AP Physics C Questions on Transformer Current and Turns Ratio

📚 Quick Study Guide

• 🔄 Turns Ratio: The ratio of the number of turns in the primary coil ($N_p$) to the number of turns in the secondary coil ($N_s$) determines the voltage and current transformation. Mathematically, $\frac{N_p}{N_s} = \frac{V_p}{V_s} = \frac{I_s}{I_p}$.
• ⚡ Voltage Transformation: In a step-up transformer, $N_s > N_p$, so $V_s > V_p$. In a step-down transformer, $N_s < N_p$, so $V_s < V_p$.
• 💡 Current Transformation: The current is inversely proportional to the turns ratio. If the voltage is stepped up, the current is stepped down, and vice versa.
• 🔋 Ideal Transformer: We assume ideal transformers are lossless (100% efficiency). Therefore, the power in the primary coil equals the power in the secondary coil: $P_p = P_s$, which means $V_pI_p = V_sI_s$.
• 🧐 Real Transformers: Real transformers have losses due to resistance in the windings, eddy currents in the core, and hysteresis. These losses reduce efficiency.

Practice Quiz

1. What is the relationship between the turns ratio and the current in an ideal transformer?
   1. A) Directly proportional
   2. B) Inversely proportional
   3. C) Exponentially proportional
   4. D) No relationship
2. A transformer has 500 turns in its primary coil and 100 turns in its secondary coil. If the primary voltage is 120 V, what is the secondary voltage?
   1. A) 24 V
   2. B) 600 V
   3. C) 120 V
   4. D) 50 V
3. If the current in the primary coil of a transformer is 2 A and the current in the secondary coil is 10 A, what is the turns ratio $N_p/N_s$?
   1. A) 5
   2. B) 0.2
   3. C) 2
   4. D) 10
4. A transformer is used to step down the voltage from 240 V to 12 V. If the primary current is 0.5 A, what is the secondary current, assuming an ideal transformer?
   1. A) 0.025 A
   2. B) 10 A
   3. C) 0.5 A
   4. D) 240 A
5. A transformer has a turns ratio of 4:1. If the secondary voltage is 50 V, what is the primary voltage?
   1. A) 200 V
   2. B) 12.5 V
   3. C) 50 V
   4. D) 4 V
6. Which of the following is NOT a source of energy loss in a real transformer?
   1. A) Resistance in the windings
   2. B) Eddy currents in the core
   3. C) Hysteresis
   4. D) Perfect insulation
7. What type of transformer is used to increase voltage?
   1. A) Step-down transformer
   2. B) Step-up transformer
   3. C) Isolation transformer
   4. D) Auto-transformer