Introduction:-
In an atom, there are 3 sub-atomic particles.
Electrons-- > carry negative charge -- > -1.6*10^-19C
Proton -- > carry positive charge -- > 1.6*10^19 C
Neutron-- > carry no charge.
All matter is made up of atoms
In this lesson we will consider only two types of charges; positive and negative.
Like charges (similar charge) repel each other
Unlike charges(opposite charges) attract each other.
SI unit of charge (electric charge) is coulomb C
If a net charge Q , flow across any cross section of a conductor in time t, then the current I through cross section is
I = Q/t
Where I = Electric current,
Q = Electric charge,
t = time
S I unit of charge is coulomb,C
One coulomb of charge is that quantity of charge which flows through a circuit when one ampere of current flows through it in one second.
1C = 1A * 1s
What is electric current (I)?
General :- Flow of charge.
Definition:- Amount of charge flowing through a particular area in unit time.
Direction of current is similar to the direction of flow of proton.
Or
Direction of current is opposite to the direction of flow of electron
S I unit of current is ampere,A
One ampere is the amount of current flowing through a circuit when one coulomb of charge flows through it in one second.
1A = 1C/1s
Instrument which measures current is called ammeter (Ammeter has low resistance)
Electric circuit:- A continuous and closed path of an electric current is called electric circuit.
In outer circuit direction of flow of electric circuit is from positive terminal of the cell to negative terminal and cell.
Conductor:- Substances through which charges can easily pass. Example:- Silver, copper etc
Numerical:- Calculate the number of electrons constituting one coulomb of charge.
Solution:-
1e- = -1.6* 10^-19 C
So 1C = 1/1.6*10^-19e-
= 0.625 * 10^19 e-
= 6.25 * 10^18 e-
Draw a schematic diagram of an electric circuit comprising - cell, electric bulb, ammeter and plug key.
Electric Potential:-
Definition 1:- Electric Potential at a point in an electric field is defined as work done in moving a unit positive charge from infinity to that point.
Electric Potential = Electric potential energy (work)/ charge
Definition 2:- Electric Potential energy per unit charge is called electric Potential.
Electric potential difference:-
For flow of charge in a conducting metallic wire, the gravity has no role to play, electrons move only if there is a difference of electric potential along the conductor.
This difference is called electric Potential difference.
Definition:- The potential difference between two points in an electric circuit is defined as the amount of work done in moving a unit positive charge from one point to another point.
Potential difference = work done (W)/ Quantity of charge moved(Q)
V = W/Q
SI unit of potential difference is volt.
One volt is the potential difference between two points in a current carrying conductor when 1 joule of work is done to move a charge of 1 coulomb from one point to other.
1volt = 1joule/1coulomb
1V = 1J/C
Voltmeter measure the potential difference.
Voltmeter is always connected in parallel across the points between which potential difference is to be measured.
Voltmeter has a high resistance so that it takes a negligible current from the circuit.
Symbols of some commonly used components in circuit diagrams:-
Ohm's law:-
At constant temperature, the current flowing through a conductor is directly proportional to potential difference across its ends.
Mathematical expression:-
I directly proportional to V
V directly proportional to I
V = IR
Where V = potential difference,
I = Current,
R = resistance.
Resistance :- It is defined as the ratio of potential difference applied between the ends of a conductor and the current flowing through it
It is a constant quantity.
SI unit is ohm
1ohm = 1volt/1Ampere.
Define 1 ohm .
The resistance of a conductor is said to be one ohm if a current of one ampere flows through it when a potential difference of one volt is applied across its ends.
Definition:-
Good conductors:- Those substances which have very low electrical resistance are called good conductors. Example:- Silver is the best conductor.
Resistor:- Those substance which have comparatively high electrical are called resistors. Example :- Nichrome (Alloy of nickel, chromium, manganese and iron metals)
Insulators:- Those substances which have infinitely high electrical resistance are called insulators. Example:- Rubber.
Draw a graph which show relationship between potential difference and current.
Variable resistance:- A component used to regulate current without changing the voltage source is called variable resistance.
Rheostat:- A device used to change resistance in the circuit.
Factors on which the resistance of a conductor depends:-
Resistance depends upon:-
Length of conductor
Area of cross section of conductor and
On nature of material.
How it depends?
Resistance is directly proportional to length of conductor
Resistance is inversely proportional to area of cross section.
Where rho is resistivity or specific resistance
SI unit of resistivity is ohm-m
Resistance also depends upon nature of material.
Resistivity range:-
Good conductors; eg metals and alloys; resistivity= 10^-8 ohm-m to 10^-6 ohm-m
Insulators; eg rubber and glass, resistivity= 10^12 ohm-m to 10^17 ohm-m
Note:- Resistance and resistivity of material vary with temperature and nature of substance.
Resistivity of alloy is generally higher than that of its constituent metals.
Applications of alloys:-
Heating elements of electrical heating appliances such as electric iron and toaster etc are made up of alloys
Reasons:-
High resistivity of alloy.
Doesn't undergo oxidation (burn) even at high temperature.
Application of metals based upon resistivity:-
Tungsten is used for filaments of electric bulbs.
