Electricity and Circuits

1.0Introduction

Have you ever seen lightning flash during a storm? Have you watched a cricket game under the bright flash lights? If so, then you have seen electricity in action. Electricity powers lights, ovens and computers. Electricity makes your clothes stick together when you take them out of the clothes dryer. The what is electricity? That's not so easy to answer. Electricity is the result of electrical charges. To understand electrical charge, we have to start with matter. Everything around us is made up of matter. This book is made of matter. You are made of matter. Like colour or hardness, electrical charge is a property of matter.

2.0Positive and negative charges

There are two types of electrical charges. These charges are called positive and negative charges. You cannot see or feel electrical charge the way you can see colour or feel hardness. However, you can observe how charges interact with each other.

A positive charge and a negative charge attract, or pull each other. Positive charges repel, or push away each other. Negative charges repel each other too. Like charges repel each other while unlike charges attract each other.

Charges add up

Matter consists of tiny particles called atoms. Atoms contain particles called protons, neutrons, and electrons. Protons and electrons have electric charge, and neutrons have no electric charge. Protons have positive electric charge and electrons have negative electric charge. The amount of positive charge on a proton equals the amount of negative charge on an electron. If an atom contains equal number of protons and electrons, the positive and negative charges cancel out and the atom has no net electric charge. Objects with no net charge are said to be electrically neutral.

Now, in an atom, protons and neutrons reside inside the nucleus while electrons moves around the nucleus. It is only the 'transfer of electrons' which makes an atom or a matter positive or negative.

• An atom becomes negatively charged when it gains extra electrons. If an atom loses electrons it becomes positively charged. An electrically charged atom is called ion.
• Just like likes poles repel and unlike poles attract each other, charges also have similar nature.

3.0Static electricity

Suppose you rub a balloon with a woolen cloth. Negative charges (electrons) would move from the wool to the balloon. This would produce a buildup of negative charges in the balloon and positive charges on woolen cloth. A buildup means that it has more of one kind of charge than the other. The buildup gives the balloon an overall negative charge. The wool would be left with a buildup of positive charges. The buildup of electrical charges on an object is called static electricity. The term static electricity is used because here the charges accumulated on the surfaces of the materials are at rest. You know that charged particles can move between objects when the objects touch each other. Even more charged particles can move when objects rub against each other. Rubbing objects together causes them to touch in more places. Rubbing produces a larger buildup of charge. The static electricity developed on the surfaces of the bodies when rubbed against each other is also called frictional electricity.

List of objects acquiring two kinds of charges on rubbing

4.0Conductors and insulators

You know that you can get a shock from a metal doorknob. If you touch the wooden door first, you can avoid that surprising shock. This is because the metal is a conductor while wood is an insulator.

Conductors: are materials that allow charges to flow through them easily. In other words 'materials which allow electric current to pass through them are conductors of electricity'. Charges move quickly from your body into the doorknob. You feel the fast discharge as a shock. Metals such as copper and silver are good conductors of electricity.

• Metals are good conductor as they have large number of free electrons. These electrons act as if they no longer belong to any one atom, but to the metal as a whole; consequently, they move freely throughout the piece of metal.
  

Insulators: are materials that do not allow charges to flow through them easily. In other words 'materials which do not allow electric current to pass through them are insulators of electricity'. When you touch the wooden door, charges move slowly onto the surface of the door. Because this discharge is slow, you don't feel it. Rubber, plastic, glass and air are also good insulators.

• Insulators have electrons that are tightly bound to the nucleus and are not free to travel within the substance i.e., they have few or negligible number of free electrons. Thus, they do not conduct electricity.
  

Conductors and insulators are equally important for us. Switches, electrical plugs and sockets are made of conductors. On the other hand, insulators like rubber and plastics are used for covering electrical wires, plug tops, switches and other parts of electrical appliances which people might touch. Human body is also a conductor. That is why, sometimes you can get a shock when you touch a charged body. Carelessness in handling electricity and electric devices can cause severe injuries and sometimes even death.

