Maharashtra Board Class 8 Science Chapter 4 Current Electricity and Magnetism PDF Download

Current Electricity And Magnetism

Which Constituents Are Present In An Atom?

An atom has same number of positively charged protons and negatively charged electrons. So an object doesn't show any charge though its atoms contain charged particles. Therefore, we can say that plenty of electrical charge is filled in the objects around us. What will happen if a glass rod is rubbed on a silk cloth? How do objects get charges? What are static and moving charges? Moving charges get transferred from one object to the other. These are negatively charged. Moving negatively charged particle are the electrons. Can this negative charge be made to flow? Can electricity be made to flow like water flowing from higher to lower level? You have learnt that a force will have to be applied to put a stationary object into motion. We get current electricity when the electrons in an electrical conductor are made to flow.

Current Electricity

A large current flows when lightning takes place from a cloud to the ground, while sensation is felt by us due to a microscopically small current flowing to the brain. You are aware of the current flowing through wires, electric bulbs, and equipments in the house. In the electric cells of a radio or in a car battery, a current is produced by the flow of both negatively and positively charged particles.

Electrostatic Potential

Water or a liquid flows from a higher level to a lower level. Heat always flows from a body at higher temperature to a body at lower temperature. Similarly, there is a tendency of the positive charge to flow from a point of higher electric level to a point of lower electric level. This electric level deciding the direction of flow of electric charges is called electrostatic potential.

Potential Difference

Similar to the height of a waterfall, the temperature difference of hot and cold bodies, the difference between the potential of two points, i.e. potential difference is interesting to us.

Take connecting copper wires and connect the 'circuit' as shown in fig 4.1 (a). No current is seen to flow in the bulb. Now connect in the same 'circuit' a 1.5 V dry cell available in the market as shown in fig 4.1 (b). Now it will be realized from glowing of the bulb that a current is flowing in the circuit. Electrons in the wire flow due to the potential difference between the two ends of the dry cell. These flow from the negative terminal of the cell to the positive terminal of the cell. Conventional current flows in the opposite direction and is shown in the figure by the sign of an arrow. We will learn about an electrical circuit later in this chapter.

In fig 4.1 (a), there is no current as there is no potential difference in the absence of any cell. Current starts flowing in the circuit as soon as the potential difference is applied. The unit of potential difference in SI system is Volt (V). We will learn about it in the next standard.

Teacher's Note

When you rub a balloon on your hair, it gets charged. This is like how electric charge moves from one thing to another. This is what we study in electricity.

Exam Trick

Remember: Potential difference is like height difference. Water flows down from high to low. Current flows from high potential to low potential. Easy to remember!

Points To Remember

An atom has protons (positive) and electrons (negative).
Moving electrons create electric current.
Potential difference means difference in electric level between two points.
Current flows from positive to negative terminal in a circuit.
Volt is the unit of potential difference.

Electric Current Measurement

How can we measure water flow emerging from a pipe? We can find it from the amount of water (litres) coming out in a specific time period. How then is the electric current measured?

We have seen that electric current is produced due to the flow of charged particles. Electrical charge flowing through a wire in 1 second can be called unit current. The SI unit of electric current is Coulomb per second or Ampere.

1 Ampere = 1A = 1 Coulomb/1 second = 1 C/s. Electric current is a scalar quantity.

Electric Cell

A source is required to produce a uniform flow of charges in a circuit. Such a general device is an electric cell. Various types of electric cells are available today. These are used in a range of machines from wrist watches to submarines. Out of these, you must be aware of solar cells. The main function of various electric cells is to maintain a constant potential difference between its two terminals. The electric cells work on the electric charges to maintain a constant potential difference, about which you will learn later. Let us learn about the electric cells that are currently in use.

Dry Cell

The dry cells are used in our radio sets, wall clocks and torches. These are available in 3-4 sizes. The construction of a dry cell is as shown in fig. 4.2.

Take a lead dry cell and remove its outer coating. Inside you will find a whitish, metal layer. This is the Zinc (Zn) metal layer. This is the negative terminal of the cell. Now, carefully break open this layer. There is another layer inside. An electrolyte is filled between these two layers. The electrolyte contains negatively charged and positively charged ions. These are the carriers of electricity. The electrolyte is a wet pulp of Zinc chloride (ZnCl\(_2\)) and Ammonium chloride (NH\(_4\)Cl). There is a graphite rod at the centre of the cell. This is positive terminal of the cell. A paste of Manganese dioxide (MnO\(_2\)) is filled outside the rod. Because of the chemical reactions of all these chemicals, electrical charge is produced on the two terminals (graphite rod and zinc layer) and an electric current flows in the circuit.