Copper and aluminium re generally used for electrical transmission lines.
Combination of resistors in series:-
When two or more resistance are combined end to end consecutively they are said to be connected in series.
Example:-
Resistors connected in series.
In series combination of resistors, current flow in each part of circuit is same. { I1 = I2 = I3 = I }
Potential difference V is equal to sum of potential difference;
Vtotal = V1 + V2 + V3....
According to Ohm's law:-
Vital = IRtotal ;. V1 = IR1 ;.
V2 = IR2 ; V3 = IR3
So put values in above equation , we get:-
IRtotal = IR1 + IR2 + IR3.....
IRtotal= I (R1 + R2 + R3.....)
Rtotal = R1 + R2 + R3.....
Thus resistance of combination is equal to sum of their individual resistances; R1, R2 and R3
Combination of resistance in parallel:-
When two or more resistances are connected between same two points, they are sad to be connected in parallel.
Example:-
In parallel combination, total current I is equal to sum of separate currents through each branch of combination.
I = I1 + I2 + I3......
According to Ohm's law:-
I1= V/R1. , I2 =V/R2. ,
I3 = V/R3. ,I = V/R
Put in 1 , we get
I = I1 + I2 + I3..
V/R = V/R1 + V/R2 + V/R3
V/R = V (1/R1 +1/R2 +1/R3)
1/R =1/R1 +1/R2 +1/R3
So Reciprocal of equivalent resistance joined in parallel is equal to sum of reciprocal of individual.
Note :-In parallel combination potential difference is same.
Disadvantages of series circuit for domestic wiring:-
In series circuit, if one electrical appliance stop working due to some defect, then all other appliances also stop working.
All appliances have only one switch due to which they cannot be turned on or off separately.
Appliances do not get the same voltage as that of power supply.
Overall resistance of circuit increases too much due to which current from power supply is low.
Advantages of parallel circuits in Domestic wiring:-
If one electrical appliance stops working due to some defects then all other appliances keep working normally.
Each electrical appliance has its own switch due to which it can be turned on or off independently without affecting other appliances.
Each electrical appliance get same voltage (220V) as that of power supply line.
In the parallel connection of electrical appliances the overall resistance of household circuit is reduced due to which current from power supply is high.
Heating effect of current:-
When an electric current is passes through a high resistance wire, like Nichrome wire, the resistance wire becomes very hot and Produce heat. This is called the heating effect of current.
When an electric charge Q moves against a potential difference V, the amount of work done is given by:- W = Q * V
We know:-
I = Q/t
Q = It
From Ohm's law:-
V/I = R
V= IR
Put the values of V and Q in
W= Q*V we get:-
W = It*IR
W = I2Rt
Assuming all electric work done is converted into heat energy:-
So H = W = I2Rt
H = I2Rt ( joules law of heating)
SI unit of heat Produced , H is joules (J)
According to joules law of heating given by the formula:-
H = I2Rt
Heat Produced in a wire is directly proportional to
Square of current (I2)
Resistance of wire (R)
Time for which current is passed.
Note:- total work done by a current in an electric circuit is called electric energy.
Applications of heating Effect of current:-
Used in working of electrical heating appliances such as electric iron, electric kettle, electric oven etc.
Used in Electric bulbs for producing light
Used in electric fuse for protecting household wiring and electrical appliance.
Electric power, P and Interrelation between P, V, I, R
The rate at which work is done by an electric current is called electric power. Or
The rate at which electric energy is consumed or dissipated is called electric power.
W = VQ
Now Q = It and V = IR
W= VIt = I2Rt
So Power = work/time
P = W/t
P = VQ/t =VIt/t = VI
P =VI
P = I2R
P= V2/R where I =V/R in eq P=VI
Note :-
P = I2R is used when I is constant;
P =V2R is used when V is constant
P = VI is used in all cases
Commercial unit of electric energy:- kilowatt hour
The electric energy consumed when an electric appliance of power one kilowatt works for one hour is called one kilowatt hour.
1 kWh = 1kW * h
1 kWh = 1000W * 3600s
1 kWh = 3.6 *10^6 joule (W-s=J)
1kWh = 3.6 MJ
Property -- > unit --- > symbol
Current -- >ampere (A) ---- > I
Charge -- > coulomb (C) --- > Q
Time -- > sec -- > t
Temperature -- > °k -- > T
Potential difference -- > volt -- > V
Work done --- > joule -- > W
Resistance --- > ohm -- > R
Resistivity -- > ohm-m -->
Power -- > watt --- > P
Heat --- > joule -- > H
Formula used = >
I = Q/t
Current = charge / time
V= W/Q
Potential
difference = work done /charge
V = IR (Ohm's law)
Potential
difference = current * Resistance
R = rho *L/A
Resistance = resistivity * length of conductor /area of cross section
For series connection=
R = R1 + R2 + R3.....
V = V1 + V2 + V3 ....
Here current flow is same
For parallel connection =
1/R = 1/R1 + 1/R2 + 1/R3
I = I1 +I 2+ I3......
Here potential difference is same
P = VI
Power = Potential difference * current
H = I^2Rt (joule's law of heating)
Heat = current ^2 *resistance*time
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