CHAPTER 1 UNITS ASSOCIATED WITH BASIC ELECTRICAL

QUANTITIES

EXERCISE 1, Page 5

1. What force is required to give a mass of 20 kg an acceleration of 30 m/s2 ?

Force = mass acceleration = 20 kg 30 = 600 N

2. Find the accelerating force when a car having a mass of 1.7 Mg increases its speed with a

constant acceleration of 3 .

Force = mass acceleration = 1.7 Mg 3 = 1700 kg 3 = 5100 N or 5.1 kN

3. A force of 40 N accelerates a mass at 5 m/s2. Determine the mass.

Force = mass acceleration, hence mass = = 8 kg

4. Determine the force acting downwards on a mass of 1500 g suspended on a string.

Force = mass acceleration = 1500 g 9.81 = 1.5 kg 9.81 = 14.72 N

5. A force of 4 N moves an object 200 cm in the direction of the force. What amount of work is done ?

Work done, W = force distance = 4 N m = 8 J

6. A force of 2.5 kN is required to lift a load. How much work is done if the load is lifted through

500 cm?

Work done, W = force distance = 2.5 kN 5 m = 12.5 kJ

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7. An electromagnet exerts a force of 12 N and moves a soft iron armature through a distance of

1.5 cm in 40 ms. Find the power consumed.

Power consumed = = 4.5 W

8. A mass of 500 kg is raised to a height of 6 m in 30 s. Find (a) the work done, and (b) the power

developed.

(a) Work done, W = force distance = (mass acceleration) distance

= (500 kg 9.81 ) 6 m = 29.43 kN m

(b) Power = = 981 W

9. What quantity of electricity is carried by electrons?

1 coulomb = electrons

Hence, electrons = C = 1000 C

10. In what time would a current of 1 A transfer a charge of 30 C?

Charge, Q = I t from which, time, t = = 30 s

11. A current of 3 A flows for 5 minutes. What charge is transferred?

Charge, Q = I t = 3 (5 60) = 900 C

12. How long must a current of 0.1 A flow so as to transfer a charge of 30 C?

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Q = I t, hence, time, t = = 300 s or 5 minutes

13. Rewrite the following as indicated:

(a) 1 000 pF = ……… nF (b) 0.02 F = ………. pF

(c) 5 000 kHz = ……… MHz (d) 47 k = …….. M

(e) 0.32 mA = ……. A

(a) 1 000 pF = = 1 nF

(b) 0.02 F = = 20,000 pF

(c) 5 000 kHz = = 5 MHz

(d) 47 k = = 0.047 M

(e) 0.32 mA = = 320 A

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EXERCISE 2, Page 7

1. Find the conductance of a resistor of resistance (a) 10 (b) 2 k (c) 2 m

(a) If resistance, R = then conductance, G = = 0.1 S

(b) Conductance, G = = 0.5 mS

(c) Conductance, G = = 500 S

2. A conductor has a conductance of 50 S. What is its resistance?

If conductance, G = then resistance, R = = 20000 = 20 k

3. An e.m.f. of 250 V is connected across a resistance and the current flowing through the resistance is 4 A. What is the power developed?

Power, P = V I = 250 4 = 1000 W or 1 kW

4. 450 J of energy are converted into heat in 1 minute. What power is dissipated?

Energy = power time, hence, power = = 7.5 W

5. A current of 10 A flows through a conductor and 10 W is dissipated. What p.d. exists across the ends

of the conductor?

Power, P = V I from which, p.d., V = = 1 V

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6. A battery of e.m.f. 12 V supplies a current of 5 A for 2 minutes. How much energy is supplied

in this time?

Energy = power time = (V I) t = (12 5) (2 60) = 7200 J or 7.2 kJ

7. A d.c. electric motor consumes 36 MJ when connected to a 250 V supply for 1 hour. Find the

power rating of the motor and the current taken from the supply.

Power = = 10000 W = 10 kW = power rating of motor

Power, P = V I, hence current, I = = 40 A

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