Electrodynamics: Grade 12 Past Paper Questions

9.1 The diagram below shows the initial position of the coil in a simple DC generator. The coil is rotated in an anticlockwise direction, as shown. 9.1.1 Name the component in this generator that ensures that the induced current in the external circuit is in one direction only. 9.1.2 Is the direction of the induced current from X to Y or from Y to X? A maximum voltage of 90 V is generated when the coil is rotating at a frequency of 20 Hz. 9.1.3 Write down the time taken for the coil to complete ONE rotation. 9.1.4 The coil starts rotating from the initial position, as shown in the diagram above. Sketch a graph of output voltage versus time for one complete rotation of the coil. Indicate the maximum voltage and the relevant time values on the graph. 9.2 Wall sockets supply rms voltage and current. A 220 V AC voltage is supplied from a wall socket to an electric kettle having a resistance of 32 Ω. Calculate the average energy dissipated by the kettle in TWO minutes.

9.1 The diagram below represents a simplified version of an AC generator. The induced current flows from X to Y in the coil when the coil is rotated clockwise, as shown in the diagram below. 9.1.1 What is the polarity of A? Choose from NORTH or SOUTH. 9.1.2 State ONE way in which the induced emf of this generator can be increased. 9.1.3 Sketch a graph of induced current versus time for ONE complete rotation of the coil from the horizontal position, as shown in the diagram above. 9.2 An AC generator produces a maximum output voltage of 340 V. A 1200 W electric kettle operates optimally when it is connected to this generator. 9.2.1 Define the term root mean square current. 9.2.2 Calculate the root mean square current passing through the kettle. 9.2.3 Calculate the resistance of the kettle.

The graph below shows how the induced current in a generator varies with time. 9.1 Name the type of generator. Choose from AC or DC. 9.2 State the energy conversion that takes place in this generator while it is in operation. 9.3 Give a reason why this generator is NOT suitable for the transmission of electricity over long distances. 9.4 Calculate the frequency at which the coil rotates in the generator. 9.5 Define the term root mean square (rms) current. 9.6 Calculate the root-mean-square current delivered by the generator. 9.7 The graph below shows how the induced current varies with time after a change was made to the operation of the generator. Fully describe the change that was made.

9.1 The simplified sketch below represents an AC generator with the coil initially horizontal between the poles of a magnet. X and Y are two points on the coil, while A is one of the poles of the magnet. When the coil of the generator rotates clockwise between the two poles of the magnet, the direction of the induced current is from X to Y, as shown above. 9.1.1 Is A the NORTH POLE or the SOUTH POLE of the magnet? 9.1.2 The coil is now rotated through 180°. Will the direction of the current be from X to Y or from Y to X? 9.1.3 Sketch an emf-time graph for TWO complete rotations of the coil, starting from the position of the coil as shown in the diagram above. 9.2 An electrical device is connected to an AC generator. The rms potential difference across the device is 200 V and the maximum current passing through the device is 6 A. Calculate the: 9.2.1 Resistance of the device. 9.2.2 Energy consumed by the device in two hours.

The speed of rotation of the coil in the generator is now DOUBLED. 9.6 Copy the set of axes below in your ANSWER BOOK and sketch the graph of output voltage versus time for 0,1 s.

A simplified diagram of an AC generator connected to a 25 Ω resistor is shown below. The coil rotates anticlockwise. 9.1 Name the component that distinguishes this generator from a DC generator. 9.2 In which direction will the induced current flow in section XY of the coil? Choose from X to Y OR Y to X. The graph below shows the output voltage of the generator for one cycle of rotation of the coil. 9.3 Define the term rms potential difference. 9.4 Calculate the rms current in the circuit. 9.5 Calculate the average power dissipated in the 25 Ω resistor.

The simplified diagram below represents an AC generator with a coil rotating clockwise. X and Y are two points in the external circuit. 9.1 What is the direction of the current in the external circuit? Write either X to Y or Y to X. 9.2 State the energy conversion that takes place in this generator. The maximum voltage produced by the generator is 125 V. 9.3 Define the term root mean square voltage. 9.4 Calculate the root mean square voltage of the generator. 9.5 The total resistance in the external circuit is 42,4 Ω. Calculate the maximum current induced. 9.6 The generator induces current at a frequency of 20 Hz. The coil started rotating from the initial position, as shown in the diagram above. Sketch a graph of induced current versus time for two complete rotations of the coil. Indicate the following on the graph: the time taken for two rotations, and the maximum current induced by the generator.

The diagrams below show different positions of the coil, rotating in a clockwise direction, in an AC generator: Position A: 0°, Position B: 45°, Position C: 90°, Position D: 135°, Position E: 180°. 9.1 Use the principle of electromagnetic induction to explain how an AC generator works. 9.2 At which position(s) during the rotation of the coil will a maximum emf be induced? 9.3 Draw a graph of the induced emf versus time. Indicate the positions A, B, C, D and E on the graph. 9.4 A 100 Ω resistor is connected to this generator. If the peak voltage produced by the generator is 170 V, calculate the rms current passing through the resistor. 9.5 This generator is now changed to a DC generator. The coil is rotated at the same speed as before. 9.5.1 What change was made to the AC generator to convert it to a DC generator? 9.5.2 State the energy conversion in a DC generator. 9.5.3 Will the peak voltage of this generator still be 170 V? Choose from YES or NO.

