electricity - physics

The rate of work, or the rate of dissipation of energy, is called electric power. It is denoted by P, i.e.,

Here, I is the current and V 1s the potential difference.

Units of electric power:

Electric power is measured in watt (W). If V is measured in volt and I in ampere, then

1 watt = 1 ampere x 1 volt

The power expended by a source is said to be 1 watt if one ampere of current flows through it under a potential difference of 1 volt.

We define the electric potential difference between two points in an electric circuit carrying some current as the work done to move a unit positive charge between the two points.

Potential difference (V) between two points = Work done (W)/Charge (Q)

If a work W is done in bringing a charge Q from infinity to a point in an electric field, then potential at that point is given

V = W / Q

Since work is measured in joule and charge in coulomb, therefore, electric potential is measured in joule per coulomb (J c-1). This unit occurs so often in our study of electricity, that it has been named as volt, in honour of the scientist Alessandro Volta (the inventor of the Voltaic cell). Thus,

1 Volt = 1 joule / 1 coulombElectric energy is defined as the total amount of work done to maintain current in a circuit in a given time.

Therefore,

W = VIt joule

Since VI = P, therefore, the above expression can be written as

W = Px t

Electrical energy consumed in a circuit is measured in kilowatt hour (kW h) or Board of trade unit (B.O.T.U.). It is defined as the amount of work done when a power of one kilowatt is consumed for one hour.

1 kW h = 1 kW x 1 hour

= 1000 W X 3600 S

= 1000 J S-l X 3600 S

= 3.6 X 106 JThe electricity generated by a power plant is usually alternating (ac). Its voltage is 11000 volt to 22000 volt. The power stations are located far away from the cities and industrial area and hence transmission of electricity is done with the help of high tension cables by stepping up the voltage up to 300000 - 200000 volt. Before distribution the voltage and electric voltage is brought down to 11000 V, 220 V or 220 Vas per the requirement using step down transformers. At our home we receive 220 V ac.

The electric energy consumed in our houses is measured by an electric meter fixed at a convenient location in the house. This meter now a days is digital in nature (Fig. 8.2). The meter reads the electric energy consumed in kW h also called the Board of trade unit. An electric meter is connected in series in the household circuit.

Different electrical appliances in the house consume different electric energy. The values of some of these are shown in table be

low.The electric energy consumed and its cost is calculated by using the following procedure:

1. Calculate the total power consumed. This is found out in watt.

2. Now, the power is converted into kilowatt by dividing the power consumed by 1000.

3. This power consumed in kilowatt is multiplied by the time in hours for which the power is used.

4. This gives the electric energy consumed in kilowatt hour.

5. This is then multiplied by the cost of one unit of electric energy to give the total cost of electricity consumed.

Mathematically, we can express this as

Total Cost

= Power (W) x t (hour) / 1000 X cost of one unit

Suppose an air conditioner of rating 3 kW, 220 Vis used for 6 hours daily for a month consisting of 30 days, then the electric energy consumed by the air conditioner in one day is

E = P x t = 3000 x 6 = 18,000 W h

or 18 kWh or 18 units

The total units of electric energy consumed in 30 days is

Energy consumed = 18 kWh x 30 = 540 kWh

If electric energy costs ₹ 2.50 per unit, then the expenditure incurred for the month is = 540 X 2.50 = ₹1350In a household circuit, three cored cables are used. Each core cable is a copper wire capable of withstanding about 15 A of current. Normally three coloured cables are used. In the old colour convention the red wire is the live wire (phase), black wire is the neutral wire and the green is earth wire. In the new colour convention Brown, called live wire (phase), Light blue, called neutral wire and Green or yellow, called earth wire. The earth wire is for the safety of the user and saves from electric shocks.

The first part of the house wiring system consists of the supply of electrical energy by the electrical company to the premises of the

consumer. The company supplies electric power through overhead or underground cables to the meter box.

The power is supplied through a cable consisting of three wires;

(i) live or phase wire, (ii) neutral wire and (iii) the earth wire.

The neutral and the ground wires are at the same potential as that of the earth at the local substation, i.e., they are at zero potential.

Live wire: The live wire carries current to the appliance at a high voltage.

Neutral wire: The neutral wire completes the circuit and carries current away from the appliance.

Ground wire: The third wire, called the earth wire (green/yellow) is a safety wire and connects the metal case of the appliance to the earth. This stops a fault making the case of the appliance live.

