AP Physics C Questions: Power and Efficiency in Transformers

📚 Quick Study Guide

• ⚡ A transformer changes AC voltage from one level to another.
• 🧲 The core of a transformer enhances the magnetic flux linkage between the primary and secondary coils.
• 🔢 The turns ratio ($N_p/N_s$) determines the voltage transformation: $V_p/V_s = N_p/N_s$.
• 💡 Power in an ideal transformer remains constant: $P_{in} = P_{out}$, so $V_p I_p = V_s I_s$.
• 🔥 Real transformers have power losses due to eddy currents and hysteresis in the core.
• 📉 Efficiency ($\eta$) is the ratio of output power to input power: $\eta = (P_{out}/P_{in}) \times 100\%$.
• 🛡️ Losses can be minimized by using laminated cores and high-quality materials.

Practice Quiz

1. A transformer has 500 turns on its primary coil and 100 turns on its secondary coil. If the input voltage is 120 V, what is the output voltage?
   1. 24 V
   2. 600 V
   3. 12 V
   4. 30 V
2. A transformer has an efficiency of 90%. If the input power is 100 W, what is the output power?
   1. 111.1 W
   2. 90 W
   3. 10 W
   4. 80 W
3. In a step-up transformer, which of the following is true?
   1. $N_p > N_s$ and $V_p > V_s$
   2. $N_p < N_s$ and $V_p < V_s$
   3. $N_p > N_s$ and $V_p < V_s$
   4. $N_p < N_s$ and $V_p > V_s$
4. What causes power loss in a real transformer?
   1. Increased voltage
   2. Eddy currents and hysteresis
   3. Decreased current
   4. Ideal core material
5. A transformer is used to step down the voltage from 240 V to 12 V. If the current in the secondary coil is 20 A, and assuming an ideal transformer, what is the current in the primary coil?
   1. 1 A
   2. 40 A
   3. 2 A
   4. 10 A
6. If the primary voltage is 200V and the secondary voltage is 50V, what is the turns ratio ($N_p/N_s$)?
   1. 1/4
   2. 4
   3. 250
   4. 1/250
7. A transformer has an input power of 500W and an output power of 450W. What is its efficiency?
   1. 111%
   2. 90%
   3. 10%
   4. 45%