V2/R
* The SI unit of electric power is watt(W).
1W = 1 volt.1ampere = 1 V A
The commercial unit of energy is kilowatt
hour(kWh),commonly known as ‘unit’
1 kW h = 1000 watt. 3600 second
= 3.6 . 106 watt second
= 3.6 . 106 joule (j)

23. Chapter 12
*
A presentation by :-
Suraj Kakkad