2003 - JAMB Physics Past Questions and Answers - page 3

2000W heater → 2000J/S
=> Heat supplied by the heater within 10 min
(600 seconds) = 2000 x 600
= 1200000J
∴ Heat capacity MC = 1200000/30
= 4000J/C
Heat capacity = 4.0 x 10⁴J/C

Generally in thermal/heat conduction, if 1.2 x 10⁶ J is the heat flow per seconds; 30KM^-1 is the temperature gradient of the materials; and 400Wm^-1K^-1 the thermal conductivity of the material, then we have that:
heat flow per seconds per unit area = thermal conductivity x temp. gradient
i.e = 1.2 x 10⁶Area
= 400 x 30
=> 400 x 30 x Area = 1.2 x 10⁶
∴ Area = 1.2 x 10⁶400 x 30
Area = 100m² or 1.0 x 10²m²

From the above diagram, if each of the resistors can dissipate a maximum of 18W, the from the relation power, P = I²R, the current in the 2Ω series resistor is given by:
I² = P/R = ¹⁸/₂ = 9 => I = √9 = 3A
The effective parallel arrangement of the two 2Ω resistors: ¹/_R = ¹/₂ + ¹/₂ = 1Ω
∴ Total resistance in the circuit = 1 + 2 = 3Ω
current flowing the circuit = 3A
∴ Maxi. power = P = I²R = 3² x 3 = 27W

Let the original length = L₁
Let the original resistance = R₁ = 5Ω
Let the original resistivity = P₁
Let the original area = a₁
Let the new length = L₂ = 2L₁

let the new area = a₂ = 1/(2a₂)
Let the new resistance = R₂
Let the new resistivity = P₂
But since the resistivity remains the same,
=> P₁ = P₂

∴ P1 =	R1 a1
_L1

= P2 =	R2 a2
_L2

But a₂ = ¹/₂a₁; and L₂ = 2L₁

∴	R1 a1
_L1

=	R2 2a1/2
_2L1

=>	5 x a1
_L1

=	R2 x a1
_4L1

∴R2 =	5 x a1 x 4L1
_a1 x L1

= 20Ω

In general, if a charged particle is accelerated between two metal plates/a region by potential difference V volts, the work done on the charge, given by the product of the charge and the potential difference, is equal to the kinetic energy of the charge
Thus eV = K.E
1.6 x 10^-19 x V = 4.8 x 10^-17

∴ V =	4.8 x 10-17
1.6 x 10-19

= 300V

A charge in motion always experienced a force on entering a magnetic field

Iron (spft iron) is usually preferred to steel in the making of electromagnets because it is more easily magnetized and equally more easily demagnetized than [II and III]