- Books Name
- CBSE Class 7 Science Book
- Param Publication
- CBSE Class 7
ELECTRIC BELL :
An electric bell is the most common application of electromagnets. It consists of an electromagnet, a springy iron strip, a hammer, a gong, twoswitches and connecting wires.
(i) Electromagnet: A coil of wire wound on an iron core acts as an electromagnet, when current is applied.
(ii) Armature: An armature with a hammer at one end is kept close to the electromagnetic facing its poles.
(iii) Interupter: To make the bell ring continuously, a device is needed to keep the hammer moving back and forth. This devide is called an interrupter.
Working of Electric bell :
Step 1: When you push the switch of the bell, the electric current flows to the electromagnet.
Step 2: The electromagnet attracts the soft iron strip. The hammer attached to the strip then hits the gong, causing a ring.
Step 3: When the soft iron strip gets attracted to the electromagnet, it no longer touches the screw (interrupter) and hence the circuit is broken, (much like a switch being turned off). This turns off the electromagnet and it can no longer attract the soft iron strip. The soft iron strip returns to its initial position, touching the screw (interrupter). This results in the circuit being complete, and current flows again.
Steps 1 to 3 repeat in quick succession as long as the switch is on. This is how we hear a continuous ring of the bell.
Calculate The Cost of Electricity :
• Commercial unit of electric energy : Kilowatt - hour (kWh) :
Electric energy is required to run the electric lamps, heaters, refrigerators, televisions and other electric appliances. The department of electricity sells the electric energy to the consumers in units called kilowatt-hours (kWh). If our electricity bill shows that we have paid for 10 units, then it means the electric appliances of our house have consumed 10 kilowatt-hours. So, 1 unit = 1 kWh.
A kilowatt-hour is the amount of electric energy used by 1000 Watt electric appliance (say a heater) when it operates for one hour.
kWh is also known as “Board of Trade Unit” (B.O.T.)
(e) Relation between kWh and Joule :
1 kWh = 1000 Wh ( 1 kW = 1000 W)
Now1 W = 1 Js–1 and 1h = 60 × 60 s = 3600 s.
∴ 1 kWh = 1000 Js-1 × 3600 s = 3600000 J = 3.6 × 106 J
∴ 1 kWh = 3.6 × 106 J
Ex.3 Calculate the cost of electricity for a house in which 7 bulbs of 100 Watt each and 3 fans of 60 Watt each are used for 5 hours a day, for a period of 30 days, if the cost of one unit is Rs. 5.
Sol. Each bulb of 100W consumes 100 Watt hour of energy, when used for 1 hour.
7 bulbs of 100W each when used for 1 hour consume, 7 × 100 = 700 Watt hour or 700 Wh of energy.
Thus, 7 bulbs of 100W each when used for 5 hours each day consume, 700 × 5 = 3500 Wh or 3.5 KWh.
In this way, we can calculate the total electricity used in 30 days i.e. Total electricity consume,
[(7 × 100) + (3 × 60)] × (5 × 30) = (700 + 180) × 5 × 30 = 132000 Wh = 132 KWh.
If the cost of each unit is Rs. 5.0, then the total cost = 132 × 5 = Rs. 660.
Danger of electricity
(i) If the current happens to pass through the heart, it cause the heart muscles to contract and generally death occurs.
(ii) A strong electric shock can give the body a big shock that can damage the body cells. Such a shock occurs on touching a live electric wire suddenly.
(iii) Handling electrical appliances in wet places is very dangerous.
(iv) Electricity could turn dangerous due to loose connections in switches, improper wiring, overloading (i.e. passing excess current above the rated capacity), improper earthing.
Safety Measures in Using Electricity :
(i) The wires used in the circuit should be of good quality and with good insulation.
(ii) Defective and damaged plugs, sockets and switches must be immediately replaced.
(iii) All connections in plugs, switches and sockets must be made of a proper insulating material.
(iv) Extension cords must not be overloaded and must be regularly tested.
(v) Switches and plugs should not be touched with wet hands.
(vi) The main switch should be immediately switched off in case of fire or short circuit.
(vii) Always use a safety fuse of proper rating and material in an electric circuit.
(viii) All appliances must be properly earthed.
(ix) Rubber sole shoes should be worn while repairing electric circuit. This protect the body from electric shocks.
(x) The inside of socket are made of conducting material and therefore must not be touched.
(xi) A proper earthing should be done at homes.