A simplified representation of a DC motor is shown in the diagram below. The current in the coil is in the direction X to Y. 9.1 Name the component that ensures that the coil rotates continuously in ONE DIRECTION. 9.2 In which direction will the coil rotate? Write down only CLOCKWISE or ANTI-CLOCKWISE. 9.3 Write down the energy conversion which takes place while the motor is working. 9.4 An AC generator, producing a maximum voltage of 311,12 V, is connected to an electric heater of resistance 30 Ω. Calculate the: 9.4.1 Root mean square (rms) value of the voltage. 9.4.2 Root mean square (rms) value of the current in the heater.

1.9 An AC generator consists of a coil which is rotated in a magnetic field. The emf-time graph for one complete rotation of the coil is shown below. If the speed of rotation of the coil is now DOUBLED, which ONE of the following graphs is CORRECT for one complete rotation of the coil? (Four emf-time graphs are given as options, each showing a different peak emf and period.)

1.8 The diagram below represents a circuit in which all the external resistors have the same resistance. Which ONE of the ammeters in the circuit will have the LOWEST reading? A. A₁. B. A₂. C. A₃. D. A₄. 1.9 A simplified diagram of an electrical machine is shown below. What type of machine is this? A. A DC motor. B. An AC motor. C. A DC generator. D. An AC generator.

1.9 The simplified diagram below represents a DC motor. The diagrams below indicate some changes made to the above motor. Which of the changes to the motor above will change the original direction of rotation of the coil? A. (i) and (ii) only B. (i) and (iii) only C. (ii) and (iii) only D. (iii) only. 1.10 An atom has a ground state energy of x. When the atom moves to a higher energy state y, a line spectrum is observed. Which ONE of the following combinations is CORRECT for the ENERGY CHANGE of the atom and the TYPE OF LINE SPECTRUM observed during the transition? A. Energy change y – x, emission. B. Energy change x – y, emission. C. Energy change x – y, absorption. D. Energy change y – x, absorption.

1.9 In which ONE of the following electrical machines is electrical energy converted to mechanical energy? A. AC generator B. DC generator C. AC dynamo D. DC motor. 1.10 Which ONE of the following combinations correctly links an emission spectrum and an absorption spectrum to the energy transitions of an electron in an atom? A. Emission spectrum from low to high energy levels, absorption spectrum from high to low energy levels. B. Emission spectrum from low to high energy levels, absorption spectrum from low to high energy levels. C. Emission spectrum from high to low energy levels, absorption spectrum from high to low energy levels. D. Emission spectrum from high to low energy levels, absorption spectrum from low to high energy levels.

1.8 The circuit below was used in a practical investigation. The battery has emf ε and internal resistance r. Ignore the resistance of the connecting wires. Graphs K and L below were obtained from the readings taken. Which ONE of the combinations below CORRECTLY indicates the voltmeter readings taken to obtain graphs K and L? A. Graph K: V₁; Graph L: V₁. B. Graph K: V₁; Graph L: V₂. C. Graph K: V₂; Graph L: V₁. D. Graph K: V₂; Graph L: V₂. 1.9 Which ONE of the following is the function of the split-ring commutator in an electric motor? The split-ring commutator … A. rotates the coil. B. delivers current from the coil to the external circuit. C. ensures that the coil rotates continuously in one direction. D. ensures that the current in the external circuit changes direction.

1.8 The battery in the circuit below has negligible internal resistance. Switch S is closed. Switch S is now opened. How will the readings on the ammeter and voltmeter be affected? D. Ammeter reading increases; voltmeter reading increases. 1.9 The magnitude of the induced emf in an AC generator can be increased by: (i) increasing the strength of the magnetic field, (ii) increasing the speed at which the coil is rotated, (iii) increasing the number of turns in the coil. Which of the statements above are CORRECT? A. (i) and (ii) only. B. (i) and (iii) only. C. (ii) and (iii) only. D. (i), (ii) and (iii).

1.7 Point charges with magnitudes q, 2q, 3q and 4q are placed in different electric fields. The force on each point charge is measured and the results are recorded in the table below: A. a charge of 4q experiences a force of 60 N. B. a charge of q experiences a force of 20 N. C. a charge of 2q experiences a force of 25 N. D. a charge of 3q. Which point charge experiences the greatest electric field strength? 1.8 Two identical light bulbs, P and Q, are connected as shown in the circuit diagram below. The internal resistance of the battery can be ignored. When switch S is closed, which combination best represents the effect on the ammeter reading and the brightness of bulb P: A. reading on ammeter increases, brightness of bulb P stays the same. B. reading on ammeter stays the same, brightness of bulb P increases. C. reading on ammeter stays the same, brightness of bulb P stays the same. D. reading on ammeter increases, brightness of bulb P increases. 1.9 The diagram below shows a coil in a magnetic field. When the coil is part of a DC motor, which ONE of the following must be connected to X and Y: A. split ring (commutator). B. slip rings. C. AC supply. D. soft iron core.

1.9 The diagram below shows a simplified electric motor. The rotation of the coil is observed from the battery. Which ONE of the following statements is CORRECT while the motor is in operation? The coil and the … A. slip rings rotate anti-clockwise. B. slip rings rotate clockwise. C. commutator rotate clockwise. D. commutator rotate anti-clockwise. 1.10 Which of the following statements is/are TRUE for the photoelectric effect? The photoelectric effect demonstrates that: (i) Light has a wave nature. (ii) Light has a particle nature. (iii) Light energy is quantised. A. (i) only. B. (ii) only. C. (i) and (iii) only. D. (ii) and (iii) only.