The earth wire, which has insulation of green colour, is usually connected to a metal plate deep in the earth near the house. This is used as a safety measure, especially for those appliances that have a metallic body, for example, electric press, toaster, table fan, refrigerator, etc. The metallic body is connected to the earth wire, which provides a low-resistance conducting path for the current. Thus, it ensures that any leakage of current to the metallic body of the appliance keeps its potential to that of the earth, and the user may not get a severe

electric shock.

Fuse: An electric fuse is a short length of easily fusible wire put into an electrical circuit, in series, for protection purposes. It is arranged to melt (''blow") at a definite current. It is an alloy of lead and tin (63 % tin+ 37 % lead). It has a high resistivity and low melting point. As soon as the safe limit of current exceeds the fuse "blows" and the electric circuit is cut off.

A fuse is inserted with each gadget which needs protection, it is inserted in modern power plugs and it is of course there in each branch circuit and at the main supply input point to provide safety everywhere. It is connected to the live wire. These are essential for the safety of the person and building as well as for the protection of the electrical gadgets in use. Whenever there is high current (in excess of some predetermined value) the fuse blows and that part of the circuit is turned off. This prevents damage to the gadget and any fire that could have resulted from overheating due to excess current.

There are different types of fuses. The older is made of two metal clamps fixed on a porcelain base with a grove in between (Fig. 8.5(a)). The fuse wire is connected in between the metal clamps which are inserted in the appropriate section of the circuit. These days costly appliances are fitted with a cartridge type fuse (Fig. 8.5(b)). It consists of a length of fuse wire connected to metals caps at the end of a short glass tube.

Nowadays Miniature circuit breakers (MCB's) are being utilised for lighting circuits (Fig. 8.6).

Fuses blow off due to the following reasons:

1. Short circuit due to worn out insulation on connecting wires.

2. Overloading of an electrical circuit.

Grounding or earthing: By earthing or grounding we mean the connecting of a device (or a system) to the earth. To ensure good contact with the earth, a large metal plate is buried under the earth at a depth where the soil is moist. The plate is surrounded by a mixture of charcoal and common salt to ensure good contact with the wet soil and hence the ground. The earthing wire is usually bare and thick. It provides a safe and easy path for the electric charge to flow down to the earth which acts as a very large sink.

Earthing the circuit or appliance is a very important safety precaution. If the metal casing of the appliance is connected to the earth with the help of a conductor, the metal casing will be then at the same potential as the earth, i.e., zero volt. The metal cases of all electrical appliances used in the home, such as room heater, electric stove, electric iron, are connected by the earth cable E to the earth. In case, the insulation inside the appliance breaks down, or the live wire becomes loose and touches the case (body of the appliance), the case becomes 'live' and the user gets a severe shock, if the case is not earthed. Earthing provides a low resistance path for the current thus allowing the excessive current to flow to the earth and not through the body of the person using the appliance.

The power line used for operating high power gadgets such as oven, heater etc. has in addition to the two line wires - live (L) and neutral (N) - a third wire E which is the earthing wire. It is connected to the third terminal post. In American English, the word 'grounding' is used for earthingA number of simple experiments can be performed to demonstrate the existence of electric charge and forces. For example, after running a comb through your dry hair, you will find that the comb will attract small bits of dry paper. The attractive force is often strong enough to suspend the pieces of paper as shown on the title figure. The same effect occurs with other rubbed materials, such as glass and rubber or plastic. Another simple experiment is to rub an inflated balloon with wool. The balloon will then stick to the

wall or the ceiling of a room, often for hours. When materials behave in this way, they are said to be electrified, or to have some electrically charged.

In a systematic series of rather simple experiments, one finds that there are two and only two kinds of electric charges, which were given the names positive and negative by Benjamin Franklin (1706-1790).

To demonstrate this fact, let us consider a hard rubber rod that has been rubbed with fur or cat skin and a glass rod that has been rubbed with silk. If two charged rubber rods (or two charged glass rods) are brought near each other, as in Figure 8.8(a), the force between them will be repulsive. On the other hand, when a glass rod that has been rubbed with silk is brought near the rubber rod, the rubber rod is attracted towards the glass rod as in Figure 8.8(b).

This observation shows that the rubber and glass are in two different states of electrification.

On the basis of these observations, we conclude that

"Like charges repel one another and unlike charges attract one another".

This statement is sometimes termed as the law of charges.

