Question

In: Physics

Bulbs A, B, and C in the figure(Part A figure)are identical and the switch is an ideal conductor.

 

Bulbs A, B, and C in the figure(Part A figure)are identical and the switch is an ideal conductor. How does closing the switch in the figure affect the potential difference? Check all that apply.

  The potential difference across A is unchanged.
  The potential difference across B drops to zero.
  The potential difference across A increases by 50%.
  The potential difference across B drops by 50%.

 

One more bulb is added to the circuit and the location of the switch is changed. The new circuit is shown in the figure.(Part B figure)Bulbs A, B, C, and D are identical and the switch is an ideal conductor. How does closing the switch in the figure affect the potential difference? Check all that apply.

  The potential difference across A increases.
  The potential difference across B doubles.
  The potential difference across B drops to zero.
  The potential difference across D is unchanged.

 

 

Solutions

Expert Solution

Concepts and reason

The problem deals with the concept of the ohms law, series resistance and parallel resistance as the effect of closing the switch is to be determined on the voltage.

Determine the net resistance of the circuit when the switch is open and determine the change in voltage across A, B, C and D. And then clos the switch and then determine the net resistance of the circuit and the change in voltage across A, B, C and D.

Fundamentals

According to ohm’s law the current flowing through a circuit is directly proportional to the voltage across the circuit and inversely proportional to the resistance.

The ohm’s law can be represented as:

V=IRV = IR

Here II is the current, RR is the resistance and VV is the voltage.

When nn resistance R1,R2,R3.....Rn{R_1},{R_2},{R_3}.....{R_n} are in parallel in a circuit then the net resistance of the circuit will be:

1Rp=1R1+1R2+1R3+.............+1Rn\frac{1}{{{R_p}}} = \frac{1}{{{R_1}}} + \frac{1}{{{R_2}}} + \frac{1}{{{R_3}}} + ............. + \frac{1}{{{R_n}}}

Here Rp{R_p} is the net resistance.

When nn resistance R1,R2,R3.....Rn{R_1},{R_2},{R_3}.....{R_n} are in series in a circuit then the net resistance of the circuit will be:

Rs=R1+R2+R3+..........+Rn{R_s} = {R_1} + {R_2} + {R_3} + .......... + {R_n}

Here Rs{R_s} is the net resistance.

Since the resistance in the bulbs A, B, C and D is equal. So to make the calculation easy suppose the value of each resistance is 50Ω50\Omega and suppose the voltage of the battery is V=83.33VV = 83.33{\rm{V}} .

Now the resistance RB{R_B} and RC{R_C} are in series therefore the net resistance of these two resistance will be as:

Rs=R1+R2+R3+..........+Rn{R_s} = {R_1} + {R_2} + {R_3} + .......... + {R_n}

Substitute 50Ω50\Omega for RB{R_B} and 50Ω50\Omega for RC{R_C} as:

RBC=RB+RC=50Ω+50Ω=100Ω\begin{array}{c}\\{R_{BC}} = {R_B} + {R_C}\\\\ = 50\Omega + 50\Omega \\\\ = 100\Omega \\\end{array}

Now the resistance RBC{R_{BC}} and RD{R_D} are in parallel therefore the net resistance will be as:

1Rp=1R1+1R2+1R3+.............+1Rn\frac{1}{{{R_p}}} = \frac{1}{{{R_1}}} + \frac{1}{{{R_2}}} + \frac{1}{{{R_3}}} + ............. + \frac{1}{{{R_n}}}

Substitute 100Ω100\Omega for RBC{R_{BC}} and 50Ω50\Omega for RD{R_D} as:

1RBCD=1RBC+1RD=1100Ω+150Ω=1+2100Ω=3100Ω\begin{array}{c}\\\frac{1}{{{R_{BCD}}}} = \frac{1}{{{R_{BC}}}} + \frac{1}{{{R_D}}}\\\\ = \frac{1}{{100\Omega }} + \frac{1}{{50\Omega }}\\\\ = \frac{{1 + 2}}{{100\Omega }}\\\\ = \frac{3}{{100\Omega }}\\\end{array}

