3.6 DC Circuits

16. If 2 coulombs flowed through a circuit in 2 seconds, the circuit would have. Option A. 1 amp. Option B. 2 volts PD. Option C. 4 amps.

Correct Answer is. 1 amp. Explanation. Q = It so I = Q/t.

22. If two resistors of 5 and 10 ohm respectively are connected in series and the current in the 5 ohm resistor is 1A. what is the current in the 10 ohm resistor?. Option A. 1 amp. Option B. It cannot be found without knowing the applied voltage. Option C. 1/3 amp.

Correct Answer is. 1 amp. Explanation. Resistors in series - current is the same in each one.

19. In the circuit shown the 24 volt battery has an internal resistance of 1 ohm and the ammeter indicates a current of 12 amperes. The value of the load resistance is. Option A. 2 ohms. Option B. 1 ohm. Option C. 6 ohms.

Correct Answer is. 1 ohm. Explanation. Find the total resistance R = V/I = 24/12 = 2 ohms. Subtracting internal resistance leaves 1 ohm.

42. A 24V battery has an internal resistance of 1 ohm. When connected to a load, 12 amps flows. The value of the load is. Option A. 12 ohms. Option B. 1/2 ohm. Option C. 1 ohm.

Correct Answer is. 1 ohm. Explanation. Total R = V/I = 24/12 = 2 ohms. Subtract the internal resistance to get the external resistance (i.e. the load).

46. For the diagram shown find the voltage at point B. Option A. 7V. Option B. 26V. Option C. 10V.

Correct Answer is. 10V. Explanation. Each resistor drops its own proportion of the voltage. Total resistance is 60 ohms. Voltage dropped by 4 ohm resistor is 4/60 * 30. Voltage dropped by 36 ohm resistor is 36/60 * 30. Total dropped by point B is 40/60 * 30 = 20V. Therefore, remaining voltage at B = 10V.

5. A circuit has a current flow of 6A. If the voltage is trebled, the new current will be. Option A. 6A. Option B. 18A. Option C. 2A.

Correct Answer is. 18A. Explanation. NIL.

10. A 3, 5 and 2 ohms resistance is connected in series with a 10 V battery. The voltage across the 2 ohms resistor is. Option A. 2 V. Option B. 10V. Option C. 4 V.

Correct Answer is. 2 V. Explanation. Each resistor takes its relative share of the voltage drop. Total resistance is 10 ohms, so the 2 ohm resistor takes 2/10ths of the voltage.

48. What is the voltage dropped across resistor 'A'?. Option A. 2 V. Option B. 20 V. Option C. 10 V.

Correct Answer is. 20 V. Explanation. 28-(2 + 6) = 20 V (voltage across both the parallel resistors is the same 20 V).

37. Referring to the drawing, if the volts dropped across the 20 ohm resistor is 10 volts, the resistance of R1 is. Option A. 2 ohms. Option B. 16 ohms. Option C. 20 ohms.

Correct Answer is. 20 ohms. Explanation. Find the current through the 20 ohm resistor (I = V/R = 10/20 = 0.5). The same current goes through all parts of a series circuit. Next find total circuit resistance (R = V/I = 28/0.5 = 56 ohms). Subtract 20 and 16 from 56.

26. A circuit consists of 3 ohm, 5 ohm and 12 ohm resistors in series. The current flowing in the 5 ohm resistor is 10 amps. What is the applied voltage?. Option A. 10 V. Option B. 100 V. Option C. 200 V.

Correct Answer is. 200 V. Explanation. NIL.

36. The source voltage in the circuit shown is. Option A. 200V. Option B. 50V. Option C. 150V.

Correct Answer is. 200V. Explanation. This is a standard potentiometer. It splits the voltage by the same ratio as the number of turns. So 50V is 300/1200ths of the source voltage (50 = 3/12V). Transpose for V.

35. What is the voltage at A?. Option A. 26V. Option B. 2V. Option C. 28V.

Correct Answer is. 26V. Explanation. 4 ohm resistor will drop 4/62ths of the 28V supply (i.e. 1.8V). So there will be approximately 26V left.

12. Three branches in a circuit have currents entering of 3A, 4A and 5A. A forth branch has 10 A leaving. A fifth branch must have. Option A. 22A leaving. Option B. 2A leaving. Option C. 2A entering.

Correct Answer is. 2A leaving. Explanation. Basic Kirkchoff's current law. Sum of currents entering = sum of currents leaving.

40. The unknown current in the network below is. Option A. 22A. Option B. 3A. Option C. 47A.

Correct Answer is. 3A. Explanation. Kirchhoff's current law.

24. If voltage is 100V, resistance is 25 ohms, what is the current?. Option A. 4 amperes. Option B. 0.4 amperes. Option C. 2500 amperes.

Correct Answer is. 4 amperes. Explanation. Ohms Law I = V/R.

49. What is the current through resistor C?. Option A. 4.5A. Option B. 18A. Option C. 9A.

Correct Answer is. 4.5A. Explanation. You need to know what the ohmic value of resistor C is to know the current through it, or at least the relative value of C compared to D - as the 9A current splits through C and D. It is 9A/2 (4.5A) only if C and D are the same. Besides, it cannot ever be 9A or 18A.

