Deck 1: Some Mathematics Revision

Question 1

An electromagnet exerts a force of 15 N and moves a soft iron armature through a distance of 12 mm in 50 ms. Determine the power consumed.

Answer 1

$\text { force } \times \text { distance }=15 \mathrm{~N} \times 12 \times 10^{-3} \mathrm{~m}=0.18 \mathrm{~N} \mathrm{~m} \text { or } 0.18 \mathrm{~J}$
$\text { Power consumed }=\frac{\text { work done }}{\text { timetaken }}=\frac{0.18 \mathrm{~J}}{50 \times 10^{-3} \mathrm{~s}}=3.6 \mathrm{~J} / \mathrm{s} \text { or } 3.6 \mathrm{~W}$

Question 2

A d.c. motor consumes 47.25 MJ when connected to a 250 V supply for 1 hour 45 minutes. Determine the power rating of the motor and the current taken from the supply.

Answer 2

$\text { Power }=\frac{\text { energy }}{\text { time }}=\frac{47.25 \times 10^{6} \mathrm{~J}}{105 \times 60 \mathrm{~s}}=7500 \mathrm{~W}$
$\text { i.e. power rating of motor is } 7.5 \mathrm{~kW}$
$\text { Power } \mathrm{P}=\mathrm{VI} \text { from which, current } \mathrm{I}=\frac{\mathrm{P}}{\mathrm{V}}=\frac{7500}{250}=30 \mathrm{~A}$
$\text { i.e. the current taken from the supply is } 30 \mathrm{~A}$

Question 3

A 100 W electric light bulb is connected to a 200 V supply. Calculate (a) the current flowing in the bulb, and (b) the resistance of the bulb.

Answer 3

Question 4

Determine the charge transferred when a current of 5 mA flows for 10 minutes.

Question 5

A current of 12 A flows in the element of an electric fire of resistance 25 . Determine the power dissipated by the element. If the fire is on for 5 hours every day, calculate for a one week period (a) the energy used, and (b) cost of using the fire if electricity cost 13.5p per unit.

Question 6

Calculate the resistance of 1200 m of copper cable of cross-sectional area 15 . Take the resistivity of copper as

Question 7

At a temperature of 40C, an aluminium cable has a resistance of 25 . If the temperature coefficient of resistance at 0C is 0.0038/C, calculate it's resistance at 0C.

Question 8

(a) Determine the values of the resistors with the following colour coding:
(i) red-red-orange-silver
(ii) orange-orange-black-blue-green
(b) What is the value of a resistor marked as 47KK ?

Question 9

Four cells, each with an internal resistance of 0.40  and an e.m.f. of 2.5 V are connected in series to a load of 38.40 . (a) Determine the current flowing in the circuit and the p.d. at the battery terminals. (b) If the cells are connected in parallel instead of in series, determine the current flowing and the p.d. at the battery terminals.

Question 10

(a) State six typical applications of primary cells. (b) State six typical applications of secondary cells. (c) State the advantages of a fuel cell over a conventional battery and state three practical applications.

Question 11

State for lithium-ion batteries
(a) three typical practical applications
(b) four advantages compared with other batteries
(c) three limitations

Question 12

Name six alternative, renewable energy sources and give a brief description of each.

Question 13

For solar power, state (a) seven advantages (b) five disadvantages