1 of 11 Current Concept Tests CRKT 1 Vote TRUE A if both statements below are always true Otherwise vote FALSE B For resistors in series the current through each resistor is the same For resistors in parallel the voltage across each resistor is the same Answer True CRKT 2 A 1 resistor is placed in parallel with a 10 000 resistor as shown 1 I 10 000 The total equivalent resistance of these two resistor in parallel is closest to A a little less than 1 B a little more than 1 C 5000 D a little less than 10000 E a little more than 10000 Answer a little less than 1 You could use the formula R tot R 1 1 1 R1 R 2 1 0 9990 Or just think 1 is a very low resistance compared to 1 1 10000 1 10000 so almost all the current will flow through the 1 resistor the circuit will behave almost as if the 10000 resistor is not present and the equivalent resistance is close to 1 The question is is the equivalent resistance a little less or a little greater than 1 Adding the 10000 resistor in parallel provides another current path of the flow of charge More flow means lower resistance M Dubson Phys1120 Notes University of Colorado at Boulder 2 of 11 CRKT 3 The circuit below consists of a battery attached to two resistors in series Resistor R1 is variable When R1 is decreased the voltage V2 across R2 A increases B decreases C stays the same V1 R1 I1 V R2 I2 V2 Answer increases This is easiest to see if you let R1 go all the way to zero Kirchhoff s 2nd Law says that the battery voltage V V1 V2 The voltage drop is split between resistors 1 and 2 So V2 is smaller than the battery voltage V But if R1 0 then the full battery voltage V is across R2 V2 increased as R1 goes down CRKT 4 Two resistors R1 and R2 are hooked to a battery in parallel R1 is twice as large as R2 How does the current IB from the battery compare to the current I1 though R1 Hint IB I1 I2 IB V A R1 2 R2 IB I1 D IB 4I1 I1 R2 B IB 2I1 I2 C IB 3I1 E None of these Answer IB 3I1 The voltage across R1 is the same as the voltage across R2 but R1 is twice as large as R2 so I1 is half the size of I2 since I V R same V R twice as big means I half as large So if I1 1A I2 would be 2A and Itot IB I1 I2 1A 2A 3A which is three times as large as IB IB 3I1 M Dubson Phys1120 Notes University of Colorado at Boulder 3 of 11 CRKT 5 The four light bulbs shown are identical Which circuit puts out more light Hint more power more light V 12V V 12V A B C They both put out the same amount of light Answer The total equivalent resistance which the battery in the A circuit sees is R 2 two resistors each of resistance R in parallel The total equivalent resistance which the battery in the B circuit sees is 2R two resistors in series The total power coming V2 from the battery is P Smaller Rtot with fixed V results in a larger power P R tot So A puts out more light M Dubson Phys1120 Notes University of Colorado at Boulder 4 of 11 CRKT 6 In the circuit below what happens to the brightness of bulb 1 when bulb 2 burns out When a bulb burns out its resistance becomes infinite 2 A Bulb 1 gets brighter B Bulb 1 gets dimmer C Its brightness remains the same 3 1 Hint What happens to the current from the battery when bulb 2 burns out V 12V Answer When bulb 2 burns outs the filament inside breaks and R2 becomes infinitely large The total equivalent resistance which the battery sees increases since bulb 2 is gone there are fewer paths for the current flow so less flow more total resistance Since the battery sees a larger Rtot the current from the battery Itot V Rtot is reduced Less current from the battery means less current through bulb 1 less light Bulb 1 gets dimmer thus B M Dubson Phys1120 Notes University of Colorado at Boulder 5 of 11 CRKT 7 The three light bulbs A B and C are identical How does the brightness of bulbs B and C together compare with the brightness of bulb A A A Total power in B C power in A B Total power in B C power in A C Total power in B C power in A C B V 12V Answer If each light bulb has the same resistance R the series resistance of B and C is 2R Power P V2 R Larger total resistance for the B C pair means less power Total power in B C power in A therefore the correct answer is C CRKT 8 Two glowing light bulbs are in a battery operated circuit Light bulb A has greater resistance than light bulb B Which light bulb is brighter A B C Depends on the circuit Answer Depends on the circuit If the resistors the light bulbs are in series the larger R will be brighter But if the resistors are in parallel the smaller R will be brighter CRKT 9 Two light bulbs are in series attached to a battery as shown The bulbs are marked 40W and 60W Which bulb is brighter Hints More power brighter When light bulbs are in series they have the same current Light bulbs are intended to operate at 120V A both have same brightness V B 40W is brighter 40W M Dubson Phys1120 Notes 60W C 60W is brighter University of Colorado at Boulder 6 of 11 Answer This is a tricky one The answer is that the 40W bulb is brighter The bulbs are labeled assuming that they would be used with a voltage of 120V So 2 Plabeled 120V So IF V 120V THEN higher R means lower P The bulb with the R smaller Plabeled has the larger R The 40W bulb has higher R than the 60W bulb These two light bulbs are in series and light bulbs are not intended to be used this way In series the current is the same so the larger R produces the larger P I2 R CRKT 10 If you wanted to measure the current through the battery where in the circuit would you place an ammeter R2 B A A B C D or E None of these will work R1 R3 D C If you wanted to measure the current through resistor R2 where would you place and ammeter A B C D or E None of these will work V Answers Use location A to measure battery current since all of the current that is going through the battery is also running through this …
View Full Document
Unlocking...