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PHYSICS, M6 2019 VCE 7*

Students in a Physics practical class investigate the piece of electrical equipment shown in the diagram. It consists of a single rectangular loop of wire that can be rotated within a uniform magnetic field. The loop has dimensions 0.50 m × 0.25 m and is connected to the output terminals with slip rings. The loop is in a uniform magnetic field of strength 0.40 T.
 

  1. What name best describes the piece of electrical equipment shown in the diagram?   (1 mark)

  2. What is the magnitude of the flux through the loop when it is in the position shown in the diagram? Explain your answer.   (2 marks)

The students connect the output terminals of the piece of electrical equipment to an oscilloscope. One student rotates the loop at a constant rate of 20 revolutions per second.

  1. Calculate the period of the rotation of the loop.   (1 mark)

  2. Calculate the maximum flux through the loop. Show your working.   (1 mark)

  3. The loop starts in the position shown in the diagram.
  4. What is the average voltage measured across the output terminals for the first quarter turn? Show your working.   (2 marks)

Show Answers Only

a.    Alternator.

b.   \(0\ \text{Wb}\)

  • This is because the plane of the area of the coil is parallel to the direction of the magnetic field, not perpendicular.

c.    \(0.05\ \text{s}\)

d.    \(0.05\ \text{Wb}\)

e.    \(4\ \text{V}\)

Show Worked Solution

a.    The device is an alternator.

  • This is due to the input being mechanical motion and the output being AC current, whereas an AC motor is the opposite.
♦ Mean mark (a) 44%.
COMMENT: Many students incorrectly answered AC motor.

b.   \(0\ \text{Wb}\)

  • This is because the plane of the area of the coil is parallel to the direction of the magnetic field, not perpendicular.

c.    \(T=\dfrac{1}{f}=\dfrac{1}{20}=0.05\ \text{s}\)
 

d.    \(\Phi=BA=0.4 \times (0.5 \times 0.25)=0.05\ \text{Wb}\)
 

e.     \(\varepsilon\) \(=\dfrac{\Delta \Phi}{\Delta t_{\text{1/4 rotation}}}\)
    \(=\dfrac{0.05}{0.0125}\)
    \(=4\ \text{V}\)
♦ Mean mark (e) 51%.

PHYSICS, M6 2019 VCE 8 MC

An electrical generator is shown in the diagram below. The generator is turning clockwise.
 

   

The voltage between \(\text{P}\) and \(\text{Q}\) and the magnetic flux through the loop are both graphed as a function of time, with voltage versus time shown as a solid line and magnetic flux versus time shown as a dashed line. Which one of the following graphs best shows the relationships for this electrical generator?
 

Show Answers Only

\(A\)

Show Worked Solution
  • The magnetic flux initially is a maximum as the plane of the coil is perpendicular to the magnetic field lines.
  • The voltage can be determined by taking the negative gradient of the magnetic flux line. This is because  \(\varepsilon=-\dfrac{\Delta \Phi}{\Delta t}\).

\(\Rightarrow A\)

PHYSICS, M6 2019 VCE 7 MC

The coil of an AC generator completes 50 revolutions per second. A graph of output voltage versus time for this generator is shown below.
 

Which one of the following graphs best represents the output voltage if the rate of rotation is changed to 25 revolutions per second?
 

Show Answers Only

\(D\)

Show Worked Solution
  • \(f \propto \dfrac{1}{T}\), so if the frequency is halved to 25 revolutions per second, the period of the wave will be doubled.
  • The induced voltage is proportional to the rate of change of flux through the coil. As the rate of change of flux is halved, the induced voltage will also be halved.

\(\Rightarrow D\)

PHYSICS, M6 2021 VCE 6

The diagram shows a simple AC generator. A mechanical energy source rotates the loop smoothly at 50 revolutions per second and the loop generates a voltage of 6 V. The magnetic field, \(B\), is constant and uniform. The direction of rotation is as shown in the diagram.
 

  1. Sketch the output EMF between \(\text{P}\) and \(\text{Q}\) versus time, \(t\), on the grid below, starting with the loop in the position shown in the diagram. Show at least two complete revolutions.   (3 marks)
     

  1. Describe the function of the slip rings shown in the diagram.   (1 mark)

  1.  i. How could the AC generator shown be changed to a DC generator?   (1 mark)

  2. ii. Sketch the output EMF versus time, \(t\), for this DC generator.
  3.     Graph the output for at least two complete revolutions.   (2 marks)

 

Show Answers Only

a.   

