This chapter explores electric current and its effects, including heating and magnetic effects, as well as applications like electromagnets and electric bells. Electric components are represented by symbols for easy circuit diagrams. The heating effect occurs when a wire gets hot from electric current, used in heaters and bulbs, with heat depending on the wire’s material, length, and thickness. The magnetic effect is when current through a wire acts like a magnet, demonstrated by compass needle deflection. Electromagnets, coils of wire around iron, are used in cranes and electric bells. Electric bells use electromagnets to strike a gong and create sound.
HEATING EFFECT OF ELECTRIC CURRENT
The heating effect of electric current refers to the phenomenon where a wire gets hot when an electric current passes through it.
Key points regarding the heating effect of electric current:
- Observation When an electric bulb lights up, it becomes warm.
- Applications Electric appliances like room heaters or electric heaters for cooking use the heating effect of electric current. These appliances contain a coil of wire called an element, which becomes red hot and gives off heat when switched on. Immersion heaters, hotplates, irons, geysers, electric kettles, and hair dryers also have elements inside them.
- Factors Affecting Heat The amount of heat produced in a wire depends on its material, length, and thickness. Different materials, lengths, and thicknesses of wires are used based on different requirements.
- Electric vs. Appliance Wires Wires used for making electric circuits typically do not become hot. However, the elements of some electric appliances become very hot and are easily visible. The filament of an electric bulb gets heated to a high temperature, causing it to glow.
- Incandescent Bulbs Incandescent electric bulbs produce light but also generate heat, wasting electricity. More electricity-efficient options include fluorescent tube-lights, compact fluorescent lamps (CFLs), and light-emitting diode (LED) bulbs. LED bulbs are increasingly preferred due to their lower electricity consumption for a given light intensity.
- Safety A large current passing through a wire can cause it to melt and break. Electric fuses use wires made from special materials that melt and break when large electric currents pass through them. Fuses are inserted in electrical circuits to prevent damages and possible fires by blowing off and breaking the circuit if the current exceeds the safe limit. Miniature circuit breakers (MCBs) are also increasingly used in place of fuses.
MAGNETIC EFFECT OF ELECTRIC CURRENT
The magnetic effect of electric current refers to the phenomenon where an electric current passing through a wire behaves like a magnet.
Key points regarding the magnetic effect of electric current:
- Hans Christian Oersted was the first person to notice the deflection of a compass needle when current passed through a wire.
- Electromagnets An electric current can be used to make magnets called electromagnets. These are coils of insulated wire wrapped around a piece of iron. Electromagnets can be very strong and lift heavy loads.
- Uses of Electromagnets Electromagnets are used in cranes to lift heavy materials, to separate magnetic materials from junk, and by doctors to remove small magnetic pieces from the eye. They are also used in many toys.
- Electric Bells Electric bells utilize electromagnets in their operation. The electromagnet pulls an iron strip with a hammer, which strikes a gong to produce sound. When the electromagnet pulls the strip, it breaks the circuit, stopping the current and causing the hammer to return to its original position. This process repeats rapidly to create the ringing sound.
ELECTROMAGNET
An electromagnet is a coil of insulated wire wrapped around a piece of iron that behaves like a magnet when an electric current flows through it. When the electric current is switched off, the coil generally loses its magnetism.
Key aspects of electromagnets:
- Construction: An electromagnet can be made by winding a long piece of insulated flexible wire around an iron nail. The free ends of the wire are connected to the terminals of a cell through a switch.
- Function: When electric current flows through the coil, the coil behaves like a magnet, and pins placed nearby cling to the tip of the nail. When the current is switched off, the coil loses its magnetism, and the pins no longer cling to the nail.
- Strength: Electromagnets can be made very strong and can lift heavy loads. The strength of an electromagnet can be compared by counting the number of pins attracted to it when varying the number of turns of wire.
- Uses: Electromagnets have various applications:
- Cranes: Electromagnets are used in cranes to lift heavy materials.
- Separating Magnetic Materials: They are used to separate magnetic materials from junk.
- Medical Applications: Doctors use tiny electromagnets to take out small pieces of magnetic material that have accidentally fallen in the eye.
- Toys: Many toys also have electromagnets inside them.
- Electric Bells: Electromagnets are a key component of electric bells.
ELECTRIC BELL
An electric bell utilizes an electromagnet to produce sound.
Key components and how it works:
- An electric bell consists of a coil of wire wound on an iron piece. The coil acts as an electromagnet.
- An iron strip with a hammer at one end is kept close to the electromagnet.
- When the iron strip is in contact with a contact screw, current flows through the coil, which becomes an electromagnet.
- The electromagnet then pulls the iron strip, and the hammer at the end of the strip strikes the gong of the bell to produce a sound.
- When the electromagnet pulls the iron strip, it also breaks the circuit, so the current through the coil stops flowing and the coil is no longer an electromagnet.
- The iron strip then returns to its original position, touching the contact screw again, which completes the circuit.
- The current flows in the coil, and the hammer strikes the gong again. This process repeats in quick succession, making the bell ring.
Questions and Answers
- Symbols for electrical circuit components:
- Connecting wires: A straight line.
- Switch in the ‘OFF’ position.
- Bulb.
- Cell.
- Switch in the ‘ON’ position.
- Battery.
- Connecting cells to make a battery: Fig. 10.22 shows four cells fixed on a board, and lines should be drawn to indicate how to connect their terminals with wires to make a battery of four cells. It is necessary to connect the positive terminal of one cell to the negative terminal of the next cell.
- Two effects of electric current:
- Heating effect.
- Magnetic effect.
- Compass needle deflection: When current is switched on through a wire, a compass needle kept nearby gets deflected from its north-south position because the electric current through a wire behaves like a magnet.
- Compass needle deflection with closed switch: The source refers to Fig 10.24.
- Fill in the blanks:
- (a) Longer line in the symbol for a cell represents its positive terminal.
- (b) The combination of two or more cells is called a battery.
- (c) When current is switched ‘on’ in a room heater, it heats up.
- (d) The safety device based on the heating effect of electric current is called a fuse.
- True or false:
- (a) To make a battery of two cells, the negative terminal of one cell is connected to the negative terminal of the other cell. (F).
- (b) When the electric current through the fuse exceeds a certain limit, the fuse wire melts and breaks. (T).
- (c) An electromagnet does not attract a piece of iron. (F).
- (d) An electric bell has an electromagnet. (T).
- Electromagnet for separating plastic bags: An electromagnet cannot be used for separating plastic bags from a garbage heap because plastic is not a magnetic material.
- Replacing fuse with wire: It is unsafe to replace a fuse with a piece of wire because the wire may not melt and break when the current exceeds the safe limit, which could cause a fire. Fuses are safety devices that prevent damage to electrical circuits and possible fires.
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