Electricity is of utmost importance in the modern world. We depend on it for almost everything in our day to day life. In order to avoid the inconvenience faced due to failure of power supply, hospitals, banks, offices and private institutions make alternative arrangements with the help of generators. Electricity is used for running electric furnaces, electric motors and several other instruments used in industries.
Domestic appliances like the fridge, electric oven, mixer, fans, washing machines, vacuum cleaner, rotimaker, etc. have helped us by saving time and labour. All these devices cannot be run without electricity.
Not only human beings but some animals also use electricity. For example, fishes such as eels use electricity to catch their prey and also for self-defence. The lightning that strikes the earth is an excellent example of natural flow of electricity. What if we could collect and store this electricity!
For the generation of electricity, water is released from a dam at a higher level and because of gravity, it falls to a lower level. Thus, as we know, the direction of flow of water between two points depends on the level of the two points.
Potential and potential difference
Equipment : Two plastic bottles, rubber tube, clamp, water.
Procedure : Set up the experiment as shown in figure 3.1. Then remove the clamp from the rubber tube. Note your observations. Answer the following questions.
- What happens when the clamp is removed?
- Does the water stop flowing? Why?
- What will you do to keep the water flowing for a longer duration? Just like water, the flow of electric charge between two points depends on a kind of electric level at those points. This level is called electric potential.
A positive charge flows from a point of higher potential to a point of lower potential. We have seen earlier that electricity flows due to the conduction of negatively charged electrons. Electrons flow from the point of lower potential to a point of higher potential. A lightning strike is the flow of electrons from point of lower (negative) potential on the clouds to the point of higher (zero) potential on the earth. We shall study the definition of electric potential in higher standards.
The difference between the values of potentials at two points A and B is called the potential difference between them. In the figure 3.2, conductor A is at a higher potential than conductor B. When these two are connected by a conducting wire, a potential difference is created between its two ends and electrons will flow from B to A through the wire. This flow will continue until the two conductors, A and B have the same potential, i.e. until their potential difference becomes zero. Only then will the flow of electrons stop.
Work has to be done against the electric field to take a positive charge from a point of lower potential to a point of higher potential.
Potential difference of a cell
The difference in potential between the positive and negative terminals of a cell is the potential difference of that cell. This potential difference is caused by chemical reactions occurring inside the cell. The potential difference sets the electrons in motion and results in the flow of electricity through a conducting wire connected to the two ends of the cell. The amount of work done to carry a unit positive charge from point A to point B is called the electric potential difference between the two points.
An introduction to scientists
The Italian scientist Alessandro Volta constructed the first electric cell. The unit of potential difference is named ‘volt’ in his honour.
Free electrons : Every atom of a metallic conductor has one or more outermost electrons which are very weakly bound to the nucleus. These are called free electrons. As shown in figure 3.3, these electrons can easily move from one part of a conductor to its other parts. The negative charge of the electrons also gets transferred as a result of this motion. The free electrons in a conductor are the carriers of negative charge.
Current flowing through a wire
As shown in the figure 3.4 A, if a conducting wire is not connected to a cell, its free electrons move randomly in all directions in the space between the atoms. When we connect the ends of the wire to the two terminals of a cell, electric force acts on the electrons. Being negatively charged, they start moving from the negative (lower potential) to the positive (higher potential) terminal of the cell, as shown in figure 3.4 B. Due to the flow of these electrons, current starts to flow through the wire. This motion of electrons is irregular but there is a definite, non-zero value to their average velocity.
An electric current is the flow of electrons through a conductor. Quantitatively, current (I) is defined as the charge passing through a conductor in unit time. If charge Q is flowing through cross-section of a conductor in time t then the
current = I = Q /t
The unit of charge in SI units is Coulomb (C). Current is expressed in Ampere (A). The charge of one electron is 1.6 x10-19 C.
Ampere : One ampere current is said to flow in a conductor if one Coulomb charge flows through it every second.
Resistance and Ohm’s Law
The relationship between the current flowing through a wire (I) and the potential difference across its ends (V) can be obtained from the law that was given by the German scientist George Simon Ohm.
If the physical state of a conductor remains constant, the current (I) flowing through it is directly proportional to the potential difference (V) between its two ends.
Resistance and resistivity of a conductor
As shown in figure 3.4, there are a large number of free electrons in a conductor. They are constantly in random motion. When a potential difference is applied between the two ends of the conductor, these electrons start moving from the end at lower potential to the end at higher potential. This directional motion of the electrons causes the flow of current. Moving electrons strike the atoms and ions which lie along their path. Such collisions cause hindrance to the flow of electrons and oppose the current. This hindrance is called the resistance of the conductor.
Resistivity : At a given temperature, the resistance (R) of a conductor depends on its length (L), area of cross-section (A) and the material it is made of.
ρ is the constant of proportionality and is called the resistivity of the material. The unit of resistivity in SI units is Ohm metre . Resistivity is a specific property of a material and different materials have different resistivity
German physicist, George Simon Ohm established a law for measuring the resistance of a conductor. In his honour, the unit of resistance is called the Ohm.
