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PHYSICS, M6 EQ-Bank 6 MC

The diagram shows an ideal transformer.
 

When the switch is closed, the pointer on the galvanometer deflects.

How could the size of this deflection be increased?

  1. Decrease the number of primary coils.
  2. Decrease the number of secondary coils.
  3. Replace the iron core with a copper core.
  4. Place a resistor in series with the galvanometer.
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`B`

Show Worked Solution
  • In order to increase the deflection of the galvanometer, the current through the secondary coil must increase.
  • The input power (`V_(p)I_(p)`) is fixed.
  • Since  `V_(p)I_(p)=V_(s)I_(s)`, the voltage of the secondary coil (`V_(s)`) must decrease if `I_(s)` increases.
  •   `(V_(p))/(V_(s))=(N_(p))/(N_(s))\ \ =>\ \ V_(s)=(V_(p)N_(s))/(N_(p))`
  • Reducing the number of secondary coils (`N_(s)`) will therefore decrease the secondary voltage (`V_(s)`) and consequently increase the current through the secondary coil.

`=>B`

PHYSICS, M6 2017 HSC 28

Contrast the design of transformers and magnetic braking systems in terms of the effects that eddy currents have in these devices.   (6 marks)

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Transformers:

  • A transformer involves primary and secondary coils wound around a laminated iron core.
  • When an AC current is applied to the primary coil, changes in magnetic flux induce a current in the secondary coil.
  • Eddy currents have undesirable effects in transformers as the iron core is a conductor.
  • So, there is induction of unwanted eddy currents which leads to energy losses in the form of heat.
  • Lamination of the iron core minimises these eddy currents and subsequent energy loss.

Magnetic Braking Systems:

  • In contrast, eddy currents are beneficial in magnetic braking systems.
  • Magnetic breaking involves using eddy currents to produce a force that stops a moving vehicle by converting kinetic energy into heat energy.
  • In order to maximise induced eddy currents, and thus the breaking effect, magnetic breaks are designed with large sheets or discs of conductive material such as copper.
Show Worked Solution

Transformers:

  • A transformer involves primary and secondary coils wound around a laminated iron core.
  • When an AC current is applied to the primary coil, changes in magnetic flux induce a current in the secondary coil.
  • Eddy currents have undesirable effects in transformers as the iron core is a conductor.
  • So, there is induction of unwanted eddy currents which leads to energy losses in the form of heat.
  • Lamination of the iron core minimises these eddy currents and subsequent energy loss.

Magnetic Braking Systems:

  • In contrast, eddy currents are beneficial in magnetic braking systems.
  • Magnetic breaking involves using eddy currents to produce a force that stops a moving vehicle by converting kinetic energy into heat energy.
  • In order to maximise induced eddy currents, and thus the breaking effect, magnetic breaks are designed with large sheets or discs of conductive material such as copper.

♦ Mean mark 49%.

PHYSICS M6 2022 HSC 22

The diagram shows features of a transformer.
 


 

For TWO features of the transformer, describe how each contributes to the transformer's efficiency.   (4 marks)

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Continuous Iron Core:

  • The continuous iron core of the transformer increases the flux linkage between the primary and secondary coils. This maximises the proportion of magnetic flux produced by the primary coil that passes through the secondary coil.

Laminations:

  • The laminations in the iron core prevent the production of large eddy currents in the iron core. This reduces energy losses in the form of heat.
Show Worked Solution

Continuous Iron Core:

  • The continuous iron core of the transformer increases the flux linkage between the primary and secondary coils. This maximises the proportion of magnetic flux produced by the primary coil that passes through the secondary coil.

Laminations:

  • The laminations in the iron core prevent the production of large eddy currents in the iron core. This reduces energy losses in the form of heat.

PHYSICS, M6 2019 HSC 24

A step-up transformer is constructed using a solid iron core. The coils are made using copper wires of different thicknesses as shown.
 

The table shows electrical data for this transformer.
 

  1. Explain how the operation of this transformer remains consistent with the law of conservation of energy. Include a relevant calculation in your answer.   (3 marks)
  1. Explain how TWO modifications to this transformer would improve its efficiency.   (4 marks)
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a.   The energy input into this transformer is `text{500 J s}^(-1)`

  • The energy output is given by:
  •    `P=V_(s)I_(s)=50xx 9=450\ \text{J s}^(-1)`
  •  This is consistent with the law of conservation of energy as `text{500 J s}^(-1)` of energy is converted into other energy forms such as heat.

b.    Modification 1:

  • Laminating the iron core prevents large eddy currents from being induced in it.
  • This reduces energy loss in the form of heat, increasing efficiency.

Modification 2:

  • Increasing the thickness of the wire in the primary coil.
  • This reduces its resistance, increasing the transformer’s efficiency.
Show Worked Solution

a.   The energy input into this transformer is `text{500 J s}^(-1)`

  • The energy output is given by:
  •    `P=V_(s)I_(s)=50xx 9=450\ \text{J s}^(-1)`
  •  This is consistent with the law of conservation of energy as `text{500 J s}^(-1)` of energy is converted into other energy forms such as heat.

b.    Modification 1:

  • Laminating the iron core prevents large eddy currents from being induced in it.
  • This reduces energy loss in the form of heat, increasing efficiency.

Modification 2:

  • Increasing the thickness of the wire in the primary coil.
  • This reduces its resistance, increasing the transformer’s efficiency.

♦ Mean mark part 48%.