Physics
PART B
40 Marks
TRIAL B FINAL
PART B Question and Answer Booklet
Question 1 [6 Marks]
The apparatus shown below consists of two long smooth parallel conducting rails separated by 0.800 m. They are joined by a resistance R of 0.050 Ω . The apparatus is in a uniform. magnetic field of induction 8.00 x 10-2 T directed upwards out of the plane of the drawing.
The bar, initially at rest, is pulled to the right with an initially increasing speed v by a constant applied force Fa.
(a) Find the magnitude and direction of the current in the bar in terms of v.
(b) Explain why the bar will reach a terminal speed if the rails are long enough.
(c) Find the terminal speed if Fa = 2.00 N.
Question 2 [6 Marks]
The windings of a motor designed to run on 240 V has a resistance of 5.0 Ω . Under normal running conditions (full speed) the motor draws 5.0 A. To protect the windings when the motor starts from rest a resistance is connected temporarily in series to limit the current to 5.0 A.
(a) Calculate the current the motor would draw at the start without the protective resistance.
(b) (i) Find the protective resistance necessary at start.
(ii) Find the back-EMF when the motor is running at operating speed.
Question 3 [8 Marks]
A pulley is made of two parts, a uniform. disc of diameter 0.30 m and mass 6.0 kg surrounded by a thin metal ring of negligible thickness and identical (0.30 m) diameter and mass 0.2 kg. It has a smooth horizontal axle. A light inextensible string passes over the pulley and masses 1.0 kg and 5.0 kg are attached to the string.
Assume that the string does not slip on the pulley. The system is released from rest.
(a) Calculate the moment of inertia of the pulley.
(b) Calculate, using energy considerations or any other method, the speed of the 5.0 kg mass when it has descended 2.0 m.
(c) Calculate the linear acceleration of the 1.0 kg mass.
(d) Determine the angular acceleration of the pulley.
Question 4 [ 10 Marks]
A Physics student assembles an experimental compound microscope using a 10.0 mm focal length objective lens and a 12.5 mm focal length eyepiece. The student places the two lenses 72.0 mm apart and places a 0.50 mm object 12.0 mm in front of the objective, perpendicular to the principal axis.
(a) Calculate the position of the image formed by the objective lens. Is this a real or a virtual image? Is it upright or inverted?
(b) Calculate the position of the final image which is formed by the eyepiece. What is the nature of this image? (real/virtual, upright/inverted).
(c) Calculate the magnitudes of the magnification of
(i) the objective lens, and
(ii) the eyepiece.
(d) What is the size of the final image?
Question 5 [ 10 Marks]
The diagram below shows a very long straight wire AA1 which carries a steady current of 10.0 A to the East. PQRS is a horizontal rigid loop of wire in which there is a small battery maintaining a constant current of 1.0 A clockwise. AA1 is in the plane of the loop.
(a) Calculate the magnetic flux density due to the current in wire AA1 0.10 m south of the wire AA1.
(b) Calculate the magnetic force exerted by the current in AA1 on PQ.
(c) Calculate the magnetic force exerted by the current in AA1 on SR.
(d) Calculate the total magnetic force experienced by the loop PQRS.
(e) Explain why the current in QR and PS can be neglected in the calculation of the net force which acts on the loop PQRS.