Electricity and magnetism

Electric current: The flow of charges per unit time is called electric current (I).

i.e. I = Q/t

Its unit is Ampere.(A).

Effects of electricity: The transformation of electric energy into other forms of energy is called effect of electricity. Some

effects of electricity are:

1. Heating effect
2. Lighting effect
3. Magnetic effect
4. Chemical effect
5. Motor effort

Heating effect: The transformation of electric energy into heat energy is called heating effect of electricity.

Heating devices: The devices that convert electric energy into heat energy are called heating devices. Eg: heater, Iron,  Kettle etc

Heating element: The conducting wire on plate in heating devices which convert electric energy into heat energy is called heating element. Eg, Nichrome wire, Tungsten wire, Constatan wire etc Heating devices which convert electric energy into heat energy is called heating element

Properties of heating element.

1. It should have high melting point.
2. It should have high resistance.
3. It shouldn’t Oxidize in high temperature.

Nichrome wire: The wire made of the alloy of Nickle (60%) and Chromium (40%) is called Nichrome wire.

Q. Can we use copper wire as heating element. Why?

Though the copper wire has high melting point, it has very low resistance, so that it doesn’t convert electric energy into heat energy. Hence., we cant use copper wire as heating element.

Q. Lighting effect of electricity

The transformation of electric energy into light energy is called lighting effect of electricity.

Lighting devices: The devices that transform electrical energy into light energy are called lighting devices. Eg, Filament lamp, Fluorescent lamp, CFL (Compact Fluorescent Lamp), LED Bulb (Light Emitting Diodes)

A. Filament lamp:

The thin tungsten filament emits light in this lamp when electricity is passed through it

Characteristics of filament lamp.

1. Tungsten filament is used as light emitting substances.
2. Nitrogen, Neon Argon etc ore filled in it.
3. It is only 10% efficient ie it converts 10% of electric energy into light energy and 90% into heat energy.
4. Its average life span is about 1000 hrs.

B. Fluorescent Lamp

Thee fluorescent powder coated in inner surface of glass tube of fluorescent lamp emits light.

Properties of fluorescent lamp.

1. Fluorescent powder emits light.
2. Mercury vapour is filled in it.
3. It is 30% efficient i.e converts 30% of electrical energy into light energy.
4. Its average life span is about 3000 hrs.

Q. Fluorescent lamp of power 60 W is more brighter than filament lamp of power 60W. Why?

The fluorescent lamp is 30%  efficient but filament lamp is 10% efficient. Hence, fluorescent lamp of power 60 W is more brighter than filament lamp of power 60W.

Q. Write any four differences between Filament and fluorescent lamp.

Q. Why is inert gases like Nitrogen, Neon, Argon etc is filled in filament lamp?

Inert gases like Nitrogen, Neon, Argon etc is filled in filament lamp to prevent from vapourization of tungsten filament with oxygen in high temperature.

Magnetic effect of electricity: When electricity is passed through a conductor wire, magnetic field is developed around it. This effect of electricity is called magnetic effect of electricity.

Solenoid: The insulated wire adjusted in the form of coil is called solenoid.

Electromagnet: When electricity is passed through solenoid, it behaves as a magnet which is called electromagnet.

Applications of electromagnet

1. Electric bell
2. Speaker
3. Microphone
4. Electric motor etc

Electric bell

The device that converts electrical energy into sound energy is called electric bell.

Q. On which effect of electricity is electric bell based ?

Electric bell is based on magnetic effect of electricity.

Structure of electric bell

Magnetic lines of force: The imaginary curve lines emerged from North pole of a magnet which enter the south pole are called magnetic lines of force

Magnetic flux: The magnetic lines of force that fall on conductor kept in magnetic field is called magnetic flux.

Faraday’s law of electromagnetic induction

When relative motion is produced between magnet or coil kept in magnetic held, the magnetic flux through coil changes and an emf is induced in coil. This is called Faraday’s law of electromagnetic induction.

