CBSE Class 6 Science Electricity and Circuits Exam Notes

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These Class 6 Science notes on Electricity and Circuits cover static and current electricity, electric potential and current, cells and the dry cell, the electric bulb, conductors and insulators, electric circuits, the torch, resistors and series and parallel combinations.

ELECTRICITY AND CIRCUITS

Electricity is a kind of energy. It comes from the charged particles called electrons and protons inside an atom.
We have two types of electricity:
a) Static Electricity: It is formed over an insulator and can't move through it.
b) Current Electricity: Here charges move in a particular direction through a conductor.

Static electricity is also known as frictional electricity. When one object is rubbed by another object, heat so produced, gets absorbed by electrons over their surface and they exchange electrons. One of them loses some electrons where the other gain some electrons equally. Both of them gain electric potential. Respectively the first one is called as positively charged object and said to be at positive potential and the second one is called as negatively charged object and is said to be in negative potential. For example when a glass rod is rubbed by fur, the glass rod loses electrons and becomes positively charged where as the fur gains electrons and becomes negatively charged equally.

[Figure: Two pairs of charged spheres mounted on insulating stands. Left: a small sphere marked + and a larger sphere surrounded by + and x signs, captioned Figure represents a positively charged object mounted on an insulator. Right: a small sphere marked - and a larger sphere surrounded by a dashed circle, captioned Figure represents a negatively charged object mounted on an insulator.]

The potential of such accumulated charge is defined as the work done in bringing unit positive charge from infinity to the given charge.

V = \( \frac{W}{Q} \) or 1V = \( \frac{1J}{1C} \)

A collection of positive charge has a potential of +1V if 1J of work is done in bringing +1C charge from infinite to that given charge.
Here V is potential measured in Volt (V); W is the work done measured in Joule (J) and Q is the charge measured in coulomb (C).
6 × 10¹⁸ electron together makes 1 coulomb charge. 1 J work done = 1 m displacement of any thing by applying 1 Newtons force.

Current Electricity: Let us consider a positively charged object and a negatively charged object are connected by conducting wire is shown below:

[Figure: A positively charged sphere A and a negatively charged sphere B, each on an insulating stand, joined by a connecting wire drawn as a coil. The label Electrons free to move in the conducting wire is placed above the wire, with an arrow e pointing from B towards A, and the label Connecting wire is placed below.]

It is obvious that at B electrons are at high pressure than at A and so electron will start drifting over the conductor from B to A. This fact is called as current of charges or electric current.

Current is defined as the rate of flow of charges and expressed as,
I (current) = \( \frac{Q\,(charge)}{t\,(time)} \)

It is 1 ampere current if at any cross-section of the conductor 1 coulomb charge passes in 1 second. So,
1A = 1C/1 Sec

Thus electric potential difference can produce electric current. Electric current can be obtained commercially in two ways. One way is from power plants which we shall learn later.
The other way is from a electric cell which is a source of electric potential difference. We shall learn more about cell and shall learn how electric circuits are made by using a cell.

ELECTRICITY

Electricity plays a very important role in our daily life. It lights, heats and cools our homes, and also performs a number of routine jobs for us. Most of the charm of modern life would be missing if there were no electricity. Electricity flows through electric wires. Electricity in motion is called current electricity.

SOURCES OF ELECTRIC CURRENT

a) Small Sources of Electric Current: Cell are small sources of current. In a cell chemical energy changes into electrical energy. A cell basically consists of two terminals - a metal cap on one side and a metal disc on the other. The metal cap is the positive terminal and the metal disc works as a negative terminal. The chemicals stored in the cell produce electricity.
Cells are of two kinds:
1. Primary Cells
2. Secondary cells or accumulators.

1. Primary Cells: These cells provide current as a result of the chemical reaction that takes place in the chemicals stored in them. When the chemicals are used up, they stop producing electricity. They cannot be recharged. It means the chemical reaction is irreversible. Simple voltaic cell, Leclanche cell, Daniel cell, dry cell, etc., are examples of primary cells.

The dry cell was discovered by French scientist G. Leclanche in 1868. It is the most commonly used cell and is used in torches, transistors, calculators, cameras, tape-recorders, etc. This cell is called dry cell because it doesn't contain any liquid chemical.

A dry cell consists of a zinc casing and a carbon rod with a brass cap at its centre. The carbon rod is surrounded with a black powder which is a mixture of manganese dioxide (MnO₂) and graphite (C). This black powder is either contained in a thin bag of cloth or is surrounded by a thin layer of sawdust. The space between the zinc casing and the sawdust is filled with a thick paste of ammonium chloride (NH₄Cl).

[Figure: Cross-section of a dry cell with labels: Brass cap (Positive Terminal), Pitch Top Seal, Zinc Casing, Paste of Ammonium Chloride, Mixture of Manganese Dioxide and Graphite, Carbon Rod, Insulating Card Covering, Negative Terminal.]

