In the previous section we saw a special property of electricity:
■ Electricity will produce heat when it passes through a conductor.
• We also saw how to calculate the 'quantity of heat' produced.
• In this section, we will see two practical applications of this property.
I HEATING COILS
We will write the steps:
1. One of the most common reasons that we want to produce heat is while cooking food. We can cook food properly only if sufficient heat is available.
• We can easily produce heat for this purpose by using a coil stove. See images of coil stoves here.
2. On many occasions we will need to heat small or large quantities of water.
• When a small quantity of water is to be heated, we can use a portable immersion type water heater. See images of such water heaters here.
3. A soldering iron is a small hand tool with a pointed tip. It is used to melt solder (a low melting alloy).
• The molten solder can be used to fill the space between metallic parts in electric circuits. Thus those metallic parts will be joined together. See images of soldering iron here.
• All the three examples given above use a 'heating coil' to produce heat.
• When a current 'I' passes through a heating coil for a time 't' seconds, heat will be produced according to the Eq.10.1: H = I2Rt
• R is the resistance of the coil.
• So it is clear that, to produce more heat, R should be high.
■ Usually Nichrome is used to make heating coils.
• Nichrome is an alloy of nickel, chromium, iron and manganese
■ Four properties possesed by nichrome, makes it suitable for making heating coils:
1. Nichrome has a very high resistivity (Details here)
• So the conductors made using nichrome will have a very high 'R' value
2. Nichrome has a very high melting point
• So nichrome will not melt even if large quantities of heat is produced in it
3. Nichrome has the ability to remain in red hot condition for a long time with out getting oxidised
• When temperature is increased, even non-reactive materials may enter into chemical reactions with the atmospheric oxygen. But nichrome does not enter into such reactions even if temperature is high
4. Nichrome has a low expansion coefficient
• When temperature increases, metals and alloys tend to expand in size. But nichrome does not expand much.
II SAFETY FUSE
• Before seeing how the safety fuse works, we must first understand the situations when safety fuse becomes a necessity.
(a) Short circuit
1. A short circuit is an 'unwanted and dangerous circuit' inside a normal circuit.
• Though it is unwanted, it some times occurs accidentally due to the defects in the wiring system.
2. 'Short' is opposite of 'long'. So 'short circuit' can be considered as the opposite of 'long circuit'.
• A long circuit is the required normal circuit designed by engineers and technicians.
• In that circuit, current flows through the required appliances like TV, refrigerator etc.,
3. But due to defects in the wiring, the positive and negative terminals come into direct contact with each other.
• In such a situation, there will be 'no resistance' to the current flow.
• Instead of flowing through the appliances, the current will flow directly between the positive and negative terminals.
4. Since there is no resistance between them, a large current will begin to flow.
• The wires in the circuit are not designed to carry such large currents.
• They will get heated, resulting in sparks and fire.
5. If the current flow can be stopped just when such high currents begin to flow, sparks and fire can be avoided
(b) Over loading
1. When engineers and technicians design a circuit, they will specify the 'exact number and types' of appliances that can be connected in that circuit.
2. They will connect all heavy appliances together in a separate circuit
• More current will flow through such a circuit
• So the wires in that circuit will be 'special wires' which can carry such heavy currents
3. They will connect all normal appliances together in a separate circuit
• Less current will flow through such a circuit
• So the wires in that circuit will be 'normal wires' which need carry only normal currents
4. If somebody connects 'more appliances' or 'heavier appliances' to the existing circuits, more current will flow. This situation is called 'over loading'
• Even 'special wires' will not be able to carry such heavy currents.
• They will get heated resulting in sparks and fire.
5. If the current flow can be stopped just when such high currents begin to flow, sparks and fire can be avoided
We will write the steps:
1. The fig.10.2 below show two types of 'fuse plugs'
♦ 'Fuse plugs' are installed inside a 'fuse box'
♦ The 'fuse box' is connected in series to the circuit
2. Consider the larger fuse plug in the fig. There are two u-shaped metallic parts
• To complete the circuit, current must be able to flow from one u-shaped portion to the other u-shaped portion.
• But they are separated by an insulating material.
• The only connection between them is a thin metallic wire. This metallic wire is called fuse wire
3. We have seen that, when a short circuit or overloading occur, heavy currents will begin to flow through the circuit.
• Since the fuse is connected in series, this heavy current will flow through the fuse wire also
4. Now, the fuse wire has a special property: Low melting point
• Indeed a fuse wire should be made of materials having low melting points
• Also a fuse wire should be thin
• So special alloys are used for making them
5. When heavy current flows through the fuse wire, it will get heated up according to
Eq.10.1: H = I2Rt.
• Since the melting point is low, the heat developed due to the heavy current is enough to melt it
6. When it melt, it will break apart.
• So the current will no longer flow between the u-shaped metallic parts
• Thus the current flow in the circuit is stopped
♦ The 'fuse box' should be connected in series to the circuit. Otherwise, the current will take an alternate path and it will not serve the purpose.
• When the current flow becomes excess, more heat is developed, which cannot be dissipated. Then the fuse wire will melt
■ Thus we can write: Safety fuse is a device that works on the 'heating effect of electric current'. It protects the appliances from short circuit and overloading.
8. When a fuse wire is included in a household wiring, the following precautions should be taken:
• The ends of the fuse wire must be connected firmly at appropriate points
• The fuse wire should not project outside the carrier base
• Length of the fuse wire should not exceed the limit
• The fuse must be connected in series in the circuit
In the next section we will see the 'lighting effect' of electric current
■ Electricity will produce heat when it passes through a conductor.
• We also saw how to calculate the 'quantity of heat' produced.
