STD 8 Science Chapter-4

Explanation
Electric current refers to the rate of flow of an electric charge. It is the movement of electrically charged particles, such as electrons, through a conductor. The flow of current is measured in amperes (A) and is the basis for the functioning of electrical devices and circuits. Electric potential, on the other hand, refers to the electric potential energy per unit charge at a certain point in an electric field. Therefore, the correct answer is Electric current.

Explanation
The SI unit of electric current is ampere. Ampere is the unit used to measure the flow of electric charge. It represents the rate at which electric charges pass through a given point in a circuit. It is named after the French physicist André-Marie Ampère, who made significant contributions to the field of electromagnetism.

Explanation
Water in a waterfall flows from a higher level to a lower level because of gravitational attraction. Gravity pulls objects towards the center of the Earth, causing them to fall downwards. In the case of a waterfall, the water at the top is at a higher elevation and is pulled downward by gravity, creating the flow of water from a higher level to a lower level. This gravitational attraction is the force responsible for the movement of water in a waterfall.

Explanation
In an electric circuit, electrons flow from a point of higher potential. This is because electrons are negatively charged particles and are attracted towards positively charged areas. Therefore, they move from a region of higher potential (positive charge) to a region of lower potential (negative charge).

Explanation
The correct answer is potential differences. The electrostatic potentials at the positive and negative ends of an electric cell can be different, and the difference between these potentials is known as the potential difference of the cell. This potential difference is responsible for the flow of electric current in a circuit.

Explanation
When three electric cells with a potential difference of 1.5V each are connected in series, the total potential difference across the battery is equal to the sum of the individual potential differences. Therefore, the potential difference of the battery will be 1.5V + 1.5V + 1.5V = 4.5V.

Explanation
When an electric current flows through a wire, it creates a magnetic field around the wire. This is due to the interaction between the moving charges (electrons) in the wire and the magnetic field they produce. This phenomenon is known as electromagnetism and is the basis for many electrical devices such as motors and generators.

Explanation
The SI unit of force is Newton. Newton is the unit of force in the International System of Units (SI). It is defined as the force required to accelerate a mass of one kilogram at a rate of one meter per second squared. This unit is commonly used in physics and engineering to measure forces such as weight, tension, and friction.

Explanation
Dry cells are used in radiosets, wall clocks, and torches. These cells are a type of electric cell that uses a paste electrolyte instead of a liquid electrolyte. They are portable and can be used in various electronic devices due to their compact size and ability to provide a steady flow of current. Dry cells are commonly used in low-power devices such as radiosets, wall clocks, and torches due to their long shelf life and ability to provide a constant voltage output. Therefore, the correct answer for this question is dry cell.

Explanation
Lead acid cells are commonly used in cars, trucks, and motorcycles as they provide a high starting current required by the vehicle's engine. These cells are rechargeable and have a high energy density, making them suitable for automotive applications. They also have a long lifespan and can withstand high temperatures and vibrations, which are common in vehicles. Therefore, the use of lead acid cells in cars, trucks, and motorcycles is a practical and reliable choice.

Explanation
Water or any liquid naturally flows from a higher level to a lower level due to the force of gravity. This is a fundamental principle in physics known as the law of gravity or the law of fluid mechanics. The force of gravity pulls the liquid downward, causing it to move from a higher elevation to a lower elevation. This is why water flows downhill, and it is also the reason why rivers and streams flow from mountains to lower-lying areas.

Explanation
When lightning strikes the ground from a cloud, a large current does flow. This is because lightning is a discharge of electricity that occurs between the negatively charged base of a cloud and the positively charged ground. The current that flows during a lightning strike can reach up to 30,000 amperes and can cause significant damage. Therefore, the statement that a large current does not flow during a lightning strike is false.

Explanation
Dry cells are commonly used in radio sets as a source of power. They provide a portable and convenient power supply for radios, allowing them to be used in various locations without the need for a direct electrical connection. The dry cell's compact size and long shelf life make it an ideal choice for powering electronic devices like radios. Therefore, the statement "The dry cells are used in our radio sets" is true.

Explanation
The statement is false because lead acid cells are not typically used in clocks and torches. These types of devices usually use smaller, lightweight batteries like alkaline or lithium-ion batteries. Lead acid cells are commonly used in larger applications such as automotive batteries or backup power systems.

Explanation
An electromagnet is a type of magnet that is created by passing an electric current through a coil of wire. The coil of wire acts as a conductor, and when the current flows through it, it creates a magnetic field. This magnetic field can attract or repel objects, depending on their properties. In the given statement, it is mentioned that the system of coil and screw is called an electromagnet, which is true because when an electric current is passed through the coil, it generates a magnetic field, making it an electromagnet.