Using the convention suggested by Franklin, the electric charge on the glass rod is called positive, and that on the rubber rod is called negative. Therefore, any charged body that is attracted to a charged rubber rod (or repelled by charged glass rod) must have a positive charge. Conversely, any charged rod that is repelled by a charged rubber rod (or attracted to a charged glass rod) has a negative charge on it.

The choice of positive and negative charge is purely arbitrary and positive and negative are just symbols used to denote the charge on glass rod and rubber rod.

Thus, we can conclude that,

1. There are only two types of charges. The type on the glass rod rubbed with silk and the type on the rubber rod rubbed with fur.

2. The bodies having a charge similar to the charge on the glass rod rubbed with silk are said to be positively charged.

3. The bodies having a charge similar to the charge on the rubber rod rubbed with fur are said to be negatively charged.

4. Unlike charges attract each other while like charges repel each other. This is call

ed the charge-force law.The net charge of an isolated system remains constant. The only way to change the net charge of a system is to bring in charge from elsewhere, or remove charge from the system.

The conservation of charge simply states that electric charge can neither be created nor destroyed.

In other words, if an object (or part of an object) gains charge, another object (or part of an object) must lose charge.

For example, if you rub a comb through your hair, it will become negatively charged. The only way for this to happen is for your hair to also become positively charged. So if the comb's charge becomes -1 mC, the charge in your hair must become + 1 mC.CONDUCTION: Conduction just means that the two objects will come into actual physical contact with each other (this is why it is sometimes called "charging by contact").

Let's assume we have a negatively charged metal object and an uncharged metal object (Fig. 8.9(a)). They are similar objects, and each is on an insulating stand so that we can move them around without them interacting with anything else. We bring the two objects close together. We will see a separation of charge happen in the neutral object as negative electrons are repelled to the right hand side (Fig. 8.9(b)). At this time, they are not touching and no charges have been transferred. We allow the two objects to touch (Fig. 8.9(c)). Some of the negative charge will transfer over to the uncharged metal object. This happens since the negative charges on the first object are repelling each other by moving onto the second object they spread away from each other. When the negative object is removed, it will not be as negative as it was (Fig. 8.9(d)).Both of the objects have some of the negative charge this depends on the size of the objects and the materials they are made of. Since they are similar in this example they have the same magnitude charge. Overall the total negative charge remains constant. We started with six negative charges, and we ended up with a total of six negative. Notice that this also means a negative object causes a negative charge on the other object.

INDUCTION: It is possible to charge a conductor without touching it. You do have to follow some special procedures. Most important is the use of a grounding wire. A grounding wire is simply a conductor that connects the object to the ground. Think of the earth as a huge reservoir of charge. It can both gain or donate electrons as needed. Depending on what the situation is, either electrons will travel up the grounding wire to the object being charged, or travel down to the ground.

Figure 8.l0(a): The neutral object is on an insulating stand. It also has a ground wire attached to it.

Figure 8.l0(b): We bring a negative object nearby. This will cause the electrons to be pushed as far away as possible, and since they are free to move, they do just that. They will travel down the ground wire.

Figure 8.l0(c): This step is very important. Keeping the negative object nearby we snip the ground wire. Now, there is no way for the electrons to travel back up the wire to the originally neutral object. If we had skipped this step and just moved the negative object away without snipping the ground wire, the negative charges would have just gone back up the wire and it would be neutral again.

Figure 8.l0(d): We remove the negative object. Now, the original object has a net positive charge.

The same sort of thing happens if you bring a positively charged object near to a grounded object. In that case, electrons would come up the grounding wire to be closer to the object. This would leave the other object with a negative charge. Notice that when you charge by induction you get the opposite charge on t

he metal object.

.An electroscope is an instrument used for detecting the presence and the sign of electric charge on a body. One such arrangement is called a Gold leaf electroscope. It was invented by Abraham Bennet, in 1787.

An electroscope is used for the following two purposes;

1. To detect whether a body has charge or not, and

2. The type of charge, i.e., positive or negative, that a charged body possesses.

8.10.1 Construction

An electroscope consists of a metal rod called stem, with a metal ball or bulb, usually made of brass, on top and two very light foils made of gold, attached to the bottom. Sometimes one leaf is fixed and the other is movable. Brass is used to prevent the corrosion of the metallic contact. The arrangement is enclosed in a glass enclosure and is protected from it with the help of a rubber cork. (Fig. 8.11). It is enclosed in a glass case so that the gold leaves are protected from the air. This also helps in capturing the charge leak through the air so that the sensitivity of the instrument can be i

ncreased.An electroscope can be used to determine whether an object is electrically charged or not. When a charged object is brought near the metal ball, the leaves separate. This is depicted in Figure 8.12(b).