Devices based on electromagnetic induction:

1. Dynamo
2. Generator

Dynamo/Generator

The device that converts kinetic energy into electrical energy is called dynamo/generator.

Q. What happens to the brightness of bulb in cycle with dynamo when its speed is increased or decreased. why?

When speed of cycle is increased, the brightness of bulb also increases because more amount current is produced in dynamo. But, when speed of cycle decreased, the brightness of bulb also decreases because low amount of current is produced in dynamo

Q. What happens to brightness of bulb when cycle is stopped. why?

When cycle is stopped the bulb does not glow because no current is produced in dynamo

Q. Differentiate between generator and dynamo.

Methods of increasing amount of current by dynamo/generator

1. by increasing the speed of dynamo / generator.
2. by increasing strength of magnet.
3. by increasing the turns of coil.
4. by decreasing distance between magnet and coil.

Q. Differences between AC and DC

Motor effect of electricity: When electricity is passed through a coil kept in magnetic field, the motion is produced either in coil or in magnet. This is called motor effect of electricity.

Motor: The device that converts electrical energy into kinetic energy is called motor.

Q. Differentiate between motor and generator.

Inverter: The device that converts AC into DC and again DC into AC is called inverter.

Working of inverter:

When there is AC supply, the inverter converts AC voltage into DC voltage and charges the battery. When power is cut off the inverter now changes DC voltage of battery into AC voltage so that electric appliances work. In this way, inverter works.

Charger: The device that charges the battery of electric devices is called charger.

Transformer: The device that converts low AC voltage into high AC voltage and high AC voltage into low AC voltage is called transformer. Types of transformer:

1. Step down transformer: The device that converts high AC voltage into low AC voltage is called step down transformer. This transformer is used in radio, TV, electric devices etc

1. Step up transformer: The device that converts low AC voltage into high AC voltage is called step up transformer.

a. What is core of transformer made of?

The core of transformer is made of by joining ‘L’ or ‘U’ shaped iron stripes.

b. What is lamination in transformer?

The process of making core of transformer by joining ‘L’ or ‘U’ shaped iron stripes is called lamination in transformer.

c. What is used in lamination in transformer?

Shellac or varnish is used in lamination in transformer.

d. Why is lamination done in transformer?

Lamination is done in transformer to prevent from heating effect due to eddy current

Laws of transformer

Input energy is equal to the output energy. The voltage is directly proportional to the no. of turns of coil.

Transformer ratio

Output voltage /  Input voltage = No. of tune in secondary coil / No. of turns in primary coil

V₂ / V₁ = N₂ / N₁

Q1. There ore 1000 turns in primary coil of transformer in which 220V AC is supplied. How many turns in secondary coil should be made to obtain 55 V AC output?

N₁ = 1000 turns

V₁  = 220 V

V₂  = 55 V

N₂ = ?

We have,

V₂ / V₁ = N₂ / N₁

55/220 = N₂ / 1000

N₂ = 250 turns

Hence, 250 turns should be made in secondary coil.

Q2. The no. of turns in primary coils is 3 times the no. of turns in secondary coil. If 220 V AC is supplied to transformer, find output voltage.

N₁ = 3x

V₁  = 220 V

V₂  = ?

N₂ = x

We have,

V₂ / V₁ = N₂ / N₁

V₂/220 = x/ 3x

V₂ = 73.33 Volt.

Safety measures in using electricity:

1. Good quality wire with proper insulation should be used.
2. Proper insulation should be done with tape in joints and naked wire.
3. Color coding of wire should be used.
4. Electric wiring should not be done in wet and damp place.
5. Fuse of proper capacity should be used.
6. Fuse and switch should always be kept in live wire.
7. Switch shouldn’t be touched with wet hands.

Electric power: The rate at which an electric device converts electrical energy into other forms of energy is called electric power.

