Electric Current And Its Effects: CBSE Class 7 Science Chapter Notes

Key Terms and Definitions

Electric Current

The flow of electric charge (electrons) through a conductor. Its conventional direction is taken from the positive to the negative terminal of the source.

Electric Circuit

A complete and continuous path for an electric current to flow. It consists of a source (cell/battery), a load (bulb), and connecting wires.

Battery

A combination of two or more electric cells connected in series to provide a potential difference for the current to flow.

Heating Effect of Current

The phenomenon where a conductor produces heat when an electric current passes through it, due to the conversion of electrical energy into heat energy.

Magnetic Effect of Current

The principle that an electric current flowing through a wire creates a magnetic field around it. Discovered by Hans Christian Oersted.

Electromagnet

A temporary magnet made by passing current through a coil of insulated wire wrapped around a soft iron core. It is magnetic only as long as current flows.

Fuse

A safety device with a special wire of low melting point that melts and breaks the circuit if the current exceeds a safe limit, preventing damage to appliances.

Filament

The thin, coiled wire inside an electric bulb (usually made of tungsten) that glows when heated by the passage of electric current.

Must-Remember Key Points

• {"point":"Electric current flows from the positive (+) terminal to the negative (-) terminal of a cell or battery in an external circuit."}
• {"point":"A switch is a simple device used to complete ('ON' position) or break ('OFF' position) an electric circuit."}
• {"point":"The amount of heat produced in a wire depends on its material (resistance), length, and thickness."}
• {"point":"Heating appliances like electric irons, geysers, and toasters use high-resistance alloys like Nichrome for their heating elements."}
• {"point":"Hans Christian Oersted discovered that a current-carrying wire behaves like a magnet and can deflect a magnetic compass needle."}
• {"point":"The strength of an electromagnet is directly proportional to the number of turns in its coil and the amount of current flowing through it."}
• {"point":"Electromagnets are crucial components in electric bells, cranes for lifting scrap iron, and various medical devices."}
• {"point":"MCB (Miniature Circuit Breaker) is a modern, reusable safety switch that automatically turns off when the current in a circuit becomes excessive."}
• {"point":"A short circuit (wires touching) or an overload (too many appliances on one socket) can cause dangerously large currents, leading to fires."}

Symbols for Electric Components

Understanding the Heating Effect of Current

The heating effect of electric current is a fundamental principle that explains how electrical energy transforms into heat. When an electric current is passed through a conductor, like a wire, the wire gets hot. This happens because the material of the wire offers resistance to the flow of electrons. As electrons move through the wire, they collide with the atoms of the conductor. These collisions transfer energy to the atoms, causing them to vibrate more rapidly. This increased atomic vibration manifests as heat.

The amount of heat generated depends on several factors:

1. Material of the wire: Wires made from materials with high resistance, such as nichrome (an alloy of nickel and chromium), produce a significant amount of heat. This property makes them ideal for the heating elements in appliances like electric heaters, toasters, and irons.
2. Amount of current: More current leads to more heat.
3. Length and Thickness: A longer and thinner wire has more resistance and thus produces more heat for the same current.

Conversely, materials like copper have very low resistance. They are used as connecting wires because they allow current to pass through without generating much heat, minimizing energy loss.

How an Electric Bell Works

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Open Circuit vs. Closed Circuit

Exam Tips for Scoring High

In exams, questions on drawing circuit diagrams are very common. Always use the standard symbols taught in your textbook. Ensure your circuit is a closed loop for any device to work, and don't forget to draw arrows indicating the conventional current direction (from + to –). When asked about a fuse, always mention three key things for full marks: it is a safety device, it works on the heating effect of current, and its wire has a low melting point.

Practice Questions with Solutions

• Why does a compass needle get deflected when brought near a current-carrying wire? This happens due to the magnetic effect of electric current. A current-carrying wire produces a magnetic field around it, which interacts with the magnetic needle of the compass, causing it to deflect.
• What is the primary purpose of an electric fuse in a household circuit? An electric fuse is a safety device used to protect electrical appliances from damage due to excessive current (from short circuits or overloading). It contains a wire with a low melting point that melts and breaks the circuit if the current exceeds a safe limit.
• Name two devices that work on the heating effect of electric current. Two common devices are an electric iron and an electric water heater (geyser).
• How can you increase the strength of a simple electromagnet? The strength of an electromagnet can be increased by: (1) increasing the number of turns in the coil of wire, and (2) increasing the magnitude of the electric current flowing through the coil.

Frequently Asked Questions

Frequently Asked Questions