5.0Electric cell

A torch is sometimes used for providing light. A torch has a bulb that lights up when it is switched on. Electricity to the bulb in a torch is provided by the electric cell. Electric cells are also used in alarm clocks, wristwatches, transistor radios, cameras and many other devices. You might have noticed that it has a small metal cap on one side and a metal disc on the other side (see figure). You may have noticed a positive (+) sign and a negative (-) sign marked on the electric cell. The metal cap is the positive terminal of the electric cell. The metal disc is the negative terminal. All electric cells have two terminals; a positive terminal and a negative terminal. A commonly used dry cell that is used in torches, wall clocks, TV remotes, etc. is shown in figure.

An electric cell produces electricity from the chemicals stored inside it. That is, in an electric cell 'chemical energy is converted into electric energy'. When the chemicals in the electric cell are used up, the electric cell stops producing electricity. Then, the electric cell has to be replaced with a new one. A combination of two or more cell is called a battery. There are many kinds of cells or batteries. They come in different sizes and shapes. Batteries are often called power source or voltage sources [see figure].

6.0Electric current

When you are in the dark, a flashlight (torch) can be used to provide light. A flow of electricity causes the bulb to light.

• A flow of electrons through a conductor is called an electric current. Electric current is measured in a unit called Ampere. The amount of electric current that can flow through a circuit depends on (i) voltage (ii) resistance. Voltage Voltage is the strength of a power source. A power source with more voltage can produce more electric current. Voltage is measured in a unit called Volt. For example, a dry cell has 1.5 volts. In India, the electricity which we get from a power plant has a voltage of 220 volts.
• Current does not exist without voltage.

Resistance

Resistance is the property of a substance to oppose or slow down electric current. Increasing the resistance of a circuit decreases the flow of electrical charges through it. Resistance allows electrical energy to be changed into other forms of energy, such as light and heat. Resistance is measured in a unit called $\mathbf{Ohm}$.

Copper wires are good conductors. They have very little resistance, so they can carry a great deal of electric current. Rubber is an insulator that has a large amount of resistance. It is difficult to make any electric current flow through rubber.

7.0Circuits

To make an electric current, you need a path to carry the current. The path along which electric current flows is called a circuit.

• A circuit is formed when an electric current passes through an unbroken path of conductors. A simple circuit has three basic parts. It has a power source, such as a battery. This powers a load, such as a lamp, a computer, television, mobile phone, etc. Connectors, such as wires, carry electrical charges between the power source and the load.
• Connecting wires, switches etc. have insulators on them so that we don't get electric shock. So definition of insulators has to be remembered.

Switch

Many circuits have a switch. A switch turns electric current on and off. The lights in your classroom are controlled by a switch. A switch is a device that can open or close the path of electric current. When the switch is closed (i.e., on), the voltage of the battery pushes on the electrons in the circuit. This causes electrons to move. Protons feel a force in the opposite direction. Protons, however, are not free to move as they reside in the nucleus of an atom. Open and closed circuits To keep charges moving, the circuit cannot have any breaks. A complete, unbroken circuit is called a closed circuit. If the circuit has any breaks or openings, it is called an open circuit. Electric current cannot flow in an open circuit.

Conventional direction of current

Electric current is always said to flow from the positive terminal to the negative terminal in a circuit. This is called conventional current. This way of describing the movement of electric current originated before scientists fully understood electricity. However, it is still the way used to describe how circuit operates. In a circuit, the movement of negatively charged electrons is from negative terminal to the positive terminal. Thus, we can say, direction of movement of electrons is opposite to the direction of electric current.

