SSLC Physics, Chapter 1 Effects of Electric Current Notes

Advantages of electricity

Electricity is a form of energy .

Electricity can be converted into any other form of energy easily.

Electricity can be transmitted from one place to another through wires

For these reasons, electricity is the most useful form of energy.

Law of conservation of energy

Energy can neither be created nor be destroyed. It can only be converted from one form to another. This is law of conservation of energy.

Intensity of current

The charge flows through a conductor in one second is the current through the conductor.

If ‘Q’ coulomb charge flows through a conductor in ‘t’ seconds , then intensity of current,

I=Q / t

Unit of current intensity is Ampere(A)

Potential difference - volt

The potential difference between two points will be one volt if one joule of work is done in moving one coulomb of charge from one point to the other.

Heating effect of electric current

In heating appliances like electric iron box, electric kettle, electric room heater, water heater, soldering iron etc. electrical energy is converted into heat energy.

In heating appliances nichrome is used as heating coil.

Nichrome has high resistance and high melting point. It becomes red hot when electricity is passed through it.

Joule heating or ohmic heating

The process by which heat is developed in a circuit on passing current through a conductor of high resistance is known as the Joule heating or Ohmic heating.

The factors influencing the heat developed when a current passes through a conductor

The current through the conductor

The resistance of the conductor

The time of flow of current through the conductor

Joule’s law

The heat generated in a current carrying conductor is directly proportional to the product of the square of the current in the conductor, the resistance of the conductor and the time of flow of current

I – current in ampere, R – resistance in Ohm, t – time in second, and H – heat in joule

Some equations to find heat developed in a current carrying conductor

Problems based on heating effect

How much will be the heat developed if 0.5A current flows through a conductor of resistance 100Ω for 10 minutes ?

A device of resistance 690Ω is working under 230V for 5 minutes. Find the quantity of heat developed?

Relation between heat developed and current

Arrangement of resistors in a circuit

Difference between resistors in series and resistors in parallel

Problems

Ten 10Ω resistors are connected in series. Calculate the effective resistance?

Ans:

Effective Resistance, R = 10×10 Ω

= 100 Ω

Ten 10Ω resistors are connected in parallel. Calculate the effective resistance?

Ans:

Effective Resistance, R = 10Ω /10 = 1 Ω

Calculate the effective resistance of the given circuit?

Ans: Effective resistance, R = R₁+ R₂+ R₃

= 10Ω+20Ω + 30Ω

= 60 Ω

Calculate the effective resistance of the given circuit?

Ans:

Effective resistance of 2 resistors which are in series connection = 6Ω + 6Ω = 12Ω

12Ω and 4Ω are in parallel

In the given circuit R₁ = 5Ω, R₂= 20Ω, R₃ = 6Ω. Calculate the effective resistance?

Ans:

4Ω and 6Ω are in series

Effective resistance = 4 Ω + 6 Ω = 10 Ω

What is the current if 6 Ω and 3 Ω resistors are connected in series and 12 V potential difference is applied?

Ans:

Effective resistance, R = R₁+ R₂

= 6 Ω + 3 Ω = 9 Ω

V = 12 V

I = 𝑉/𝑅= 12/9=4/3𝐴

What is the current if 12 Ω and 4 Ω resistors are connected in parallel and 6 V potential difference is applied?

Electric heating appliances

Electric heating appliances are instruments that make use of the heating effect of electricity. In these appliances electrical energy is converted into heat energy. The part which converts electrical energy into heat energy are called heating coils. Generally heating coils are made of nichrome, an alloy of nickel, chromium and iron.

Example : electric heater, immersion heater, soldering iron, electric kettle etc.

Peculiarities of Nichrome

What are the peculiarities of nichrome?

• High resistivity.

• High melting point.

• Ability to remain in red hot condition for a long time without getting oxidised.

Safety fuse

Safety fuse is a device which protect us and the appliances from danger when an excess current flows through the circuit. This device works on the heating effect of electric current. Fuse wire in safety fuse is made up of an alloy of Tin and Lead.

Fuse wire has high resistance and low melting point. When the current flows through a circuit exceeds the permissible limit, excessive heat is generated and the fuse wire melts. Thus the circuit breaks and appliances are protected from the damage due to the flow of excessive current.

When a fuse wire is included in a household wiring, what are the precautions to be taken?

The ends of the fuse wire must be connected firmly at appropriate points. The fuse wire should not project out of the carrier base. Fuse wire of appropriate amperage should be used

In a safety fuse thick copper wire was used as fuse wire. Give your opinion.

Safety fuse is intended to break the circuit when excess current is happened to flow through the circuit due to short circuiting or overloading. If thick copper wire is used instead of fuse wire the circuit will not break when excess current flows. This will damage the electrical appliances in that circuit.

Resistors in series and parallel

Amperage

Amperage(A) is the ratio of the power of an equipment to the voltage applied. Amperage of a conductor increases with the thickness of the conductor.

Gauge

Gauge is the reciprocal of the diameter of a conductor. As the gauge increases the thickness of the conductor decreases and hence the amperage of the conductor also decreases.