         

b.    The function of the slip rings:

  • Produce an AC voltage output by maintaining a constant connection between the loop and the external circuit.
     

c.i.   Changing the slip rings to a split ring commutator.

c.ii. 

         

Show Worked Solution

a.    \(\text{Period}\ (T)=\dfrac{1}{f}=\dfrac{1}{50}=0.02\ \text{s}\).

b.    The function of the slip rings:

  • Produce an AC voltage output by maintaining a constant connection between the loop and the external circuit.
♦ Mean mark (b) 54%.

c.i.   Changing the slip rings to a split ring commutator.
 

c.ii.  The output of a DC generator is positive only.

   

♦ Mean mark (c)(i) 55%.

PHYSICS, M6 2022 VCE 5 MC

A simple electricity generator is shown in the diagram below. When the coil is rotated, the output voltage across the slip rings is measured. The graph shows how the output voltage varies with time.
 

The frequency of rotation of the generator is now doubled.

Which one of the following graphs best represents the output voltage measured across the slip rings?
 

Show Answers Only

\(C\)

Show Worked Solution
  •  If the frequency is doubled than the coil will rotate twice as fast, hence two periods will occur instead of one.
  • The induced emf is proportional to the frequency of the rotation of the coil, as the frequency is doubled so will the induced emf.

\(\Rightarrow C\)

PHYSICS, M6 2023 VCE 6

Kim and Charlie are attempting to create a DC generator and have arranged the magnets along the axis of rotation of the wire loop, J, K, L and M, as shown in the diagram. They are having some trouble getting it to work. They rotate the loop in the direction of the arrow, as shown in the diagram.
 

  1. Using physics concepts, explain why this orientation of the magnets will not generate an EMF.   (2 marks)
  1. Kim and Charlie decide to move the magnets so that an EMF is generated. On the diagram, draw the positions of the magnets to ensure that an EMF is generated.   (1 mark)

Show Answers Only

a.    EMF will not be generated:

  • The generator has the magnetic field parallel to the plane of the loop. 
  • Therefore, there is no flux cutting through the loop of wire.
  • Further, as the loop rotates there is no change in flux and no current or EMF produced.

b.   The magnets should be rotated by 90°.
 

Show Worked Solution

a.    EMF will not be generated:

  • The generator has the magnetic field parallel to the plane of the loop. 
  • Therefore, there is no flux cutting through the loop of wire.
  • Further, as the loop rotates there is no change in flux and no current or EMF produced.
♦ Mean mark (a) 46%.

b.   The magnets should be rotated by 90°.
 

PHYSICS, M6 2016 HSC 29

Explain how different discoveries in physics led to the development of THREE technologies, including the electric generator.   (6 marks)

Show Answers Only

Electric Generator

  • The discovery that a change in magnetic flux would induce an emf in a conductor was discovered by Faraday.
  • This discovery led directly to the development of the electric generator.

GPS Tracking

  • Einstein discovered the Theory of Special Relativity that is an explanation of how speed affects mass, time and space.
  • The Hafele-Keating atomic clock experiment involved flying atomic clocks at high speeds on aircraft and comparing them with synchronised clocks on the surface of Earth. This experiment helped validate time dilation which is a feature of Special Relativity.
  • This theory and its subsequent validation has seen it applied to GPS technology. Without adjustments for special relativity, GPS would not work with any meaningful degree of accuracy.

Television

  • Physicists discovered that electrons could be made to travel through evacuated tubes.
  • They were deflected by electric and magnetic fields and could cause a fluorescent screen to emit light.
  • The electrons could be manipulated precisely to form an image on the screen leading to the development of television.
Show Worked Solution

Electric Generator

  • The discovery that a change in magnetic flux would induce an emf in a conductor was discovered by Faraday.
  • This discovery led directly to the development of the electric generator.

GPS Tracking

  • Einstein discovered the Theory of Special Relativity that is an explanation of how speed affects mass, time and space.
  • The Hafele-Keating atomic clock experiment involved flying atomic clocks at high speeds on aircraft and comparing them with synchronised clocks on the surface of Earth. This experiment helped validate time dilation which is a feature of Special Relativity.
  • This theory and its subsequent validation has seen it applied to GPS technology. Without adjustments for special relativity, GPS would not work with any meaningful degree of accuracy.

