Download Special Assignment_1_EEE

NIRMA UNIVERSITY
INSTITUTE OF TECHNOLOGY
B.Tech. Semester- I (ME, CL, EC, CH), July-Dec, 2014
EE101: ELEMENTS OF ELECTRICAL ENGINEERING
SPECIAL ASSIGNMENT – I
A: NUMERICALS
1
A circuit consists of three resistances of 12 Ω, 18 Ω and 36 Ω respectively joined in parallel is
connected in series with a fourth resistance. The whole circuit is supplied at 60 V dc and it is found
that the power dissipated in 12 Ω resistance is 48 watt. Determine the value of the fourth resistance
and the total power dissipated in the circuit.
2
An electric radiator is required to dissipate 1 kW when connected to a 230 V supply. If the coils of
the radiator are of wire 0.5 mm in diameter having resistivity of 60 μΩ-cm. Calculate the
necessary length of the wire.
3
A copper wire of diameter of 1.2 cm has got a resistance of 0.3 Ω and it has been drawn so that its
diameter is reduced to 50 %. What will be its new resistance?
4
Calculate the value of power delivered by 8 V source in the following circuit.
2
R
3
1
4
8
5V
1
10V
8V
2
1
1
5
A lamp rated 100 V, 75 W is to be connected across 230 V supply. Find value of resistance to be
connected in series with the lamp. Also find the power loss occurring in the resistance.
6
Two Heater A and B are in parallel across supply voltage V volt. Heater A produces 500 kcal in 200
min, and Heater B produces 1000 kcal in 10 min. The resistance of A is 10 ohm. What is the
resistance of B ? If the same heaters are connected in series across the voltage V, how much heat will
be produced in kcal in 5 min ?
7
A electric kettle has a rating 230V, 500W is found to take 15 minutes to raise the temperature of 1
litre of water from 15 oC to boiling point. Calculate the percentage energy used for heating the water.
8
Determine the KW input necessary for an electric heater to raised the temperature of 4.536kg litres of
water from 15 oC to 90 oC in 10 minutes. Heat loss in radiation during the period is 65000 J and
water equivalent of heater as 130 gm. Calculate the efficiency of the operation. How long will it take
to heat the water through the same range if the input KW were halved? Take 1 Kcal = 4186 joules.
9
An electric Kettle is required to heat 0.6 litre of water from 10 oC to the boiling point in 5 minutes,
the supply voltage being 240V. The efficiency of the kettle is 78%. calculate (I) The resistance of the
kettle (ii) the cost of energy consumed at 40 p Per kWh.
10 Ten electric heaters, each taking 200 W were used to dry out on site an electric machine which had
been exposed to a water spray. They were used for 60 hours on a 240 V supply at a cost of twenty
paise/kWh. Calculate (a) Current (b) power in kW (c ) energy in kWh (d) cost of energy
1
2
3
4
5
B: OBJECTIVES
The temperature coefficients of resistance is positive in case of
(A) Insulators
(B) conductors
(C) electrolytes
(D) both A and C
The temperature coefficient of resistance is defined as
(A) increase in resistance per ohm per oC. (B) increase in resistance per oC.
(C) decrease in resistance per ohm per oC. (D) the ratio of decrease in resistance per oC to
the resistance at 0oC.
A battery is made by combination of 6 cells in series, each capable to deliver to 4A at 2 V.
How much voltage and current can it deliver?
(A) 12 V and 24 A
(B) 2 V and 24 A
(C 12 V and 4 A
(D) 2 V and 4 A
A certain piece of aluminium is to be shaped into a conductor of minimum resistance, its
length and cross-sectional area shall be respectively
(A) L and A
(B) 2L and A/2
(C) L/2 and 2A
(D) L/2 and A/2
Kirchhoff’s voltage law is concerned with
(A) IR drops
(B) battery emfs
(C) junction voltages
(D) both (a) and (b)
6 One kWh is equal to
(A) 4186 kcal
(B) 9.81 kcal
(B) 860 kcal
(D) 36 * 105 kcal
7 If a 200 V heater is fed from a 100 V supply, the heat produced is
(A) one half
(B) one fourth
(C) the same
(D) four times
8 The SI unit of Heat energy is
(A) Joules
(B) kWh
(C ) kg m
(D) kcal
9 One H.P. Is equal to
(A) 710 watt
(B) 735 watt
(C ) 500 watt
(D) 9.81 watt
10 For a given line voltage, four heating coils will produce maximum heat when connected
(A) All in parallel
(B) all in series
(C ) with two parallel pairs in series
(D) One pair in parallel with the other two in
series.
C: SHORT QUESTIONS
1. Derive the transformation of Delta circuit to Star circuit and vice versa.
2. How are the domestic appliances connected in practice ? Justify your answer.
3. Explain the Current and Voltage divide rules with the help of suitable example.
4. State Joule's law and give relationship between electrical, Mechanical and thermal system of unit.
5. Give practical applications of heating effect of electric current.
6. Define 'power and energy'. State their electrical units.
7. Establish the relation between
i) kcal and kwh
ii) Horse power and watts
iii) kW and Joule