Ch 3 - L4 Electromagnetic Induction in Transformers

A step-down transformer changes the potential difference of an alternating current from 10‎ ‎000 V to 250 V.If it has 25 turns on its secondary coil, how many turns does it have on its primary coil?

• 250

• 1000

• 400

• 100

Consider a non-ideal transformer of efficiency 85%. When an ac voltage of 9 V is applied across the primary coil, a current of 2 A passes through the primary coil. The power lost due to the inefficiency of the transformer equals

• 18 W

• 3.2 W
• 2.7 W

• 1.5 W

The graph below represents the relation between the input power and the output power The voltage applied to the primary coil is 15 V, and the voltage induced across the secondary coil is 72 V. If the current in the secondary coil is 2 A, calculate the current in the primary coil.

• 2.5 A

• 7.68 A

• 9.6 A

• 12 A

A non-ideal transformer with an efficiency of 80%.An AC voltage is applied across its primary coil. The ratio between the current in the secondary coil and that in the primary coil is 4/25. The ratio between the voltage applied across the primary coil and that induced across the secondary coil is

• 1/5

• 4/25

• 5/1

• 25/4

a transformer that gives a voltage of 360 V across its secondary coil when an ac voltage with a peak value of 18 V is applied across its primary coil. If the current in its primary coil is 12 A and the current in its secondary coil is 0.45 A,then the efficiency of this transformer is equal to

• 133.3%

• 27 %

• 75%

• 63.

a 90% efficient transformer. If the voltage across the secondary coil is 4 V and the power consumed at the primary coil is 80 W,then the current in the secondary coil is

• 5A

• 18A

• 9A

• 20A

a non-ideal transformer of efficiency 80% with 30 turns on its primary coil and 600 turns on its secondary coil. If a power of 600 W is applied to the primary coil, then the output power will be equal to

• 480 w

• 600 w

• 12000 w

• 96000 w

Which of these values decreases in the secondary coil of an ideal step-up transformer?

• Potential difference

• Current frequency

• Current intensity

• Electric power

The diagram shows a direct current motor. The blue arrows shown represent forces that act on the coil. Which of the terminals 𝑎 or 𝑏 is the positive terminal of the motor?

• A

• B

Would the direct current motor shown in the following figure rotate clockwise or counterclockwise?

• Clockwise

• Counterclockwise