10 minutes maximum! Can you do it in 5?

1-3. These questions are about RMS values.

An A.C. generator produces an emf as shown by this graph: It is connected to a 2Ω resistor.

What is the RMS voltage Vrms and the RMS current Irms produced in the resistor?

Vrms Irms
A
 4 √2
 8 √2
B
 8 √2
 2 √2
C
 4 √2
 2 √2
D
 8 √2
 8 √2

2. What is the RMS power in this circuit?

• A. 2√2 W
• B. 2 W
• C. 4√2 W
• D. 4 W

3. Which of these statements gives the best definition of RMS voltage from an A.C. supply?

• A. The arithmetic mean voltage from the supply.
• B. The voltage that is equal to half the peak supply voltage.
• C. The voltage that is equal to double the peak supply voltage.
• D. The voltage that has the same heating effect as the AC supply.

4. An AC generator is connected to a heating element with a fixed resistance of 10Ω. The average power output from the heater is 500 W.

What is the peak voltage output from the supply?

• A. 100 V
• B. 200 V
• C. 10 000 V
• D. 20 000 V

5-8. These questions are about transformers: 5. A transformer has an input voltage of 20 V and an output of 200 V.
For this transformer, which of these statements is true?

• A. This is known as a 'step-up' transformer, however the current is decreased.
• B. This is known as a 'step-up' transformer, because the current is increased.
• C. This is known as a 'step-down' transformer, because the current is decreased.
• D. This is known as a 'step-down' transformer, however the current is increased.

6. The transformer described in question 5 is connected to a heating element. The input power to the transformer is 20 W. Assuming the transformer is 100% efficient, the power produced as heat in the element will be:

• A. 2 W
• B. 10 W
• C. 20 W
• D. 200 W

7. A transformer is required to charge a 5 V phone from a 100 V power supply. If the transformer has 1000 turns on the primary coil, then the number of turns on the secondary coil must be:

• A. 50
• B. 500
• C. 2000
• D. 20000

8. Transformers are used to increase the AC voltage for transmission over long distances through cables. A high voltage is used because:

• A. The current is lower, causing less power lost as heat in the cables.
• B. The current can be higher, transmitting more power.
• C. The transformers are more efficient with a high voltage.
• D. The power transmitted is proportional to the square of the voltage.

9 & 10. A diode bridge rectifier circuit can be used to change an AC voltage into a DC voltage. 9. Which of the following diagrams shows the output of this diode bridge circuit? 10. A capacitor can be used to make the output 'smoother' with less variation. Which of the diagrams below correctly shows the typical placement of this capacitor? 