Fig. 1 shows a simplified circuit on an electric train. Energy from an overhead wire is transferred to a motor. The motor turns the train wheels. When the t...
Question 1 Report
Fig. 1 shows a simplified circuit on an electric train. Energy from an overhead wire is transferred to a motor. The motor turns the train wheels. When the train slows down on a hill, the motor can operate as a generator and transfer some kinetic energy to electrical energy for the battery. The train controller records a current of 150 A at a potential difference of 600 V while the train is accelerating. It travels at a constant velocity for part of the route, so the driving force and resistive forces are balanced.
(a) What useful energy store increases as the train accelerates? [1]
(b) Calculate the electrical power supplied to the motor. [2]
(c) Calculate the electrical energy supplied in 30 s. [3]
(d) Describe the energy transfer when the train uses regenerative braking. [2]
(e) Which statement is correct when the train moves at constant velocity: the resultant force is zero, or the resultant force is forwards? [1]
(f) Give one reason why not all of the electrical energy reaches the wheels as useful kinetic energy. [1]
Answer Details
(a) As the train accelerates, its speed increases, so its kinetic energy store increases. [1]
(b) Electrical power is the rate of electrical energy transfer:
\[P=VI=600\ \mathrm{V}\times150\ \mathrm{A}=90\,000\ \mathrm{W}\]
The power supplied to the motor is 90 000 W (90 kW). [2]
(c) Use \(E=Pt\):
\[E=90\,000\ \mathrm{W}\times30\ \mathrm{s}=2\,700\,000\ \mathrm{J}\]
The electrical energy supplied is 2 700 000 J. [3]
(d) In regenerative braking, the train's kinetic energy is transferred to electrical energy as the motor operates as a generator. That electrical energy is then transferred to the battery's chemical energy store. [2]
(e) The resultant force is zero. At constant velocity there is no acceleration, so the forward driving force exactly balances the resistive forces. [1]
(f) Not all electrical energy becomes useful kinetic energy because some is dissipated, for example as thermal energy in the motor or wires. Sound and energy transferred against air resistance are also acceptable. [1]
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