Q. The power of an iron is 500W. what does it mean?

It means that an iron converts 500 Joule of electrical energy into heat energy in 1 second.

Power consumption (unit): The electrical energy consumed by an electrical device of certain power in certain time is called power consumption. Its unit is Kilowatt hour (Kwh).

1 kwh: The electrical energy consumed by a device of 1 kw power in 1 hour time is called 1 kwh.

Formula of power consumption:

Power consumption = PxNxT where,

P = Power in Kw

N = Number of device

T = time in hour

Q1. In a house, 10 bulbs of 100 watt are used for 6 hours daily, a rice cooker of 500 watt is used for 2 hrs daily and two TVs of 50 watt for 7 hrs daily. Calculate units consumed in a month.

For bulbs,

P = 100W = 100/1000 kw = 0.1 kw

N = 10

T = 6 hrs

Units in a day = PxNxT = 0.1x10x6 = 6 units

For rice cooker,

P = 500 W = 500/1000 kw = 0.5 kw

N = 1

T = 2 hrs

Units in a day = PxNxT = 0.5x1x2 = 1 units

Unit in a daya PXNXT

For TV

P = 80W= 80/1000 kW  = 0.08 kw

N = 2

T = 7 hrs

Unit in a day = PxNxT = 0.08x2x7 = 1.12 unit

Total unit in a day = 6+1+1.12 = 8.12 unit

Total unit in a month = 8.12×30 = 243.6 units

Hence, total units consumed in a month is 243.6 units.

Q2. In a house, 10 tube lights of 40 watt are used for 3 hours daily, 4 tv of 40 watt are used for 4 hrs daily, 3 iron of 800 W are used for 2 hours a week and 3 bulbs of 100 Watt are used for 3 hrs daily. Calculate total unit consumed in a month. Also find the amount to be paid at the rate of Rs 8 per unit.

For tubelights,

P = 40W = 40/1000 kw = 0.04 kw

N = 10

T = 3 hrs

Units consumed in a day = PxNxT = 0.04x3x10 = 1.2 units

For tv,

P = 40 W = 40/1000 kw = 0.04 kw

N = 4

T = 4 hrs

Units consumed in a day= PxNxT = 0.04x4x4 = 0.64 units

For iron,

P = 800W= 800/1000 kW  = 0.8 kw

N = 3

T = 2 hrs

Unit consumed in a week = PxNxT = 0.8x2x3 = 4.8 unit

Unit consumed in a month = PxNxT = 4.8×4 = 19.2 unit

For bulb,

P = 100 W = 100/1000 kw = 0.1 kw

N = 3

T = 3 hrs

Units consumed in a day= PxNxT = 0.1x3x3 = 0.9 units

Total units consumed in a day (except iron) = 0.64+0.9+1.2 = 2.74 unit

Total units consumed in a month (with iron) =  2.74×30+19.2 = 101.4 unit.

Now, Amount to be paid = Rs 8×101.4 = Rs 811.2

Hence, total unit consumed in a month is 101.4 unit and amount to be paid is Rs 811.2.

Q3. Study and answer

i. What type is transformer is it? How do you know it?

It is step down transformer. I know it because secondary coil has less no. of turns.

ii. What is its function? Where is it used?

It converts high AC voltage to low AC voltage. It is used in substation and in electrical appliances like radio, TV etc.

iii. What will happen if it is connected in reverse order?

It will become step-up transformer.

Q4. The metre reading of a house in the beginning of Baisakh is 1032 units. What will be metre reading on 30th of Baisakh, if 4 bulbs of 60 watts are used for 4 hours every day, 4 fans of 75 watts for 11 hrs everyday.

