1 of 7 I 1 Inductor 1 consists of a single loop of wire Inductor 2 is identical to 1 except it has two loops How do the self inductances of the two loops compare I Coil 2 Coil 1 I A L2 2 L1 B L2 2L1 C L2 2L1 Answer L2 2L1 in fact L2 4 L1 The self inductance L increases by 4 L I If we keep I fixed but double the number N of turns increases by 4 Total flux total N 1 N doubles but 1 BA also doubles because when we double the number of turns that doubles the current and so B is doubled I 2 Two long solenoids each of inductance L are connected together to form a single very long solenoid of inductance Ltotal What is Ltotal A 2L B 4L C 8L D none of these L L Ltotal PHYS1120 Concept Tests M Dubson University of Colorado at Boulder 2 of 7 Answer 2L The inductance of a solenoid is L on2A z where n is the number of turns per length n N z and z is the length In this case we did not change n but z length doubled so L doubles I 3 The same current I is flowing through solenoid 1 and solenoid 2 Solenoid 2 is twice as long and has twice as many turns as solenoid 1 and has twice the diameter Hint for a solenoid B o n I I 1 2 I What is the ratio of the magnetic energy contained in solenoid 2 to that in solenoid 1 that is what is A 2 B 4 C 8 U2 U1 D 16 E None of these Answer There is 8 times as much magnetic field energy in the large solenoid as in the small solenoid The B field is the same in both solenoids same n turns length so same B o n I so both solenoids contain the same energy per volume u U vol B2 2 o The larger solenoid has 8 times the volume 2X the length 4X the cross sectional area so it has 8 the energy PHYS1120 Concept Tests M Dubson University of Colorado at Boulder 3 of 7 I 4 An LR circuit is shown below Initially the switch is open At time t 0 the switch is closed What is the current thru the inductor L immediately after the switch is closed time 0 A Zero B 1 A C 0 5A D None of these R1 10 V 10V L 10H R2 10 After a long time what is the current from the battery A 0A B 0 5A C 1 0A D 2 0A E None of these Now suppose the switch has been closed for a long time and is then opened Immediately after the switch is opened the current thru R2 is A zero B not zero Answers Q1 zero The current thru an inductor cannot change instantly Q2 1A After a long time the current I is constant the emf across the inductor is zero and the inductor acts like a wire a short The circuit acts as shown below The resistor R2 is shorted out by the inductor no current flows thru R2 and I V R1 10 10 1A R1 10 V 10V R2 10 Q3 Not zero After the switch is opened the battery and R1 are no longer in the circuit The circuit is now as shown below The current thru the inductor will continue it can t change instantly and the current thru the inductor must flow thru the resistor since they are in series PHYS1120 Concept Tests M Dubson University of Colorado at Boulder 4 of 7 L 10H I R2 10 I 5 The switch in the circuit below is closed at t 0 R 20 V 10V L 10H What is the initial rate of change of current dI dt in the inductor immediately after the switch is closed Hint what is the initial voltage across the inductor A 0 A s B 0 5A s C 1A s D 10A s E None of these Answer 1A s By the Loop Law V battery voltage IR LdI dt Immediately after the switch is closed the current is zero because the current thru the inductor cannot change instantly so IR is zero so V L dI dt So dI dt V L 10V 10H 1 A s PHYS1120 Concept Tests M Dubson University of Colorado at Boulder 5 of 7 I 6 A transformer is connected to a battery as shown The voltage difference across the resistor R is A V N2 N1 B V N1 N2 C V D zero E not enough information to answer V N2 N1 R iron core Answer zero It s a trick question Transformers only work with AC voltages The DC voltage V from the battery produces a DC current in the primary coil but produces no voltage of any kind in the secondary coil Transformers work because of Faraday s Law the changing flux produced by the AC current in the primary coil produces an emf in the secondary coil If the flux is not changing there is no emf I 7 The primary coil of a transformer is connected to a battery a resistor and a switch The secondary coil is connected to an ammeter When the switch is thrown closed the ammeter shows A a zero current all the time B a non zero current for a brief time when the switch is closed C a steady non zero current after the switch is closed V A R iron core Answer There is a non zero current briefly as the switch is closed but then after a short time there is no current As the switch is closed the current in the primary changes from zero to some non zero value While the current is changing there is a changing B field and a changing flux which causes an emf in the secondary and a current flow in the secondary The time period over which the current changes from zero to non zero as the switch is closed is L R where L is the inductance of the primary coil and R is its resistance PHYS1120 Concept Tests M Dubson University of Colorado at Boulder 6 of 7 I 8 A step down transformer is attached to an AC voltage source and a resistor as shown How does the current in the resistor I R compare to the current drawn from the AC source Iinput With AC circuits we always use rms values of I and V A I R I in B I R I in C I R I in I in IR D Depends on the value of Iin Answer IR Iin For a step down transformer the voltage is stepped down But power in power out so IPVP ISVS so when the voltage is stepped down the current must be stepped up I 9 An electrical engineer at a power plant wants to reduce the energy wasted during power transmission from the plant to the …
View Full Document
Unlocking...