Television

  • Physicists discovered that electrons could be made to travel through evacuated tubes.
  • They were deflected by electric and magnetic fields and could cause a fluorescent screen to emit light.
  • The electrons could be manipulated precisely to form an image on the screen leading to the development of television.

PHYSICS, M6 2016 HSC 16 MC

The cone of a speaker is pushed so that the coil moves in the direction shown.
 

Which row of the table correctly identifies the behaviour of the speaker and the direction of the current through the conductor?

\begin{align*}
\begin{array}{l}
\rule{0pt}{1.5ex}\textit{} & \textit{} \\
\textit{}\rule[1ex]{0pt}{0pt}& \textit{} \\
\rule{0pt}{2.5ex}\textbf{A.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{B.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{C.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{D.}\rule[-1ex]{0pt}{0pt}\\
\end{array}
\begin{array}{|c|c|}
\hline
\rule{0pt}{1.5ex}\textit{The speaker} & \textit{The direction of the} \\
\textit{behaves like a ...}\rule[1ex]{0pt}{0pt}& \textit{current is from ...} \\
\hline
\rule{0pt}{2.5ex}\text{generator}\rule[-1ex]{0pt}{0pt}&\text{\(X\) to \(Y\)}\\
\hline
\rule{0pt}{2.5ex}\text{generator}\rule[-1ex]{0pt}{0pt}& \text{\(Y\) to \(X\)}\\
\hline
\rule{0pt}{2.5ex}\text{motor}\rule[-1ex]{0pt}{0pt}& \text{\(X\) to \(Y\)} \\
\hline
\rule{0pt}{2.5ex}\text{motor}\rule[-1ex]{0pt}{0pt}& \text{\(Y\) to \(X\)} \\
\hline
\end{array}
\end{align*}

Show Answers Only

\(A\)

Show Worked Solution
  • Motion of the coil causes the induction of a current causing the speaker to behave like a generator.
  • A magnetic north pole is induced on the right hand side of the speaker coil in order to oppose the motion of the coil away from the magnet by creating an attractive force between the coil and the magnet (Lenz’s Law).
  • Using the right hand grip rule the direction of induced current is from \(X\)  to \(Y\).

\(\Rightarrow A\)


♦♦♦ Mean mark 25%.

PHYSICS, M6 2017 HSC 10 MC

The diagram shows a model of a generator connected to a galvanometer.
 

The loop is rotated continuously in a clockwise direction as viewed from the end nearest the galvanometer.

Which row of the table correctly identifies the type of generator and the movement of the needle of the galvanometer?
 

\begin{align*}
\begin{array}{l}
\rule{0pt}{1.5ex} \ \rule[-0.5ex]{0pt}{0pt}& \\
\rule{0pt}{2.5ex}\textbf{A.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{B.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{C.}\rule[-1ex]{0pt}{0pt}\\
\rule{0pt}{2.5ex}\textbf{D.}\rule[-1ex]{0pt}{0pt}\\
\end{array}
\begin{array}{|c|c|}
\hline
\rule{0pt}{1.5ex}\textit{Type of generator}\rule[-0.5ex]{0pt}{0pt}& \textit{Movement of the needle} \\
\hline
\rule{0pt}{2.5ex}\text{DC}\rule[-1ex]{0pt}{0pt}&\text{Swings between 0 and +}\\
\hline
\rule{0pt}{2.5ex}\text{AC}\rule[-1ex]{0pt}{0pt}& \text{Swings between – and 0}\\
\hline
\rule{0pt}{2.5ex}\text{DC}\rule[-1ex]{0pt}{0pt}& \text{Swings between + and –} \\
\hline
\rule{0pt}{2.5ex}\text{AC}\rule[-1ex]{0pt}{0pt}& \text{Swings between – and +} \\
\hline
\end{array}
\end{align*}

Show Answers Only

\(A\)

Show Worked Solution
  • The generator has a split ring commutator so it is a DC generator.
  • Using the right hand palm rule, induced current will flow clockwise as viewed from above through the external circuit.
  • This means current will flow into the positive terminal of the galvanometer causing a deflection towards +.

\(\Rightarrow A\)