The electroscope is a device that can be used to demonstrate the characteristic of electric charge (Fig. 8.11). When charged objects are brought close to the bulb, electrons in the bulb are either attracted or repelled, according to the charge force law. For example, if a negatively charged rod is brought near the bulb, electrons are repelled, and the bulb is left with a positive charge. The electrons are conducted to the metal foil leaves, which separate because of a mutually repulsive electric force between them (Fig. 8.12). Similarly, if a positively charged rod is brought near the bulb, the leaves also diverge. (Can you explain, why?)

Notice that the net charge of the electroscope remains zero in these cases because it is isolated (insulated) - only the distribution of charge is changed. However, it is possible to give a net positive or negative charge to an electroscope by electrostatic charging.

The degree of divergence of gold leaves in a gold leaf electroscope is an indicator of the amount of charge that is transferred to the gold leaves.

The following results hold for an elect

roscope.

An electroscope can be charged in two ways;

1. By Conduction and

2. By Induction.

8.12.1 Charging by Conduction

Electrons can be transferred from one metal body to the other if they are touched. This is made use of, in charging an electroscope by conduction as illustrated in Figure 8.13.

When a rubber rod is rubbed with fur it attains a negative charge. Suppose this rod is brought in contact with the metal ball of the electroscope. The electrons in the rod are mutually repelled by one another. Some will be able to move onto the electroscope. Thus, the electroscope will attain a negative charge. In this case we say that the electroscope has been charged by conduction. Conduction refers to the flow of charge for a short time. Since both the leaves of the electroscope will now have a negative charge, therefore, they will repel each other and will open up as shown in Figure 8.13 (b).

It should be remembered that an electroscope can also be given a positive charge. This can be done by bringing a positively charged rod in contact with the metal ball. Also, in charging by conduction the same charge is attained by the electroscope as that on the charging rod.

8.12.2 Charging by Induction

Another way of charging an electroscope is by induction. Induction is the process in which an uncharged insulated metallic conductor gets electrically charged when a charged conductor is brought near it. The charge on the uncharged conductor is equal and opposite of that on the charged conductor.

When an electroscope is touched with a finger, the electroscope is grounded, that is the finger provides a path for the electrons to can escape to the earth from the metal ball.

Now, when a negatively charged rod is brought close to the metal ball, without touching it, the rod repels electrons from the metal bulb into the finger and down into the Earth (Fig. 8.14(a)). Removing the finger without removing the charged rod (Fig. 8.14(b)) will leave a net positive charge on the electroscope, because the removed electrons have no way of flowing back into the electroscope once the ground path is removed.

This leaves a net positive charge on the electroscope. Thus, the electroscope has been charged by induction.

It should be remembered that an electroscope can also be given a negative charge. This can be done by bringing a positively charged rod near the metal ball. Also, in charging by induction the equal and opposite charge is attained by the electroscope as that on the charging rod.

The gold leaves in an electroscope fold back when they are touched with hands because the charge is earthedEverybody has seen lightning and has probably been frightened by this spectacular display of energy at sometime or other.

Lightning is the spectacular bluish white light produced in the clouds.

Lightning is unpredictable, and seems to occur randomly and instantaneously. Everything but the thunder is usually over in less than half a second.

Benjamin Franklin was the first to demonstrate the electrical nature of lightning. He performed an experiment with a kite made of silk which he flew during a thunderstorm. The central spar of this kite was a 30 cm long iron wire. It was connected to a strong silk thread, the other end of which was connected to an iron key.

Benjamin brought his knuckles near the lower end of the key, but nothing happened. However, when it started raining, he again brought his knuckles near the lower end of the key. This time, he drew sparks to his knuckles from the key. This happened because on being wet the silk thread allowed electricity to flow from the clouds to the key, then to the knuckles and finally through his body into the earth.

Benjamin was lucky that he was not electrocuted.

The following safety measure should be taken during a lightning storm.

1. Stay indoors and away from open windows.

2. Avoid open areas and stay away from isolated trees, towers, or electrical poles as lightning tends to strike the tallest object in an area.

