Electricity Worksheet-26

Electricity Worksheet-26

1. Draw a schematic diagram of a circuit consisting of a battery of five 2 V cells, a 5 Ω resistor, a 10 Ω  resistor and a 15 Ω resistor, and a plug key, all connected in series.

2. In a household, 5 tube lights of 40 W each are used for 5 hours and an electric press of 500 W for 4 hour each day. Calculate the total energy consumed by the tube lights and press in a month of 30 days.

3. In the circuit shown in the Fig calculate :

(a) total resistance in arm CE, (b) total current drawn from the battery, and (c) current in each arm, i.e., AB  and CE of the circuit.

4. Two lamps, one rated 60 W at 220 V and the other 40 W at 220 V, are connected in parallel to the electric supply at 220 V. (a) Draw a circuit diagram to show the connections, (b) Calculate the current drawn from the electric supply, (c) Calculate the total energy consumed by the two lamps together when they operate for one hour.

5. Distinguish between the terms 'overloading' and 'short-circuiting' as used in domestic circuits.

6. For the circuit shown in Figure calculate

(a) the value of current through each resistance

(b) the total current in the circuit

(c) the total effective resistance of the circuit

7. Two resistors, with resistances 5 Ω and 10 Ω, are to be connected to a battery of emf 6 V so as to obtain:

(i) minimum current flowing (ii) maximum current flowing.

(a) How will you connect the resistances in each case ?

(b) Calculate the strength of the total current in the circuit in the two cases.

8. Electrical resistivity of some substances at 20°C are given below :

Silver                 

Copper              

Tungsten           

Iron                    

Mercury            

Nichrome         

Answer the following questions in relation to them :

(i) Among silver and copper, which one is a better conductor ? Why ?

(ii) Which material would you advise to be used in electrical heating devices ? Why ?

9. Why does the connecting cord of an electric heater not glow hot while the heating element does ?

10. What do the following symbols mean in circuit diagram ?

11. An electric circuit consisting of a 0.5m long nichrome wire XY, an ammeter, a voltmeter, four cells of 0.5V each and a plug key was set up.

Draw a diagram of this electric circuit to study the relation between the potential difference maintained between the points 'X' and 'Y’ and the electric current flowing through XY.

Graph shown here was plotted between V and I values.

What would be the values of V/I ratios when the potential difference is 0.8 V, 1.2 V and 1.6 V respectively ? What conclusion do you draw from these values?

Answer:

1. The schematic diagram of the required circuit is shown in                                                                

2. Electricity energy consumed by 5 tube lights of 40 W each for 5 h per day

          = 5 × 40W × 5h

          = 1000 Wh

          = 1 kWh

Electricity energy consumed by electric press of 500 W for 4 h per day

          = 500 W × 4h

          = 2000 Wh

          = 2 kWh

Total electrical energy consumed per day

          = 1 kWh + 2 kWh

          = 3 kWh

Total electrical energy consumed in a month of 30 days

          = 3kWh × 30

          = 90 kWh (90 units)

3. (a)    R₁ = 2 Ω + 4 Ω = 6 Ω 

(b)    total current,

          I = I₁ + I₂ = 0.5 A + 0.5 A = I A

 (c)  

4. The circuit diagram showing the connections is shown below

(b)  As  p = v I,  

Thus,   and 

Current drawn from the electric supply,

I = I₁ + I₂

(c) Total power in the circuit, P = P₁ + P₂ = 60 W + 40 W = 100 W

Total energy consumed by the two lamps in one hour = 100 W × 1h = 100 Wh

5. Short circuiting occurs when a live wire comes in contact with a neutral wire.

Overloading occurs when many electrical appliances of high power rating are operated at the same time.

In short-circuiting, a large current flows through the circuit because the resistance of the circuit formed when two wires touch each other is very small.

In overloading, a large amount of current is drawn from the electric supply due to use of many electrical appliances.

6. (a)

Potential difference across each resistor,  v = 6 V

(b) Total current, I = I₁ + I₂ + I₃ = 1.2A + 0.6A + 0.2A = 2A

 (c) Since R₁, R₂ and R₃ are in parallel, total effective resistance of the circuit, i.e., R_p is given by

or      R_p = 3 Ω

7. To obtain minimum current, the two resistors R₁ and R₂ should be connected in series

Equivalent resistance,

Potential difference across the combination V = 6 V

To obtain maximum current, the two resistors R₁ and R₂ should be connected in parallel 

Equivalent resistance,

Thus,

8. (i) Silver is a better conductor than copper. This is due to the reason that the resistivity of silver is less than that of copper.

(ii) Nichrome should be used for making electrical heating devices because :

(a) It has very high resistivity which produces high temperature when current flows through it.

(b) It does not react with air when it is red hot (800°C).

9. The connecting cord of an electric heater is usually made up of copper which has a low resistivity and does not produce much heat as compared to the heating element. The resistivity of the heating element, on the other hand, is high and it produces large amount of heat which is retained by the element which thus gets very hot and emits light.

10. This symbol represents a Variable resistance or rheostat

This symbol represents on plug key or switch which is closed.

11. The circuit diagram to study the relation between V and I is shown here

When

Thus,

When

Thus,

When

Thus, 

It is clear that      or   = constant