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

But a₂ = ¹/₂a₁; and L₂ = 2L₁

= 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

= 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]