3. Disconnect all electronic devices.

4. Do not use wired devices.

5. Water pipes also conduct electricity, so taking a shower or using the plumbing must be avoided during a storm.

6. While outside, take shelter in vehicles with their windows closed.

7. Stay away from conductors such as metallic fences and wires. These objects act as conductors of lightning.

8. Install lightning conducto

rs on tall buildings.

It is a device used to protect a building from damage by lightning during a thunderstorm.

A lightning conductor reduces the possibility of lightning or prevents lightning by neutralising the charge on the clouds with opposite charge sent up from its pointed spokes at the top of the building. In spite of this if lightning strikes, it provides a safe path for the discharge to flow down to the earth below, leaving the building unaffected and safe.

Description: A lightning conductor consists of a thick copper strip on the outside of the building which connects metal spikes at the top to a metal plate in the ground as shown in Figure 8.15. The metal plate (usually copper) is buried deep in wet soil to ensure good conductivity between the plate and the earth.

The lightning strikes the lightning conductor which, being made of copper, allows the entire discharge from the clouds to safely flow into the ea

rth.A cell is an arrangement in which chemical energy is converted into electrical energy. In a cell chemicals are stored. When used, the chemical reactions occurring in the cell produce electricity. Cells are of two types:

(i) Primary and

(ii) Secondary.

A primary cell is one which once used cannot be reused, whereas a secondary cell is one which can be used again and again by re-charging.

A cell is used to operate small devices like torches, toys, radio, TV remote and small tape recorders. Cells come in a number of shapes and sizes. Some of these shapes and sizes are shown on the title page. A special cell called a button cell is nowadays used in calculators and watches.

8.15.1 A Battery

A cell produces very small current and voltage. In order to obtain a large voltage and current, cells are connected one after the other, i.e., in series. This combination of cells is called a battery. A battery can provide a large current and a large voltage. A battery is as shown in fi

gure 8.16.

Dangers of electricity include a variety of hazards that include Electric Shock, Psychological Damage, Physical Burns, Neurological Damage and Ventricular fibrillation resulting in death.

The risks inherent with electric power can generally be divided into two categories:direct and indirect. The direct danger is the damage that the power itself can do to the human body, such as stoppage of breathing or regular heartbeats, or burns. The indirect

dangers of electricity include the damages that can result to the human body as a result of something caused by electric shock, such

as a fall, an explosion, or a fire.

Electricity at any voltage can be dangerous and should always be approached with caution. An electric shock can occur upon contact of a human or animal body with any source of voltage high enough to cause sufficient current flow through the muscles or nerves. The minimum current a human can feel is thought to be about 1 milliampere (mA). As little as 80 milliampere, can seize the heart muscle. The current may cause tissue damage or heart fibrillation if it is sufficiently high. A fatal electric shock is referred to as

electrocution.

Burns: Dangers of electricity include physical burns.

Ventricular fibrillation: A low voltage (110 to 220 V), 50 or 60 Hz AC current travelling through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 60 mA. With DC, 300 to 500 mA is required. If the current has a direct pathway to the heart (e.g., via a cardiac catheter or other kind of electrode), a much lower current of less than 1 mA, (AC or DC) can cause fibrillation. Fibrillations are usually lethal because all the heart muscle cells move independently. Above 200 mA, muscle contractions are so strong that the heart muscles cannot move at all.

Neurological effects: Other Dangers of Electricity cause interference with nervous control, especially over the heart and lungs. Repeated or severe electric shock which does not lead to death has been shown to cause neuropathy.

When the current path is through the head, it appears that, with sufficient current, loss of consciousness almost always occurs swiftly.It goes beyond doubt that electricity plays a very important role in our lives. It is one of the most important and a convenient source

of energy at homes and industries. However we daily come across of reports regarding hazards of electricity. Use of electricity can

prove to be very dangerous if certain precautions and safety measures are not observed in the design of electrical devices and in handling them. The precaution must be observed.

1. Use wires of high quality, proper amperage and good insulating material.

2. Cover all naked wires and joints with insulating tape.

3. All connections at plugs, switches, sockets must be tight.

4. Replace any defective plugs switches and sockets.

5. Never touch any part of the circuits without rubber shoes or rubber gloves.

6. Use fuse (now-a-days MCB) of proper rating and material.

7. All electrical appliances must be properly earthed.

8. Connect switches and fuse to live wires.

9. Put out the main switch in case of short circuiting and fire.

10. Do not use water as fire extinguishers in case of fire due to electricity.