The construction of the dry cell is shown in Figure. The zinc casing is surrounded by thick insulating paper. A layer of pitch seals the top of the cell and prevents the leakage of the contents of the cell.

2. Secondary Cells or Accumulators: These cells also provide current as a result of chemical reaction taking place in the chemicals stored in them. But unlike primary cells, they can be recharged as and when required. Here the chemical reaction is reversible and the electrical energy can be stored in them. Lead accumulator, nickel-iron accumulator and alkali accumulator are examples of secondary cells.

[Figure: Photograph of a car battery, a lead accumulator, with terminals on top.]

b) Bigger Source of Electric Current: The electric current used in our homes for running fans, refrigerators, heaters, etc. is of very large magnitude. Similarly, the electrical energy used in factories, electric trains, etc. is also of very large magnitude. Cells cannot provide so much energy. For producing a large amount of energy, big power houses are constructed in which the energy of flowing water or the energy of steam is converted into electric energy.

An electric generator consists of two parts namely, turbine and dynamo, which are coupled together. The energy of the flowing water or steam rotates the blades of the turbine. The energy of the turbine operates the dynamo, which finally converts the mechanical energy into electric energy.

ELECTRIC BULB

Electrical appliances we use work only when an electric current flows through them. When you switch on the light in your room, electric current flows through the bulb and enables it glow. Do you know what is inside the electric bulb?

It has a tiny coiled wire which is supported by two thick wires making a V-shape. This tiny coiled wire is called a filament. The two wires making this V-shape are known as supporting wires. The lower end of one of the wires is joined to the metal casing at the side of the bulb. The lower end of the second wire is joined to the metal tip at the base of the bulb. The metal casing at the lower and the metal tip at the base form the terminals of the bulb. On passing current through the terminals, the electric bulb produces light.

[Figure: Photograph of an electric bulb showing the glass bulb, the filament supported by two wires and the metal base.]

CONDUCTORS AND INSULATORS

We find that some substances allow electricity to flow through them while others do not. Substances which allow electricity to flow through them are called conductors. On the other hand, substances which do not allow electricity to flow through them are called insulators. Current cannot pass through a cotton thread. It requires metal wires that can allow electricity to pass through them.

Table: Conductors and Insulators

ELECTRIC CIRCUITS

The closed path in which the current flows is called in electric circuit. In other words, the complete path from one terminal of an electric cell through the electric bulb and back to the other terminal of the electric cell is known as electric circuit.

Elements of an Electric Circuit

The essential components or elements of an electric circuit are given as follows:
1. Electric sources (cell, battery, etc.
2. Electric appliances (a device for using the electric current).
3. Conductor (for providing a path for the flow of current)
4. Switch or a key (for opening and closing the circuit).

Electric Torch

In a pocket torch, usually two or more cells are connected end to end (in series) to increase the brightness of the bulb. Figure shows the arrangement of cells and the bulb in a torch.

[Figure: Cut-away diagram of a torch with labels: sealing ring (water resistant), plastic casing, metal switch contacts, slide switch, reflector, lamp filament, C-size cells connected in series, metal spring, lamp contact.]

Resistors

We shall now learn about some conductors which conduct but offers some resistance to the motion of charges and are called as resistors. Without resistor no electric appliance can be constructed.
The symbol of resistance is a zig-zag line.
When electric current (I) passes through any resistor (R) a potential difference (V) is formed across its two end points and behaves like a cell.

[Figure: A resistor R with current I entering from the left; the potential difference V is marked across its two ends by a double arrow between two terminals.]

According to ohm's law, V = I R. 1V = 1A × 1Ω. (Ω stands for ohm and is the unit of resistance). 1Ω resistance is that which when connected across a cell of 1V gives only 1A current will pass through it.

A resistance is found in electric bulb, heater and in many other applications. The resistance can be combined in the following two manners.

[Figure: Two circuits driven by a cell of voltage V. Series Combination: resistors r₁ and r₂ in one loop with the same current I through both, and potential differences V₁ and V₂ across them. Parallel Combination: resistors r₁ and r₂ in two branches with currents I₁ and I₂ and the same potential difference V across both, and the total current I.]

1. In series combination
i) Same current passes through all the resistance. But the voltage get divided across them so that V = V₁ + V₂. That is why resistances some time called as voltage divider.
ii) The total resistance of the circuit becomes (r₁ + r₂).
2. And in parallel combination,
i) The voltage across them remain same but the current get distributed through them.
ii) Total resistance becomes \( \frac{r_1 r_2}{r_1 + r_2} \).

Note: In house wiring, all the applications are connected in a parallel manner. Ask your teacher why?