Electricity: Magnetic and Heating Effects Study Notes
Magnetic Effect of Electric Current
When an electric current flows through a conductor, such as a wire, it produces a magnetic field around it. This phenomenon is known as the magnetic effect of electric current.
The magnetic field is a region around a magnet or a current-carrying wire where its magnetic effect can be felt, such as by the deflection of a magnetic compass needle.
A magnetic compass needle is a tiny magnet. It deflects when another magnet is brought near it or when it is placed near a current-carrying wire.
This magnetic effect can act through any non-magnetic materials placed in between the conductor and the compass.
The magnetic field created by an electric current disappears once the flow of current stops, causing the compass needle to return to its original direction.
Discovery of the Connection Between Electricity and Magnetism
The link between electricity and magnetism was discovered in by Hans Christian Oersted ( ), a professor at a university in Denmark.
Oersted noticed that the needle of a magnetic compass lying nearby deflected whenever an electrical circuit was closed or opened.
Following his investigation, he published his findings, which led other scientists to repeat his experiments and further explore the electromagnetism connection.
The magnetic effect of electric current is utilized in various modern devices including electromagnets, electric bells, motors, fans, and loudspeakers.
Electromagnets and Their Properties
A current-carrying coil that behaves as a magnet is called an electromagnet.
In practical applications, most electromagnets include an iron core inside the coil to make the magnetic field stronger.
When current passes through a cylindrical coil, it adopts magnetic properties. If an iron nail is inserted into the core of this coil, the deflection of a nearby compass needle increases significantly, indicating a stronger magnetic field.
Properties of electromagnets include:
Two poles: Like a bar magnet, an electromagnet possesses a North pole and a South pole.
Polarity determination: If the North pole of a magnetic compass is attracted to one end of an electromagnet, that end is the South pole.
Polarity reversal: The poles of an electromagnet can be reversed by changing the direction of the electric current.
Adjustable strength: The strength of an electromagnet can be modified by changing the amount of electric current flowing through the coil, changing the number of turns in the coil, or both.
Temporary magnetism: The magnetic effect exists only while the current is flowing and disappears when the current stops.
Earth as a Magnet
The Earth behaves like a giant magnet, which explains why a freely suspended magnet always rests in a North-South direction.
The Earth's magnetic field is generated deep within its core by the movement of liquid iron, which creates electric currents.
Functions of the Earth's magnetic field:
Navigation: Many migratory birds, fish, and animals use the field to navigate across oceans and continents.
Protection: It acts as a shield, blocking harmful particles from space and protecting life on Earth.
Industrial Applications of Electromagnets
Lifting electromagnets are powerful magnets attached to cranes, used in factories and scrap yards.
A crane operator can control the magnet by switching the current on and off.
When the current is switched on, the electromagnet lifts iron and steel objects; when switched off, the field disappears, and the objects are released.
Heating Effect of Electric Current
The heating effect of electric current occurs when an electric current passes through a conductor and causes it to get heated.
This happens because every conductor offers some opposition, known as resistance, to the flow of current. This resistance converts some electrical energy into heat energy.
The amount of heat generated in a conductor depends on multiple factors:
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 (e.g., a battery with cells produces more heat than a single cell).
Resistance and Heating Materials
Different conductors offer different levels of resistance. For example, nichrome wire offers higher resistance than copper wire of the same size and length.
Nichrome wire (thickness approximately or gauge) is commonly used in heating experiments and devices because it heats up significantly when current passes through it.
A heating element is a rod or a coil of wire found in electrical appliances. In many appliances, this element glows red-hot when in use.
Common household appliances using the heating effect include:
Electric room heaters
Electric stoves
Electric kettles
Electric irons
Water heating immersion rods
Hair dryers
Incandescent lamps (the filament is heated until it glows).
Safety and Industrial Uses of the Heating Effect
Industrial applications include steel manufacturing, where high-temperature furnaces use electric current to generate heat to melt and recycle scrap steel.
Overheating hazards: Excessive heat can cause energy loss during transmission, melt plastic parts in plugs and sockets, or lead to fires.