A continuous path of an electric current through conducting wires connected to the two ends of a cell and other resistances is called an electric circuit. A circuit is depicted by a figure. This figure shows how different components are to be connected in the circuit, by using special symbols for each of the components. Such a figure is called an electric circuit diagram.
In the circuit in figure 3.5, an ammeter is used to measure current and a voltmeter to measure the potential difference between the two ends of a resistor. As the voltmeter has a very high resistance, only very small current flows through it.
Conductors and insulators
We have learnt about the resistance to an electric current. We can divide substances into conductors and insulators (bad conductors).
Conductors : Those substances which have very low resistance are called conductors. Current can flow easily through such materials.
Insulators : Those substances which have extremely high resistance and through which current cannot flow are called insulators.
- Why are some substances conductors while others are insulators?
- Why can our body conduct electricity? Make a list of conductors and insulators you see around you.
Experimental proof of Ohm’s law :
Material : 4 cells of 1.5 V each, ammeter, voltmeter, conducting wires, nichrome wire, plug key.
Procedure : 1. Set up the circuit as shown in figure 3.7. 2. Use the nichrome wire XY as the resistance. 3. Connect one of the 4 cells as shown in figure 3.7 (a.) Take readings of ammeter and voltmeter and enter them in the table below. 4. Now add the rest of the cells one by one as shown in figures 3.7 (b, c and d). Enter the readings in the table for each case. 5. Determine the values of for each case. 6. Draw a graph between current and potential difference and study it.
System of resistors and effective resistance
A resistor is a two ended component having a given amount of resistance between its two ends. In several electrical devices, a number of resistors are connected together in different ways. Ohm’s law is applicable to all such connected resistors.
Resistors in series
Study figure 3.8. You can see that the ends of the three resistors are connected so that they follow one after the other in a single line. These resistors are said to be connected in ‘series.’ In such an arrangement, the same current flows through each resistor. The value of current is I and the potential difference between C and D is V.
The three resistors, R1 , R2 and R3 are connected in series in the circuit. If V1 , V2 , V3 are potential differences of every resistor R1 , R2 and R3 respectively, then
V = V 1 + V 2 + V 3 ——–(1)
If RS (S for series) is the effective resistance between C and D, then, according to Ohm’s law,
V = I RS V 1 = I R 1 , V2 = I R 2 and V 3 = I R 3 . Substituting all these in equation (1) we get,
I RS = I R 1 + I R 2 + I R 3 RS = R 1 + R 2 + R 3
If n resistors are connected in series then,
Rs = R 1 + R 2 + R 3 +——-+ Rn
Resistors in parallel
Resistors are said to be connected in parallel when their ends are connected at both sides as shown in figure 3.9. The figure shows three resistors R1 , R2 and R3 connected in parallel between points C and D. V is the potential difference between C and D. Let I1 , I2 and I3 be the currents flowing through R 1 , R2 , and R3 respectively.
Then, the total current flowing through the circuit is
If a number of resistors are connected in parallel,
- The inverse of the effective resistance is equal to the sum of the inverses of individual resistances.
- The current flowing through an individual resistor is proportional to the inverse of its resistance and the total current flowing through the circuit is the sum of the currents flowing through individual resistors.
- The potential difference across the end of all resistors is the same.
- The effective resistance of resistors connected in parallel is less than the least resistance of individual resistors.
- This arrangement is used to reduce the resistance in a circuit
Domestic electrical connections
The electricity in our homes is brought through the main conducting cable either from the electric pole or from underground cables. Usually, there are three wires in the cable. One is called the live wire which brings in the current. It has a red or brown insulation. The other wire is called neutral wire through which the current returns. It is blue or black. In India, the voltage difference between the live and neutral wires is about 220V. Both these wires are connected to the electric meter through a fuse. Through a main switch, they are connected to all the conducting wires inside the home so as to provide electricity to every room. In each separate circuit, various electrical appliances are connected between the live and neutral wires. The different appliances are connected in parallel and the potential difference across every appliance is the same. The third wire is called the earth wire and is of yellow or green colour. This is connected to a metal plate buried deep underground near the house and is for safety purposes.
Fuse wire : Fuse wire is used to protect domestic appliances. It is made of a mixture of substances and has a specific melting point. It is connected in series to the electric appliances. If for some reason, the current in the circuit increases excessively, the fuse wire gets heated up and melts. The circuit gets broken and the flow of current stops, thus protecting the appliance. This wire is fitted in a groove in a body of porcelain – like non-conducting material. For domestic use, fuse wires with upper limits of 1 A, 2 A, 3 A, 4 A, 5 A and 10 A are used.
Precautions to be taken while using electricity
- Electric switches and sockets should be fitted at a height at which small children cannot reach and put pins or nails inside. Plug wires should not be pulled while removing a plug from its socket. Pull a plug.
- Before cleaning an electrical appliance it should be switched off and its plug removed from the socket.
- One’s hands should be dry while handling an electrical appliance, and, as far as possible, one should use footwear with rubber soles. As rubber is an insulator, it prevents the current from flowing through our body, thereby protecting it.
- If a person gets an electric shock, you should not touch that person. You should switch off the main switch and if the switch is too far or you do not know where it is located, then you should remove the plug from the socket if possible. If not, then you should use a wooden pole to push the person away from the electric wire.