Electricity: Magnetic and Heating EffectsClass 8 Science Notes
Does an Electric Current Have a Magnetic Effect?
When an electric current flows through a wire, it creates something unexpected: a magnetic field around it. This connection between electricity and magnetism might seem surprising, but it's a fundamental principle that underlies many technologies we use every day.
To understand this effect, imagine a simple experiment. A magnetic compass, which contains a tiny magnet, is placed near a wire. When an electric current flows through the wire, the compass needle moves, or deflects, from its original direction. This deflection indicates that the current-carrying wire is exerting a magnetic force on the compass needle. When the current stops, the magnetic effect disappears, and the compass needle returns to its original position.
The area around a magnet or a current-carrying wire where its magnetic effect can be felt is called a magnetic field.
This phenomenon, where electric current produces a magnetic field, is known as the magnetic effect of electric current.
Be a scientist
In 1820, Hans Christian Oersted (1777-1851), a professor in Denmark, made this same discovery. He noticed that a compass needle deflected whenever an electrical circuit was closed or opened nearby. Oersted's discovery led to further investigations into the connection between electricity and magnetism, paving the way for many practical applications.
The magnetic effect of electric current is used in devices like:
- Electromagnets
- Electric bells
- Motors
- Fans
- Loudspeakers
Electromagnets
An electromagnet is a type of magnet that is created by passing electric current through a coil of wire. The magnetic field produced by the coil is similar to that of a regular magnet, but it can be turned on and off by controlling the current.
To make an electromagnet, you can wrap a wire around an iron nail and connect the ends of the wire to a battery. When the current flows through the wire, the nail becomes magnetized and can pick up iron paper clips. When the current is stopped, the nail loses its magnetic effect, and the clips fall off.
For practical applications, most electromagnets have an iron core to make them stronger. The iron core concentrates the magnetic field, allowing the electromagnet to lift heavier objects.
Electromagnets also have two poles, North and South, just like a regular magnet. The polarity of the electromagnet depends on the direction of the current flowing through the coil.
The strength of an electromagnet can be changed by:
- Changing the amount of electric current flowing through the coil
- Changing the number of turns of the coil
The poles of an electromagnet can be reversed by changing the direction of the current.
A Step Further
The Earth itself behaves like a giant magnet. Deep inside the Earth, the movement of liquid iron in the core creates electric currents, which generate a magnetic field. This magnetic field is what causes a freely suspended magnet to align itself along the north-south direction.
Earth’s magnetic field:
- Helps migratory birds, fish, and animals navigate
- Acts as a shield, blocking harmful particles from space
- Helps protect life on Earth
Lifting Electromagnets
Lifting electromagnets are strong electromagnets that are attached to cranes. Crane operators use them to move heavy iron and steel objects in factories and scrap yards. By switching the current on and off, the operator can pick up and release the objects as needed.
Does a Current Carrying Wire Get Hot?
When an electric current flows through a wire, the wire gets hot. This is because the wire offers some resistance to the flow of current. This resistance causes some of the electrical energy to be converted into heat energy. This warming effect is known as the heating effect of electric current.
Different conductors offer different levels of resistance to the flow of current. A nichrome wire, for example, offers higher resistance compared to a copper wire of the same size and length.
The amount of heat generated in a wire depends on:
- The material of the wire
- The thickness of the wire
- The length of the wire
- The duration for which the current flows
- The magnitude of the electric current
Many household appliances, such as:
- Electric room heaters
- Stoves
- Irons
- Immersion rods
- Water heaters
- Kettles
- Hair dryers
Work on the principle of the heating effect of electric current. All these devices contain a heating element, which is a rod or coil of wire that gets hot when current passes through it.
A Step Further
It is important to use appropriate wires, plugs, and sockets that are rated for the specified electric current of the connections to prevent unnecessary heating in household switchboards.
The heating effect of electric current can sometimes cause problems, such as:
- Energy loss in wires during transmission
- Damage to plugs and sockets
- Fires
Safety devices are placed in household circuits to minimize such incidents.
Ever Heard Of...
The heating effect of electric current has industrial applications, such as in steel manufacturing industries. Specially designed high-temperature furnaces use electric current to produce heat. This heat is used to melt and recycle scrap steel, converting it into usable steel.
How Does a Battery Generate Electricity?
Batteries are portable sources of electricity that power many of our devices. But how do they actually generate electricity?
Voltaic Cell
One of the earliest types of electric cells was the Voltaic cell, also known as the Galvanic cell. It contains two metal plates made of different materials and a liquid called an electrolyte, placed in a container. The plates, called electrodes, are partly dipped in the electrolyte, which is usually a weak acid or salt solution.
A chemical reaction between the plates and the electrolyte produces electricity. When the circuit is connected, electric current flows from the positive terminal through the circuit to the negative terminal. Over time, the chemicals get used up, and the cell stops working. It is then called 'dead' and cannot supply any more electricity.
Ever Heard Of...
The Voltaic or Galvanic cells get their names from two Italian scientists, Alessandro Volta and Luigi Galvani.
Galvani noticed that a dead frog's leg kicked when touched with two different metals. Volta believed the electricity came from the metals, leading to the invention of the first battery.
A Step Further
Some common metal pairs for Voltaic cells are:
- Zinc/copper
- Zinc/silver
- Aluminium/copper
- Iron/copper
- Magnesium/copper
- Lead/copper
Some metals, like copper, act as positive electrodes, while others, like zinc, act as negative electrodes, due to their chemical properties.
Dry Cells
Dry cells are one of the most widely used electric cells today. They are called 'dry' because the electrolyte is not a liquid but a thick moist paste.
A dry cell consists of:
- A zinc container (negative terminal)
- A carbon rod at the center covered with metal cap (positive terminal)
- A paste-like electrolyte surrounding the carbon rod
The dry cell is a single-use cell, meaning it has to be disposed of once it is used up.
Rechargeable Batteries
Rechargeable batteries can be recharged and reused multiple times. This prevents wastage and saves money over time.
There are many different kinds of rechargeable batteries:
- Small batteries used in watches and phones
- Batteries used in laptops and tablets
- Bigger batteries that run inverters or drive electric vehicles
Rechargeable batteries do not last forever. After being charged and used many times, they slowly wear out.
A Step Further
Lithium-ion (Li-ion) batteries are the most common type of rechargeable battery today. Scientists are working on solid-state batteries, which would be safer, charge faster, and last longer.