Safety measures: Circuit safety devices are used to minimize risks. It is essential to use wires, plugs, and sockets rated for the specific electric current of the connection to prevent unnecessary heating.
Generation of Electricity in Cells and Batteries
Cells and batteries are devices that generate electric current through internal chemical reactions.
Modern portable electricity sources allow for lighting lamps, making magnets, and heating wires.
The Voltaic Cell
A Voltaic cell, also called a Galvanic cell, was one of the earliest types of electric cells.
Components of a Voltaic cell:
Electrodes: Two metal rods made of different materials partly dipped in a liquid.
Electrolyte: A liquid, such as a weak acid or salt solution, that conducts electricity.
Container: Usually made of glass or plastic.
How it works: A chemical reaction between the electrodes and the electrolyte produces electricity. Current flows from the positive terminal through the circuit to the negative terminal.
End of life: When the chemicals are used up, the cell is considered "dead" and cannot supply more electricity.
Historical background: Named after Italian scientists Alessandro Volta and Luigi Galvani. Galvani noticed a dead frog's leg kicked when touched by copper and iron. Volta proved that the electricity came from the combination of metals and liquid (like saltwater-soaked paper) rather than the frog, leading to the first battery.
Common metal pairs for electrodes: Zinc/copper, zinc/silver, aluminium/copper, iron/copper, magnesium/copper, and lead/copper.
DIY Lemon Cell
A simple cell can be constructed using lemons, copper wire/strips ( thick), iron nails, and an LED.
In this setup:
Electrodes: Copper wire and iron nails.
Electrolyte: Lemon juice.
Assembly: Copper wires and iron nails are inserted into five or six juicy lemons and connected in series.
LED polarity: Current passes through an LED only when its positive terminal (longer wire) is connected to the battery's positive terminal and its negative terminal (shorter wire) is connected to the negative terminal.
Dry Cells
Dry cells are widely used because they are more convenient than liquid Voltaic cells.
The electrolyte is a thick, moist paste rather than a liquid.
Structure of a dry cell:
Zinc container: Acts as the negative terminal.
Carbon rod: Located at the center with a metal cap, acting as the positive terminal.
Paste electrolyte: Surrounds the carbon rod.
Dry cells are generally single-use and must be disposed of once they are used up.
Rechargeable Batteries
Rechargeable batteries can be reused multiple times, saving money and preventing waste.
Common applications: Laptops, mobile phones, cameras, inverters, and electric vehicles.
Lithium-ion () batteries: The most common type of rechargeable battery today, relying on metals like lithium and cobalt.
Future technology: Scientists are developing solid-state batteries that replace liquid/paste electrolytes with solid materials to be safer, charge faster, and last longer.
Battery degradation: Rechargeable batteries eventually wear out after many charge cycles.
Environmental Safety and E-waste
Dead batteries are not completely inert; they contain acids and metals like lead, cadmium, nickel, or lithium.
Improper disposal in regular garbage can cause fires or environmental harm.
Valuable materials in batteries can be recycled and reused at specialized e-waste recycling facilities.
Questions & Discussion
Why does an incandescent torch lamp get warm? It is due to the heating effect caused by the resistance of the filament to the electric current.
If a lifting electromagnet stops working but the wire is still warm, what happened? Possible reasons include the battery being nearly exhausted (too weak to lift but still providing some current for heat) or a partial short circuit.
What happens to the deflection if battery terminals are reversed? The direction of the deflection will reverse because the poles of the electromagnet change with the current direction.
Will a coiled wire without an iron core still deflect a compass? Yes, but the deflection will be significantly less because the iron core concentrates and strengthens the magnetic field.
Which coils produce a magnetic effect? All conductive coils (iron, copper, aluminium, and nichrome) will produce a magnetic effect and deflect a compass needle when current flows through them.
Fill in the blanks:
The solution used in a Voltaic cell is called an electrolyte.
A current-carrying coil behaves like a magnet.
True or False:
Dry cells are less portable compared to Voltaic cells (False).
A coil becomes an electromagnet only when electric current flows through it (True).
An electromagnet with a single cell attracts more clips than one with a battery of cells (False).