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Alternating Currents & Transformers (AQA A Level Physics): Exam Questions

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Alternating Currents & Transformers

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11 mark

An alternating voltage is displayed on the screen of a cathode–ray oscilloscope as shown in the image below.

qu1

The oscilloscope time–base is set to 0.50 ms div–1.

What is the frequency of the alternating voltage?

  • 2 Hz 

  • 2 × 10­–3 Hz

  • 500 Hz

  • 0.50 Hz

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21 mark

Which statement about alternating current is incorrect?

  • The rms current is the value of direct current that would produce an equivalent heating effect

  • The peak value of an alternating current is half of the peak–to–peak value of current

  • The rms current I subscript r m s end subscript = fraction numerator I subscript 0 over denominator square root of 2 end fraction where I subscript 0 is the peak current

  • The rms current is greater than the peak current

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31 mark

Many household appliances are connected to a 230 V r.m.s. mains supply.

What is the peak voltage of a household’s mains supply?

  • 0 V

  • 115 V

  • 230 V 

  • 325 V

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41 mark

A working transformer is set up such that the ratio of the number of turns on the primary coil to the number of turns on the secondary coil is 5:1.

The voltage across the primary coil is 10 V.

Which of the statements below is incorrect?  

  • The transformer is a step–up transformer

  • The ratio of the primary voltage to the secondary voltage is 5:1

  • N subscript P over N subscript S equals V subscript P over V subscript S

  • This transformer is connected to an ac power supply

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51 mark

An alternating voltage is displayed on an oscilloscope, as shown in the image below:

qu5

The y–gain of the oscilloscope is set to 2 V div–1.

What is the r.m.s. voltage of the signal?

  • 2.0 V

  • 2.8 V

  • 4.0 V

  • 8.0 V

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11 mark

A sinusoidal current has a time period of T. The peak current is I subscript 0 and the r.m.s. current I  flows in a resistor such that the mean power dissipated in the resistor is P.

Which of the following statements is correct?

  • The frequency of the alternating current is fraction numerator 2 straight pi over denominator T end fraction

  • The r.m.s. voltage V = P over I

  • The peak current  I subscript 0 = begin inline style fraction numerator I over denominator square root of 2 end fraction end style

  • The mean current is begin inline style I subscript 0 over 2 end style

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21 mark

A transformer has 500 turns on the primary coil and 1500 turns on the secondary coil.

The primary coil draws a current of 0.5 A from a 230 V ac supply. The current in the secondary coil is 0.15 A.

What is the efficiency of the transformer? 

  • 2%

  • 45%

  • 90%

  • 99%

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31 mark

Transferring electrical energy efficiently between the coils in a transformer requires various engineering considerations.

Which of the following statements would not help reduce power losses in a transformer?

  • Using a material for the core that is easily magnetised

  • Laminating the core 

  • Using coils made of thinner wires

  • Using a core that increases flux linkage across the primary and secondary coils

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41 mark

A direct current I develops a certain power in a variable resistor, which is set at 9 Ω.

To obtain the same power when a sinusoidal alternating current is used, the resistor has to be set at 3 Ω.

What is the r.m.s. value of the alternating current?

  • fraction numerator 1 over denominator square root of 3 end fraction I

  • I

  • square root of 3 I

  • 3 I

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51 mark

A coil P is connected to a 230 Hz mains supply of constant peak voltage. Coil P is magnetically linked to coil Q via a soft iron core, which is connected to a cathode–ray oscilloscope, as shown in the image below:

0glsVDZV_qu10

Which line, A to D, correctly describes the effect on the oscilloscope trace if the soft iron core is removed?

 

Height of the trace

Number of cycles on screen

A

decreases

stays the same

B

stays the same

decreases

C

stays the same

stays the same

D

increases

decreases

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    11 mark

    An alternating current I in Amps varies with time t in seconds according to the equation:

    I = 3sin (50t)

    What is the mean power dissipated by the alternating current in a resistive load of 10 Ω?

    • 21 W

    • 30 W

    • 44 W

    • 810 W

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    21 mark

    The voltage V of an a.c. source varying with time t is shown the figure below: 

    q12-7-10-hard-mcq-alternating-currents-and-transformers-1

    Which of the following gives the correct expression for the root–mean–square voltage, V subscript r m s end subscript if the peak voltage is V subscript 0?

    • 0

    • V subscript 0 over 2

    • V subscript 0

    • fraction numerator V subscript 0 over denominator square root of 2 end fraction

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    31 mark

    A sinusoidal alternating current flow through a resistor of 50 kΩ, as shown in the image below:

    q13-7-10-hard-mcq-alternating-currents-and-transformers-1

    Which graph correctly shows the power dissipated in the resistor? 

    q13-7-10-hard-mcq-alternating-currents-and-transformers-2

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      41 mark

      The voltage V of an a.c. source varying with time t is shown the figure below:

      aqa-a-7-10-hard-mcq-qu4

      The source is applied across a resistive load of 4.0 Ω.

      What is the rms current Irms in the load?

      • 0.0375 A

      • 0.5 A

      • 0.006 A

      • 1.2 A

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      51 mark

      For a transformer, the ratio of turns on the primary coil to the secondary coil is 10:1.

      The primary voltage and current are 15 V and 2 A respectively.

      If the efficiency of the transformer is 90%, which statement is incorrect?

      • The secondary voltage is 1.5 V

      • The output power is 30 W

      • The secondary current is 18 A

      • The energy lost in one hour is about 11 000 J

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