A LevelPhysicsAQAExam QuestionsFields & Their ConsequencesAlternating Currents & Transformers

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

Exam code: 7408

3 hours31 questions

Select a download format for Alternating Currents & Transformers

Scroll for more

Select an answer set to view for
Alternating Currents & Transformers

Easy
Medium
Hard
1
1 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

Choose your answer

2
1 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

Choose your answer

3
1 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

Choose your answer

4
1 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

Choose your answer

5
1 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

Choose your answer

1
1 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

Choose your answer

2
1 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%

Choose your answer

3
1 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

Choose your answer

4
1 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

Choose your answer

5
1 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

    Choose your answer

    6
    Sme Calculator
    1 mark

    An alternating voltage is displayed on an oscilloscope. The Y-gain is set to 5.0 V per division and the timebase is set to 2.0 ms per division.

    Oscilloscope screen with a square grid. A sinusoidal waveform is centred on the horizontal axis. The waveform peaks at 3.0 divisions above the centre line and the troughs are 3.0 divisions below (peak-to-peak height = 6.0 divisions). One complete cycle of the waveform spans 4.0 divisions horizontally. At least two full cycles are visible on the screen

    Which row correctly gives the peak voltage and the frequency of the signal?

    Peak voltage / V

    Frequency / Hz

    A

    15

    125

    B

    15

    250

    C

    30

    125

    D

    30

    250

      Choose your answer

      7
      Sme Calculator
      1 mark

      A transformer has a primary coil of 600 turns connected to a 120 space straight V supply. The secondary coil has 100 turns. The current in the primary coil is 2.0 space straight A.

      The transformer has an efficiency of 75%.

      What is the current in the secondary coil?

      • 1.5 space straight A

      • 9.0 space straight A

      • 12 space straight A

      • 16 space straight A

      Choose your answer

      1
      1 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

      • 45 W

      • 810 W

      Choose your answer

      2
      1 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

      Choose your answer

      3
      1 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

        Choose your answer

        4
        1 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

        Choose your answer

        5
        1 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

        Choose your answer