• In this section, we will see two practical applications of this property.
I HEATING COILS
We will write the steps:
1. One of the most common reasons that we want to produce heat is while cooking food. We can cook food properly only if sufficient heat is available.
• We can easily produce heat for this purpose by using a coil stove. See images of coil stoves here.
2. On many occasions we will need to heat small or large quantities of water.
• When a small quantity of water is to be heated, we can use a portable immersion type water heater. See images of such water heaters here.
3. A soldering iron is a small hand tool with a pointed tip. It is used to melt solder (a low melting alloy).
• The molten solder can be used to fill the space between metallic parts in electric circuits. Thus those metallic parts will be joined together. See images of soldering iron here.
• All the three examples given above use a 'heating coil' to produce heat.
• When a current 'I' passes through a heating coil for a time 't' seconds, heat will be produced according to the Eq.10.1: H = I2Rt
• R is the resistance of the coil.
• So it is clear that, to produce more heat, R should be high.
■ Usually Nichrome is used to make heating coils.
• Nichrome is an alloy of nickel, chromium, iron and manganese
■ Four properties possesed by nichrome, makes it suitable for making heating coils:
1. Nichrome has a very high resistivity (Details here)
• So the conductors made using nichrome will have a very high 'R' value
2. Nichrome has a very high melting point
• So nichrome will not melt even if large quantities of heat is produced in it
3. Nichrome has the ability to remain in red hot condition for a long time with out getting oxidised
• When temperature is increased, even non-reactive materials may enter into chemical reactions with the atmospheric oxygen. But nichrome does not enter into such reactions even if temperature is high
4. Nichrome has a low expansion coefficient
• When temperature increases, metals and alloys tend to expand in size. But nichrome does not expand much.
II SAFETY FUSE
• Before seeing how the safety fuse works, we must first understand the situations when safety fuse becomes a necessity.
(a) Short circuit
1. A short circuit is an 'unwanted and dangerous circuit' inside a normal circuit.
• Though it is unwanted, it some times occurs accidentally due to the defects in the wiring system.
2. 'Short' is opposite of 'long'. So 'short circuit' can be considered as the opposite of 'long circuit'.
• A long circuit is the required normal circuit designed by engineers and technicians.
• In that circuit, current flows through the required appliances like TV, refrigerator etc.,
3. But due to defects in the wiring, the positive and negative terminals come into direct contact with each other.
• In such a situation, there will be 'no resistance' to the current flow.
• Instead of flowing through the appliances, the current will flow directly between the positive and negative terminals.
4. Since there is no resistance between them, a large current will begin to flow.
• The wires in the circuit are not designed to carry such large currents.
• They will get heated, resulting in sparks and fire.
5. If the current flow can be stopped just when such high currents begin to flow, sparks and fire can be avoided
(b) Over loading
1. When engineers and technicians design a circuit, they will specify the 'exact number and types' of appliances that can be connected in that circuit.
2. They will connect all heavy appliances together in a separate circuit
• More current will flow through such a circuit
• So the wires in that circuit will be 'special wires' which can carry such heavy currents
3. They will connect all normal appliances together in a separate circuit
• Less current will flow through such a circuit
• So the wires in that circuit will be 'normal wires' which need carry only normal currents
4. If somebody connects 'more appliances' or 'heavier appliances' to the existing circuits, more current will flow. This situation is called 'over loading'
• Even 'special wires' will not be able to carry such heavy currents.
• They will get heated resulting in sparks and fire.
5. If the current flow can be stopped just when such high currents begin to flow, sparks and fire can be avoided
■ So we see that, it is essential to stop the current just when short circuit or over loading occurs.
• How does the safety fuse help us to achieve this objective? We will write the steps:
1. The fig.10.2 below show two types of 'fuse plugs'
♦ 'Fuse plugs' are installed inside a 'fuse box'
♦ The 'fuse box' is connected in series to the circuit
2. Consider the larger fuse plug in the fig. There are two u-shaped metallic parts
 |
| Fig.10.2. Source: By Anihl - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5001280 |
• But they are separated by an insulating material.
• The only connection between them is a thin metallic wire. This metallic wire is called fuse wire
3. We have seen that, when a short circuit or overloading occur, heavy currents will begin to flow through the circuit.
• Since the fuse is connected in series, this heavy current will flow through the fuse wire also
4. Now, the fuse wire has a special property: Low melting point
• Indeed a fuse wire should be made of materials having low melting points
• Also a fuse wire should be thin
• So special alloys are used for making them
5. When heavy current flows through the fuse wire, it will get heated up according to
Eq.10.1: H = I2Rt.
• Since the melting point is low, the heat developed due to the heavy current is enough to melt it
6. When it melt, it will break apart.
• So the current will no longer flow between the u-shaped metallic parts
• Thus the current flow in the circuit is stopped
♦ The 'fuse box' should be connected in series to the circuit. Otherwise, the current will take an alternate path and it will not serve the purpose.
7. Even when the circuit is performing in a normal condition, some heat is generated in the fuse wire
• But this heat is small and is dissipated into the surroundings• When the current flow becomes excess, more heat is developed, which cannot be dissipated. Then the fuse wire will melt
■ Thus we can write: Safety fuse is a device that works on the 'heating effect of electric current'. It protects the appliances from short circuit and overloading.
8. When a fuse wire is included in a household wiring, the following precautions should be taken:
• The ends of the fuse wire must be connected firmly at appropriate points
• The fuse wire should not project outside the carrier base
• Length of the fuse wire should not exceed the limit
• The fuse must be connected in series in the circuit
In the next section we will see the 'lighting effect' of electric current
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