8.0Torch bulb

A torch bulb or any other electric bulb has an outer case of glass that is fixed on a metallic base (see figure). Inside the bulb, there is a thin wire that gives off light which is called the filament of the bulb. The filament is fixed to two thicker wires, which also provide support to it. One of these thick wires is connected to the metal case at the base of the bulb. The other thick wire is connected to the metal tip at the centre of the base. The base of the bulb and the metal tip of the base are the two terminals of the bulb. These two terminals are fixed in such a way that they do not touch each other.

• The filament generally made of tungsten metal of an electric bulb has high resistance. When an electric current flows to the filament, electrical energy converts into heat energy. Then, the heated filament produces light.
  
  An electric bulb
• An electric bulb don't have fixed polarity on its terminals. When it is connected to a cell or battery its's terminals becomes positive and negative.

9.0Electric circuit of a torch (flashlight)

The negative terminal of the cell 1 presses against the spring at the base of the torch. Positive terminal of the cell 1 touches the negative terminal of the cell 2 . The positive terminal of the cell 2 presses against the metal tip at the base of the bulb. There are three metal strips. Strip A is connected with the metal case of the bulb, strip B is connected with the spring and strip $C$ is a part of the switch. While metal strips $A$ and $B$ are fixed, strip C can be pressed and made to slide. When you press the switch, the strip C touches strips A and B and the circuit is complete. Then current flows in the circuit and makes the bulb glow.

• In the diagram of a torch, always connect positive terminal of cell 1 to the negative terminal of cell to and positive terminal of cell 2 with bulbs terminal to complete the circuits

10.0Concept Map

• Have you ever taken clinging clothes from a clothes dryer? Why this happens? How can this cling can be reduced or avoided?
  
  Electrons move between clothes in a dryer. A positively charged sock will stick to a negatively

• In a cloth dryer, when clothes are rubbed against each other, some clothes gain electrons while some clothes loose electrons. Thus, they get oppositely charged. Since opposite (or unlike) charges attract each other, the clothes cling to one another. This tendency for clothes to stick (cling) to other clothes because of development of opposite charges on them is called static cling. To reduce or avoid the static cling of clothes, put the synthetic materials and the natural materials for separate drying. Combining synthetic materials in the dryer, such as nylon, often leads to static cling with natural materials, such as cotton. Also, do not dry clothes for too long as over drying also leads to static cling.

• Why a negatively charged balloon sticks to the uncharged wall?

• When you hold a negatively charged balloon near a wall, it repels the negative charges (electrons) present in the wall. This makes the wall near the balloon positive. As we know, negative charge and positive charge attract each other, the negative charged balloon attracts the positively charged portion of the wall. This pull makes the balloon "stick" to the wall (see figure).
  
• A charged body is attracted towards an uncharged body by process called induction. The process of charging an uncharged body from a charged body without touching them together is called induction.

• When a light bulb burns out (get fused), it stops emitting light. Why?

• When a light bulb burns out, it makes an open circuit. This happens because a wire inside the bulb breaks in two. The circuit no longer has a complete path, so electric current cannot flow through it. Hence, it does not emit light.

• Both the electric cell and the bulb have two terminals each. Why do they have these two terminals?

• The two terminals are required in electric cell or a bulb or any other electric device to make the circuit complete. From one terminal, current enters into the device while the current leaves from the other terminal of the device. Also, one terminal must be positive while other terminal must be negative.

11.0Some basic terms

• Magnitude - size of some quantity.
• Inflated - Increases in size when filled with air.
• Conventional - based on or in accordance with what is practiced generally.
• Cling - to hold on tightly to somebody/something.
• Amber - when rubbed, amber becomes negatively charged and attracts lightweight particles such as pieces of straw, fluff, or dried leaves.
• Socket - a place in a wall or extension board where a piece of electrical equipment can be connected to the electricity supply.
• Polarity - it is an entity contains two distinct and opposite poles that can either attract or repel each other.
• Connectors - parts or devices used for electrically connecting or disconnecting circuits.
• Buildup - an increase of something over a period.
• Static - "Static" means stationary or at rest.