Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes

Science Class 7

Class 7 : Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes

The document Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes is a part of the Class 7 Course Science Class 7.
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Electric Current and its Effects

Electric Components


Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes


An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields.


Circuit Diagram

A simplified conventional pictorial representation of an electrical circuit, using standard symbols for electric components, is called a circuit diagram. A 'circuit diagram' is also known as an electrical diagram, wiring diagram, elementary diagram or electronic schematic.


Electrical Circuit

A closed path formed by the interconnection of electrical components through which electric current flows is called an electrical circuit.
For a bulb to glow, it must be connected to battery rather than a cell, because a bulb will require more power. So if a circuit requires more power, then a battery should be connected.

Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes


Open Circuit

If current does not flow through a circuit, then it is said to be an 'open' circuit or incomplete. Its switch is in 'off' position.

Closed Circuit

A circuit is said to be a closed circuit or complete when current flows through it. Its switch is in 'on' position.

Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes


Cell Holder

A compartment that holds two or more cells together to form a battery is called a cell holder.


Battery

A combination of two or more cells connected together is called a battery. It is formed by connecting the positive terminal of one cell to the negative terminal of another. To identify the positive and negative terminals, they are denoted as + and -, respectively. These batteries are used in many devices, such as torch lights, mobile phones, calculators and even automobiles.


Symbols

Different symbols are used to represent different components of electrical circuits, but the symbols used must be standardised. These symbols are easy to understand, remember and draw.

Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes

Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes

Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes


Heating Effect of Electric Current

In the 19th century, James Joule studied a property, which says that "when an electric current flows through the filament of a bulb, it generates heat, and so the bulb becomes hot". This property is named the heating effect of electric current.


Compact Fluorescent Lamps (CFL's)

We use electric bulbs to obtain light. Due to the heating effect, some part of the energy received by the bulb is used up, and hence, some electricity is wasted. CFL's do not depend on the heating effect of electricity to produce light, since they do not use filaments. Using CFL's instead of ordinary bulbs minimises wastage of electricity. In CFL's, light is generated using two electrodes. The fluorescent coating inside each tube makes the light brighter.
We use every day many appliances that work on the property of the heating effect of electric current. For example, the electric room heater, electric roti maker, electric iron, toaster, hair dryer, electric stove, immersion water heater, food warmer, electric coffee maker, electric rice cooker and geyser work on the property of the heating effect of electric current.


Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes


Heating Elements

These appliances have coils of wire that produce heat, which are known as heating elements. As current flows through these electrical appliances, the coils of wire inside turn bright orange red in colour. This is because a huge amount of heat is produced. Different appliances have different types of heating elements. The type of heating element depends on the function of the appliance. Some appliances are required to produce more heat than others.


ISI Mark

Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes

You should purchase only appliances that bear an ISI mark. ISI stands for Indian Standards Institute. If an appliance bears the ISI mark, it means that it is safe and will not waste electrical energy. Moreover, it is a mark of quality.
Factors affecting production of heat:
The factors that affect the production of heat in a wire through which an electric current is passing are the length and thickness of the wire, the duration of flow of current, and the material of the wire.


Electric Fuse

The electric fuse works on the principle of the heating effect of electric current. An electric fuse is a safety device to prevent damage to an electrical circuit when excessive current flows through it. It is made of a special material. As the current increases beyond a limit, the wire in the electric fuse melts and breaks off. The fuse is then said to have blown off. The circuit is broken and current stops flowing through it. Thus, a fuse prevents fires. There are various types of fuses. Some fuses are used only in buildings, while others are used in appliances.


Reasons for Excessive Current

When all the appliances are connected to the same socket, these appliances draw more current, and so the load increases. When the insulation on the wires is torn, two wires carrying current touch each other directly. This causes a spark, which leads to fire. This is termed as a SHORT CIRCUIT. If a fuse is not used, then overloading and short circuits result in fire.


Miniature Circuit Breakers (MCB)

Instead of fuses, MCBs are used nowadays because these are switches that turn off automatically when there is an overload or a short circuit. After solving the problem in the circuit, the switch can be turned back on, and then the current flows as usual.


The Magnetic Effect of Electric Current

When electric current flows through a wire wound around an iron bar, the bar behaves like a magnet.


Electromagnet

When electric current flows through a wire wound around an iron bar, the bar behaves like a magnet. This magnet is called an electromagnet. An electromagnet is formed due to the magnetic effect of electric current. This magnetic effect of electric current was discovered by Hans Christian Oersted. Once, while preparing for a lecture, he noticed that there was a deflection in the needle of a magnetic compass kept near a wire that was connected to a battery. This deflection occurred every time the battery was switched on and off. He realised that a magnetic field is created around a current-carrying wire in a circuit. The components required to create an electromagnet are two pieces of insulated copper wire, a nail, a battery containing two cells, a bulb with a holder, and some paper clips made of iron. The bulb is also used for making an electromagnet since it serves as a resistance to the current in the circuit and it prevents the battery from quick discharge.


Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes 

Construction of an Electromagnet

  • Take a nail and wind a copper wire around it without any overlap, as shown in the figure.
  • Remove the insulation on the wire at the two ends
  • Connect one end of the wire to the battery and the other to one terminal of the bulb holder.
  • Connect one end of the second wire to the remaining terminal of the bulb holder, and the other end to the battery.
  • Place the paper clips near the nail.
  • When you switch on the current, the paper clips will cling to the nail. This is because the nail becomes an electromagnet.
  • When the battery is disconnected, the nail is no more an electromagnet. The paper clips will not cling to the nail.

Application of Electromagnet

Electromagnets are used in medicines, toys, iron industries, and most commonly in the electric bell. Cranes are used for lifting material, separating iron from scrap in industries, and to lift cars. These work on the principle of the electromagnet. Electromagnets are also used in electromagnetic trains called Maglev's.


Construction and Working of an Electric Bell Circuit

  • An electric bell consists of an iron core, on which is wound a wire as a coil.
  • One end of the coil is connected to one terminal of a battery, and the other end to a steel rod that acts like a spring for the hammer touching the screw contact.
  • The other terminal of the battery is connected to the screw contact with a switch in the middle.

Chapter Notes - Chapter 14 : Electric Current and its Effects, Class 7, Science | EduRev Notes

Working

  • Electric current flows through the coil when the switch is ON, and the iron core acts as an electromagnet.
  • The iron core attracts the hammer towards it.
  • The hammer hits the bell and produces a sound.
  • The circuit breaks at the screw contact when the hammer moves towards the iron core. At this point, the iron core ceases to be an electromagnet.
  • The hammer is pulled back to its original position due to the spring action of the steel rod, and then touches the contact again to complete the circuit.
  • The circuit is completed and current flows through the coil again, and the hammer strikes the bell again.
  • The process repeats itself and you hear a ringing sound since the hammer keeps hitting the bell, until the switch is released.
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