11. What is the PD of a circuit which has a 40 mA current and a 1 kilohm resistance?. Option A. 40 V. Option B. 400 V. Option C. 4 V.

Correct Answer is. 40 V. Explanation. V = I x R. mA * kohms cancel each others' prefix.

14. 20 amperes flow for 20 seconds. How many coulombs have flowed?. Option A. 1. Option B. 20. Option C. 400.

Correct Answer is. 400. Explanation. Q = It.

38. The current in the circuit shown is. Option A. 5 mA. Option B. 0.2 A. Option C. 5 A.

Correct Answer is. 5 mA. Explanation. Total R = 40 k ohms. I = V/R = 200/40,000 = 0.005A = 5mA. (don't forget the 'k' means times by 1000).

1. A potential difference of 50 volts produces a current of 10 milliamperes through a resistance of. Option A. 500 ohms. Option B. 5 ohms. Option C. 5,000 ohms.

Correct Answer is. 5,000 ohms. Explanation. PD = 50V, I = 0.01A, R = ?, R=PD/I.

18. The total current flowing in a circuit of 200 lamps in parallel, each of a resistance of 400 ohm and connected across an input of 100 volts is. Option A. 25 amps. Option B. 50 amps. Option C. 40 amps.

Correct Answer is. 50 amps. Explanation. Total resistance is 400 / 200 = 2 ohms. Ohms law for current I = V/R = 100/2 = 50 amps.

39. In the following circuit, the input at P is 4 amps and at Q is 5 amps. What is the voltage across the 6 ohm resistor?. Option A. 54V. Option B. 6V. Option C. 1.5V.

Correct Answer is. 54V. Explanation. V = I * R = 9 * 6 = 54V. A combination of Kirchhoff's current law and Ohm's Law.

2. A conductor with a positive Q (charge) of 4C has 12.56 x 1018 electrons added to it. It will have a Q =. Option A. 6. Option B. 2. Option C. zero.

Correct Answer is. 6. Explanation. A charge of 1 Coulomb (C) is 6.28 * 1018. Hence 4 + 2 = 6 C.

4. A current of 5A flows for 2 minutes. How many coulombs passed a point in the circuit?. Option A. 2.5. Option B. 600. Option C. 100.

Correct Answer is. 600. Explanation. Q = IT, Q = 5 * 120 = 600C.

6. A 10V battery supplies a resistive load of 10 ohms for 1 minute. What is the work done?. Option A. 60J. Option B. 600J. Option C. 10J.

Correct Answer is. 600J. Explanation. Work out power. Then Energy = Power * Time (time must be in seconds).

33. The reading on the ammeter in the circuit shown is. Option A. 3A. Option B. 12A. Option C. 6A.

Correct Answer is. 6A. Explanation. Total resistance = 2 ohms. I = V/R = 12/2 = 6 A.

44. The diagram shows a 200V long shunt generator. What is the voltage across the series resistor. Option A. 10V. Option B. 6V. Option C. 30V.

Correct Answer is. 6V. Explanation. The 0.04 and 100 ohm resistor are the field windings of the generator, so the 100 amps goes through the 0.06 ohm resistor. V = I * R = 100 * 0.06 = 6V.

8. The voltage at point A is. Option A. 28 V. Option B. 21 V. Option C. 7 V.

Correct Answer is. 7 V. Explanation. NIL.

50. Which is not thermally operated?. Option A. A limiting resistor. Option B. A current limiter. Option C. A fuse.

Correct Answer is. A limiting resistor. Explanation. NIL.

27. Two resistors are connected in series and have an e.m.f. of V volts across them. If the voltages across the resistances are V1 and V2 then by Kirchhoff's law. Option A. V2 = V1 + V. Option B. V1 = V2 + V. Option C. V = V1 + V2.

Correct Answer is. V = V1 + V2. Explanation. NIL.

20. If service No. 1 is isolated from the supply busbar shown there will be. Option A. an increase in supply voltage. Option B. a decrease in total current consumption. Option C. a decrease in supply voltage.

Correct Answer is. a decrease in total current consumption. Explanation. Shutting down a service on an aircraft (by pulling the circuit breaker for example), increases the total resistance and reduces the current consumption.

51. In a Desynn indicator system, the rotor is. Option A. an electromagnet. Option B. a permanent magnet. Option C. an AC magnet.

Correct Answer is. a permanent magnet. Explanation. NIL.

9. A loss of electrical insulation results in. Option A. an open circuit between the supply and earth. Option B. a short circuit between the supply and earth. Option C. an open circuit in the supply.

Correct Answer is. a short circuit between the supply and earth. Explanation. NIL.

25. A short circuit between the supply and earth. Option A. is not dangerous as the current drawn will be low. Option B. does not matter if the circuit uses the aircraft earth as a return. Option C. could be very dangerous as the current drawn will be very high.

Correct Answer is. could be very dangerous as the current drawn will be very high. Explanation. In a short circuit, the resistance is zero. By ohms law, I = V/R. If R = 0, I = V/0 so what is I (it is not zero!!!).