Due to the wet pulp used in this cell, the chemical reaction proceeds very slowly. Hence a large electric current can not be obtained from this. Compared to the electric cells using liquids, the shelf life of dry cells is longer. Dry cells are very convenient to use as these can be held in any direction with respect to ground and can be used in mobile instruments.

Teacher's Note

Your TV remote and wall clock use dry cells. These cells have two terminals - one positive and one negative. Just remember this simple thing!

Exam Trick

Dry cell = Zinc cover (negative) + Carbon rod (positive). Remember: Zinc is outer layer and carbon is inner rod. This helps you draw the diagram correctly in exam.

Points To Remember

Dry cell has zinc cover as negative terminal.
Carbon rod at the centre is the positive terminal.
Electrolyte is a wet paste inside the cell.
Dry cells last longer than wet cells.
Dry cells can be used in any direction.

Lead-Acid Cell

Figure 4.3 depicts the design of a Lead-Acid cell. Let us examine its principle. This type of cell can be recharged after getting electrically discharged. The lead-acid cell contains a lead electrode and a lead oxide electrode and both are dipped in dilute sulfuric acid. PbO\(_2\) carries a positive charge, while the Pb electrode carries a negative charge. The potential difference between these two is nearly 2V. Because of the chemical reaction between the substances in the cell, electrical charge is produced on both the electrodes and electric current flows through the load (e.g. bulb) in the circuit.

This kind of electric cells have a capacity to deliver large current. Hence lead-acid cells are used in cars, trucks, motorcycles and uninterrupted power supplies (UPS).

Ni-Cd Cell

These days, a variety of gadgets are available, which are required to be carried to different places. Such gadgets use Ni-Cd cells. The cells deliver 1.2 V potential difference and are rechargeable.

Teacher's Note

Your car battery is a lead-acid cell. It can be charged again and again. Just like your phone charger, you can recharge your car battery many times.

Exam Trick

Lead-Acid = 2V. Ni-Cd = 1.2V. Remember: Lead-Acid gives more voltage. This is used in big vehicles like cars and buses.

Points To Remember

Lead-acid cell has lead electrode and lead oxide electrode.
It can be recharged many times.
Potential difference is nearly 2V.
It is used in cars and motorcycles.
Ni-Cd cells are used in portable gadgets.

Electric Circuit

When a cell holder, an electric bulb and a plug key are connected by connecting wires, as shown in fig. 4.4 (b) and a dry cell is fitted in the holder (fig 4.4 (a)), then the bulb lights up by closing the plug key. This means that a current flows through the circuit and bulb lights up. On the removal of the cell, the electric current flowing through the circuit stops as indicated by the bulb which ceases to glow. This type of connection of electrical components is called an electrical circuit. A circuit is shown in fig 4.4 (b).

An electrical circuit is also used in the home supply. However, the electricity supply is made from outside, instead of the electric cells. You will learn about it later.

Connecting Cells

You must have seen more than one electric cells connected in an electrical circuit (see figure 4.5 (a)). In the transistor radio, 2-3 dry cells are seen to be connected in series. The purpose of doing this is to obtain more potential difference than that of a single cell. Therefore, it is possible to obtain higher current. If the cells are connected as seen in fig 4.5 (b), the connection of cells is known as a Battery of cells.

In this series connection, the positive terminal of one cell is connected to the negative terminal of second cell and the positive terminal of the second cell is connected to the negative terminal of the third one. Therefore, if each cell has a potential difference of 1V the total potential difference of 3 cells will be 3V.

Teacher's Note

When you put two batteries in your torch one after another, they are connected in series. This gives more power to the torch bulb. This is a real example you see every day.

Exam Trick

Series connection = Add all voltages. If you connect 3 cells of 1.5V each in series, you get 1.5 + 1.5 + 1.5 = 4.5V total. Easy math!

Points To Remember

Electric circuit has cell, wires, bulb and key connected together.
Series connection means cells are joined positive to negative terminal.
Voltages add up in series connection.
Battery is made by connecting multiple cells in series.
Current flows through the circuit when key is closed.

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