Eng phys exam questions Flashcards
State the law of conservation of angular momentum.
(1 mark)
The (total) angular momentum (of a system) remains constant
provided no external torque acts (on the system)
State the energy transfers that take place from the moment the wheel is released until
the mass hits the floor. (2 marks)
-(Gravitational potential energy of falling mass) is converted to
linear/translational ke of mass and rotational ke of whee
-and internal energy in bearings/air around wheel
Explain why the indicated power for the air motor is different from the output power.
[1 mark
Indicated power (is developed by air in cylinder and) does not take into account frictional losses at bearings/between cylinder and piston/in opening valves
An electric motor drives a machine which stamps out shapes from sheet steel. The
machine is fitted with a flywheel of moment of inertia 25 kg m2 which is accelerated
uniformly until it is rotating at 640 rev min-1
. The machine then starts a stamping
operation which reduces the flywheel’s angular speed to 360 rev min-1
.
Explain why a flywheel is fitted between the motor and the stamping machine. ( 2 marks)
- Flywheel acts as store of energy
- Able to deliver large amount of EK in short time
- Without flywheel motor would stall during stamping (as load torque excessively high)
A student suggests that setting the time-base to 0.2 ms (from 0.5) division–1 might reduce
uncertainty in the determination of the time constant.
State and explain any possible advantage or disadvantage in making this
suggested adjustment ( 3 marks)
- idea that the waveform will stretch horizontally
- by a factor of 0.5/0.2 = 2.5
- Measuring larger distance reduces uncertainty
Connecting an identical resistor in parallel to a resistor in series:
halfs the resistance
Explain why the graph you plotted confirms that x decreases exponentially with n.
graph is linear and has negative gradient
Describe and explain two procedures the student should take to reduce
uncertainty in the measurements of p. (4 marks)
- Repeat experiment and calculate average
- To reduce impact of human error
- Repeat readings to detect anomalies
- So that they can be discarded
Explain why the rapid compression of the gas can be assumed to be an adiabatic
process. (2 marks)
- in adiabatic compression there is no heat transfer/Q = 0
- If compression is quick there is no time for heat transfer
When the lid is closed slowly, the compression can be assumed to be isothermal.
The gas can be compressed either isothermally or adiabatically from the same
initial conditions to the same final volume.
Compare without calculation the work done in each process (3 marks)
(think p-v graphs)
- Final pressure for adiabatic change greater
- In the same volume boundary in the p-v graph, greater area under adiabatic change curve
- So more work would be done adiabatically
Compare the theoretical and real engine cycles. In your answer you should:
• state and explain the differences between the cycles
• explain why the work output per cycle of the real engine is less than that
predicted by an analysis of the theoretical cycle. (6)
-Real engine needs induction and exhaust
strokes
-Ideal cycle needs no pumping loop/same air
used repeatedly
-Corners rounded on real cycle
Reason: valves take finite time to open and
close/combustion not instantaneous
-Heating/cooling cannot occur at constant
volume in real cycle
Reason: piston would have to stop
-In real cycle expansion & compression are not
adiabatic
Reason: heat transfer takes place to cooling
medium during these strokes
-In ideal cycle air only is taken through cycle
-In real engine some exhaust gas/fuel vapour is
present
-Max pressure is lower in real engine
Fuel may not be completely burnt
-Ideal cycle makes no reference to any
mechanism
-Area of loop is smaller for real engine, so less
work done per cycle
-Area of pumping loop has to be subtracted from
main loop, reducing work done
-Friction between moving surfaces/between
piston & cylinder/in bearings has to be
overcome
-energy expended in opening valves
