AP Physics C Questions: Step-Up and Step-Down Transformers

📚 Quick Study Guide

• ⚡ Transformer Basics: Transformers change AC voltage from one level to another through electromagnetic induction. They consist of two or more coils electrically isolated but magnetically linked.
• 🔢 Turns Ratio: The ratio of primary to secondary turns ($N_p/N_s$) determines the voltage transformation.
• 📈 Step-Up Transformer: Increases voltage ($N_s > N_p$). Voltage is stepped up proportionally to the turns ratio: $V_s/V_p = N_s/N_p$. Current is stepped down: $I_s/I_p = N_p/N_s$.
• 📉 Step-Down Transformer: Decreases voltage ($N_s < N_p$). Voltage is stepped down proportionally to the turns ratio: $V_s/V_p = N_s/N_p$. Current is stepped up: $I_s/I_p = N_p/N_s$.
• 💡 Ideal Transformer Equation: $V_p I_p = V_s I_s$ (Power in equals power out, assuming 100% efficiency).
• 💯 Efficiency: Real transformers have losses (e.g., hysteresis, eddy currents). Efficiency ($\eta$) is given by: $\eta = (P_{out}/P_{in}) \times 100\%$.

🧪 Practice Quiz

1. Which of the following is the primary function of a transformer?
   1. A) To convert AC voltage to DC voltage.
   2. B) To change the frequency of AC voltage.
   3. C) To step up or step down AC voltage.
   4. D) To amplify the current in a DC circuit.
2. In a step-up transformer, how does the number of turns in the secondary coil compare to the primary coil?
   1. A) The secondary coil has fewer turns than the primary coil.
   2. B) The secondary coil has the same number of turns as the primary coil.
   3. C) The secondary coil has more turns than the primary coil.
   4. D) The number of turns is unrelated to the type of transformer.
3. If a transformer has a turns ratio ($N_p/N_s$) of 1:5, and the primary voltage is 120V, what is the secondary voltage?
   1. A) 24V
   2. B) 120V
   3. C) 600V
   4. D) 1200V
4. What happens to the current in a step-down transformer?
   1. A) It increases.
   2. B) It decreases.
   3. C) It remains the same.
   4. D) It fluctuates randomly.
5. An ideal transformer has a primary voltage of 240V and a primary current of 2A. If the secondary voltage is 120V, what is the secondary current?
   1. A) 1A
   2. B) 2A
   3. C) 3A
   4. D) 4A
6. Which of the following factors contributes to energy losses in a real transformer?
   1. A) Hysteresis losses.
   2. B) Eddy current losses.
   3. C) Resistance in the windings.
   4. D) All of the above.
7. A transformer has an efficiency of 90%. If the input power is 500W, what is the output power?
   1. A) 400W
   2. B) 450W
   3. C) 500W
   4. D) 550W