Transformer

Which one of the following statements concerning transformers is false?

1 Their operation makes use of mutual induction.
2 They are an application of Faraday's and Lenz's laws.
3 A transformer can function with either an ac current or a steady dc current.
4 A transformer that steps down the voltage, steps up the current.
5 A transformer that steps up the voltage, steps down the current.

The current in the secondary coil of a step-up transformer is 0.86 A when the current in the primary coil is 4.8 A. Determine the turns ratio, Ns/Np, of the transformer.

1 5.6
2 0.24
3 0.12
4 4.1
5 0.18

A transformer has 450 turns in its primary coil and 30 turns in its secondary coil. Which one of the following statements concerning this transformer is true?

1 This is a step-up transformer.
2 The turns ratio is 15 for this transformer.
3 The ratio of the voltages Vs / Vp is 15 for this transformer.
4 The ratio of the currents Is / Ip is 0.067 for this transformer.
5 The power delivered to the secondary must be the same as that delivered to the primary.

A step-down transformer is used to:

1 increase the power
2 decrease the power
3 increase the voltage
4 decrease the voltage
5 change ac to dc

A step-up transformer is used to:

1 increase the power
2 decrease the power
3 increase the voltage
4 decrease the voltage
5 change ac to dc

Iron, rather than copper, is used in the core of transformers because iron:

1 can withstand a higher temperature
2 has a greater resistivity
3 has a very high permeability
4 makes a good permanent magnet
5 insulates the primary from the secondary

The core of a transformer is made in a laminated form to:

1 facilitate easy assembly
2 reduce i2R losses in the coils
3 increase the magnetic flux
4 save weight
5 prevent eddy currents

A generator supplies 100V to the primary coil of a transformer. The primary has 50 turns and the secondary has 500 turns. The secondary voltage is:

1 1000V
2 500V
3 250V
4 100V
5 10V

The primary of a 3 : 1 step-up transformer is connected to a source and the secondary is connected to a resistor R. The power dissipated by R in this situation is P. If R is connected directly to the source it will dissipate a power of:

1 P/9
2 P/3
3 P
4 3P
5 9P

A device that transforms electrical energy to mechanical energy is a Choose one answer.

1 generator.
2 transformer.
3 magnet.
4 motor.
5 none of the above

A certain transformer doubles input voltage. If the primary coil has 12 A of current, then the current in the secondary coil is Choose one answer.

1 2 A
2 6 A
3 12 A
4 24 A

The primary coil of a transformer has 100 turns on it and the secondary coil has 50 turns on it. This is Choose one answer.

1 a step-down transformer.
2 a step-up transformer.
3 either of the above, depending on relative input and output currents

A transformer works because

1 electricity flows from a primary coil to a secondary coil.
2 electricity is induced in a rotating coil.
3 a changing magnetic field is transferred to a coil via an iron core.
4 electricity is transferred to a coil via an iron core


Point 1: X = Y = slope = Point 2: X = Y = I = Ampere