Here, Meter reading at beginning of Baisakh = 1032 units

For bulb,

P = 60 W = 60/1000 kW  = 0.06 kw

N = 4

T = 4 hrs

Unit consumed  = PxNxT = 0.06x4x4 = 0.96 unit

For fan,

P = 75 W = 75/1000 kW  = 0.075 kw

N = 4

T = 11 hrs

Unit consumed  = PxNxT = 0.075x4x11 = 3.3 unit

For rice cooker,

P = 750 W = 750/1000 kW  = 0.75 kw

N = 1

T = 1 hrs

Unit consumed  = PxNxT = 0.75x1x1 = 0.75 unit

Total unit consumed in a month = 30(0.75+0.96+3.3) = 150.3 unit

Total units = 1032 + 150.3 = 1182.3 units

Cost of electricity = Rs 15×1182.3 units = Rs 17734.5

Therefore, metre reading will be 1182.3 unit and total cost will be Rs. 17734.5

Q5. The ratio of primary and secondary windings of transformer is 12:1. Calculate input voltage to obtain 11KV output voltage.

N₁ = 12x

V₁  = ?

V₂  = 11x1000V = 11000 V

N₂ = x

We have,

V₂ / V₁ = N₂ / N₁

11000/V₁ = x/12x

V₁ = 11000×12 = 132000 V

Therefore, input voltage is 132 kV

Q6. In a house 10 lamps of 60 watt each are used for 6 hr daily, 2 heaters of 1000 W are used 3 hr daily and 2 irons cf 5000 W are used 3 hrs for 5 days. Calculate energy consumed in 1 month and cost of electricity used.

For lamp,

P = 60 W = 60/1000 kW  = 0.06 kw

N = 10

T = 6 hrs

Unit consumed  = PxNxT = 0.06x6x10 = 3.6 unit

For heater,

P = 1 kw

N = 2

T = 3 hrs

Unit consumed  = PxNxT = 1x2x3 = 6 unit

For iron,

P = 5000 W = 5000/1000 kW  = 5 kw

N = 5

T = 3 hrs

Unit consumed  = PxNxT = 5x3x5 = 75 unit

Unit consumed in month = 4×75 = 300unit

Total unit consumed in a month = 300 + 30(3.6+6) = 588 unit

Cost of electricity = Rs 10×588 units = Rs 5880

Therefore, energy consumed is 588 unit and cost of electricity is Rs 5880.

Q7. In a house, five lamps of 15 watt are used 6 hr and 2 electric kettles of 1500 W are used for 30 minutes daily for 20 days. Calculate total electricity consumed and total cost of electricity at rate of Rs 15 per unit

For lamp,

P = 15 W = 15/1000 kW  = 0.015 kw

N = 5

T = 6 hrs

Unit consumed  = PxNxT = 0.015x5x6= 0.45 unit

For heater,

P = 1500 w = 1500/1000 kW  = 1.5 kw

N = 2

T = 30/60 hrs =0.5 hrs

Unit consumed  = PxNxT = 1.5×0.5×2 = 1.5 unit

Unit consumed in month = 30(0.45+1.5) = 39 unit

Total cost of electricity = Rs 15×39 = Rs 585

Therefore, energy consumed is 39 unit and cost of electricity is Rs 585.

Short answer questions

a. What is fluorescent material?

Codmium silicate

b. What is the purpose of using inverter at home?

Purpose of using inverter at home to run electrical appliances though the power is cut off.

c. Why should electric appliances be earthed?

Electric appliances should be earthed to present from electric accidents when there is leakage of electric current in metallic part of electric device,

d. How does a fluorescent lamp produce visible light?

When electricity is passed through a fluorescent lamp, it produces UV rays due to mercury vapour in it. UV rays thus produced strikes fluorescent powder. So that it converts into visible lights.

e. Why is a step up transformer used in power house?

The step up transformer is used in a power house to convert low AC voltage into high AC voltage. So that loss of energy is prevented when electric current is transmitted to greater distances.

f. Fuse and switch should be kept in live wire. why?

Fuse and switch should be kept in live wire in order to prevent from electrical accident and electrical damage when there is overload in current or switch is off.