Electric power

Power of an electric appliance

The amount of energy consumed by an electrical appliance in unit time is its power. Unit of power is Watt.

If W joule work is done in t seconds,

then power, P = W/t

¢ P = I² R

¢ P = VI

¢ P = V²/R

An appliance of power 740 W is used in a branch circuit if the voltage is 230 V. What is the amperage?

Amperage = 𝑤𝑎𝑡𝑡𝑎𝑔𝑒/𝑣𝑜𝑙𝑡𝑎𝑔𝑒

= 740/230 = 3.217 ≈ 3.22 A

¢ A heating appliance has a resistance of 125Ω. If 2 A current flows through it, what is the power of the appliance?

R = 125 Ω

I = 2A

P = I² R

= 2² ×125

= 4 × 125 = 500W

¢ A current of 0.4 A flows through an electric bulb working at 230V. What is the power of the bulb?

P=VI

= 230 ×0.4

= 92W

¢ 230 V is applied between the ends of an appliance of resistance 460 Ω. What is the power of the appliance?

An electrical appliance of power 1kW works for 10minutes. Calculate the heat developed in the appliance?

H = I² Rt

= P ×𝑡

= 1000 × 600

= 600000 W

= 600kW

Lighting effect of electric current

Why is the bulb of an incandescent lamp filled with inert gas / nitrogen?

If the bulb is evacuated the filament vapourises and ultimately the filament breaks. To avoid this the bulb is filled with inert gas / nitrogen. Inert gas/ nitrogen does not react with filament even in hot condition.

lighting effect of electric current

What are the advantages of tungsten to make it suitable for being used as a filament in incandescent lamps?

Tungsten is used as filament in incandescent lamps because of the following properties.

High resistivity

High melting point

High ductility

Ability to emit white light in the white hot condition

Nichrome is not used in incandescent lamps. Why?

When electricity is passed through nichrome wire it becomes red hot and heat energy is produced. It will not emit white light.

What is the disadvantage of incandescent lamp?

A major part of the electrical energy supplied to an incandescent lamp is lost as heat. Hence the efficiency of these devices is less.

Working of discharge lamp

Discharge lamps are glass tubes fitted with two electrodes. They emit light as a result of discharge of electricity through the gases filled in tubes. When a high potential difference is applied the gas molecules get excited . To attain stability these gas molecules radiate energy as light.

Advantages of led

What are the advantages of LED(Light Emitting Diode) over other lamps?

As there is no filament in LED, there is no lose of energy in the form of heat.

They work using low power.

Since there is no mercury in it, it is not harmful to environment.

It has high efficiency and high longevity.

Let us assess - questions and answers

Ans:

Fuse wire of suitable amperage should be selected considering the connected load in the circuit and voltage applied. Amperage is calculated by dividing wattage with voltage.

Amperage of available fuse wire are 0.1A, 0.2A, 0.5A, 1.5A, 3A, 5A, 10A etc.

Ans:

I = 0.5A

V = 230V

a) t = 5minutes = 5 ×60 seconds = 300seconds

Q = I × t

= 0.5 × 300 = 150C

b) R = V/ I

= 230/ 0.5

= 2300/ 5

= 460Ω

c) I = 0.5A

R = 460Ω

t = 5minutes = 5 ×60 seconds = 300seconds

H = I² Rt

= 0.5² × 460 × 300

= 0.25 × 460 × 300

= 34500J = 34.5kJ

d) P = I² R

= 0.5² × 460

= 0.25 × 460

= 115W

Ans:

No, the heat developed will not increase on increasing the resistance without changing the voltage. Actually the heat developed will decrease because the current will decrease as the resistance increases. That is H ∝ 1/ 𝑅

Ans:

As the voltage increases the power of the device also increases and vice versa.

Ans: Power decreases

Ans: Reduce their resistance

Ans: iii) between 500 W and 510 W

The maximum power that the fuse wire can draw

= 230 × 2.2

= 506 W

Ans:

a) P = VI

I = P/ V = 115/ 230 = 1/ 2 = 0.5A

b) Q = I×t

= 0.5 ×10 × 60

= 300 C

Ans: R = 𝑉/ 𝐼 = 60/ 4 =15 Ω

if V = 120 V,

I = 𝑉/ 𝑅 = 120/15 = 8A

Ans:

a) Highest resistance is obtained when all the resistors are connected in series.

Effective resistance, R = R₁+ R₂+ R₃

= 2Ω + 3Ω + 6Ω

= 11 Ω

c) Yes, we can make a resistance of 4.5Ω using these three.

3 Ω is connected in series with 1.5 Ω

∴ Effective resistance of the circuit, R = 3 + 1.5 = 4.5 Ω

Effective resistance of two 2 Ω resistors in parallel = 1 Ω

Effective resistance of the circuit = 4 × 2 Ω + 1 Ω = 9 Ω

Ans:

Intensity of light increases. As the broken filament joined, length of the filament decreases. So resistance also decreases. As a result the power increases.