52. A load that is subject to a potential difference with a current running through it will. Option A. have a magnetic field around it with the magnitude and direction of the field determined by the corkscrew rule. Option B. dissipate power in the form of heat. Option C. will move in a direction shown by the right hand rule.

Correct Answer is. dissipate power in the form of heat. Explanation. NIL.

21. If the cross-sectional area of a conductor is doubled, with voltage constant, the current will. Option A. remain constant. Option B. double. Option C. halve.

Correct Answer is. double. Explanation. Get the formula for resistance of a conductor - and apply ohms law to it.

30. The current flowing through a circuit can be increased to four times its original value by. Option A. doubling the applied voltage and halving the resistance. Option B. doubling the resistance and doubling the applied voltage. Option C. halving the applied voltage and halving the resistance.

Correct Answer is. doubling the applied voltage and halving the resistance. Explanation. NIL.

47. In a series circuit with 3 resistors, the current is. Option A. lower than the smallest. Option B. the total of all 3 using ohms law to find the current. Option C. even in all 3.

Correct Answer is. even in all 3. Explanation. The current is the same in all components in a series circuit.

45. To find the internal resistance of a battery in a circuit of known current. Option A. use the formula R = V * I. Option B. find the lost volts of the circuit. Option C. find the emf of the circuit.

Correct Answer is. find the lost volts of the circuit. Explanation. Find the difference between the Voltage On-load and the Voltage Off-load (the volts 'lost' across the internal resistance), then use R = V/I (Ohm's Law).

28. A voltmeter is connected. Option A. in parallel. Option B. in series or parallel. Option C. in series.

Correct Answer is. in parallel. Explanation. Voltmeters are connected 'across' the component (i.e. in parallel with it. Ammeters are connected in series.

31. In a circuit containing three resistors of equal value connected in series and one of the resistors short circuits, the effect is for the current in the other two resistors to. Option A. decrease. Option B. increase. Option C. remain the same.

Correct Answer is. increase. Explanation. Reducing one resistor to zero ohms (short circuit) means the source emf is now applied to the two remaining resistors, and causes a greater current flow through them.

23. The voltage in a series circuit. Option A. is different in each component. Option B. is the same in each component. Option C. is less than it would be in a parallel circuit.

Correct Answer is. is different in each component. Explanation. NIL.

7. A galvanometer measures. Option A. millivolts. Option B. megohms. Option C. milliamps.

Correct Answer is. milliamps. Explanation. A galvanometer measures milliamps.

41. In a balanced Wheatstone bridge, across the centre of the bridge there is. Option A. current and voltage at maximum. Option B. no current flow. Option C. no voltage present at either end.

Correct Answer is. no current flow. Explanation. The ammeter reads zero when it is balanced.

29. Since electrical supplies taken from a bus-bar are in parallel, isolating some of the services would. Option A. reduce the current consumption from the bus-bar. Option B. increase the current consumption from the bus-bar. Option C. not affect the current consumption, it would reduce the voltage.

Correct Answer is. reduce the current consumption from the bus-bar. Explanation. Isolating some of the services increases the resistance of the total circuit similar to removing a resistor from a parallel circuit. So total current consumption reduces.

13. In a circuit containing three resistors of equal value connected in parallel, one resistor goes open circuit. The current in the other two resistors will. Option A. decrease. Option B. increase. Option C. remain the same.

Correct Answer is. remain the same. Explanation. NIL.

34. An ammeter is connected into a circuit in. Option A. series. Option B. shunt. Option C. parallel.

Correct Answer is. series. Explanation. ammeter is always put into a circuit in series with the component it is measuring the current through.

17. If the voltage across a resistor is doubled. Option A. the current is doubled. Option B. the current is halved. Option C. the resistance is halved.

Correct Answer is. the current is doubled. Explanation. NIL.

15. If the resistance of an electrical circuit is increased. Option A. the current will increase. Option B. the voltage will increase. Option C. the current will decrease.

Correct Answer is. the current will decrease. Explanation. NIL.

43. A parallel circuit with any number of 2 terminal connections. Option A. the individual voltage drops is equal to the emf. Option B. has the same current throughout. Option C. the resistance is dependent on current.

Correct Answer is. the individual voltage drops is equal to the emf. Explanation. Parallel circuit - the voltage drops across each branch is the source voltage (regardless of the resistance in the branches).

32. In a series resistive circuit. Option A. the total voltage is equal to the sum of the individual voltages. Option B. the total voltage is the same as the highest individual. Option C. the total voltage equals the difference between the individual voltages.

Correct Answer is. the total voltage is equal to the sum of the individual voltages. Explanation. NIL.

3. The power dissipated by a load of known resistance connected across a known potential difference can be calculated by. Option A. dividing the square of the potential difference by the value of the load resistance. Option B. multiplying the square of the potential difference by the value of the load resistance. Option C. working out the current flowing in the circuit by using ohms law and multiplying the result by the resistance.

Correct Answer is. working out the current flowing in the circuit by using ohms law and multiplying the result by the resistance. OR dividing the square of the potential difference by the value of the load resistance. Explanation. NIL.