Rearrange the equation to determine RBCD{R_{BCD}} as:

1RBCD=3100ΩRBCD=100Ω3RBCD=33.33Ω\begin{array}{l}\\\frac{1}{{{R_{BCD}}}} = \frac{3}{{100\Omega }}\\\\{R_{BCD}} = \frac{{100\Omega }}{3}\\\\{R_{BCD}} = 33.33\Omega \\\end{array}

Now the resistance RBCD{R_{BCD}} and RA{R_A} are in series therefore the net resistance of these two resistance will be as:

Rs=R1+R2+R3+..........+Rn{R_s} = {R_1} + {R_2} + {R_3} + .......... + {R_n}

Substitute 33.33Ω33.33\Omega for RBCD{R_{BCD}} and 50Ω50\Omega for RA{R_A} as:

RABCD=RBCD+RA=33.33Ω+50Ω=83.33Ω\begin{array}{c}\\{R_{ABCD}} = {R_{BCD}} + {R_A}\\\\ = 33.33\Omega + 50\Omega \\\\ = 83.33\Omega \\\end{array}

Therefore the net resistance of the circuit when the switch is open is 83.33Ω83.33\Omega .

From the ohm’s law determine the current from the circuit as:

I=VRI = \frac{V}{R}

Substitute the value of voltage V=83.33VV = 83.33{\rm{V}} and the net resistance of the circuit R=83.33ΩR = 83.33\Omega as:

I=VR=83.33V83.33Ω=1A\begin{array}{c}\\I = \frac{V}{R}\\\\ = \frac{{83.33{\rm{V}}}}{{83.33\Omega }}\\\\ = 1{\rm{A}}\\\end{array}

Therefore again from the ohm’s law the voltage across the resistance RA{R_A} is as:

V=IRV = IR

Substitute the value of current I=1AI = 1{\rm{A}} and the value of resistance A RA=50Ω{R_A} = 50\Omega as:

VA=IRA=(1A)(50Ω)=50V\begin{array}{c}\\{V_A} = I{R_A}\\\\ = \left( {1{\rm{A}}} \right)\left( {50\Omega } \right)\\\\ = 50{\rm{V}}\\\end{array}

Since in this case the switch is closed so no current will flow from bulb C as the current will by pass from the alternative switch path. So there is no use of presence of resistance RC{R_C} in the circuit in this scenario.

Now the resistance RB{R_B} and RD{R_D} are in parallel therefore the net resistance will be as:

1Rp=1R1+1R2+1R3+.............+1Rn\frac{1}{{{R_p}}} = \frac{1}{{{R_1}}} + \frac{1}{{{R_2}}} + \frac{1}{{{R_3}}} + ............. + \frac{1}{{{R_n}}}

Substitute 50Ω50\Omega for RB{R_B} and 50Ω50\Omega for RD{R_D} as:

1RBD=1RB+1RD=150Ω+150Ω=1+150Ω=250Ω\begin{array}{c}\\\frac{1}{{{R_{BD}}}} = \frac{1}{{{R_B}}} + \frac{1}{{{R_D}}}\\\\ = \frac{1}{{50\Omega }} + \frac{1}{{50\Omega }}\\\\ = \frac{{1 + 1}}{{50\Omega }}\\\\ = \frac{2}{{50\Omega }}\\\end{array}

Rearrange the equation to determine RBD{R_{BD}} as:

1RBD=250ΩRBD=50Ω2RBD=25Ω\begin{array}{l}\\\frac{1}{{{R_{BD}}}} = \frac{2}{{50\Omega }}\\\\{R_{BD}} = \frac{{50\Omega }}{2}\\\\{R_{BD}} = 25\Omega \\\end{array}

Now the resistance RBD{R_{BD}} and RA{R_A} are in series therefore the net resistance of these two resistance will be as:

Rs=R1+R2+R3+..........+Rn{R_s} = {R_1} + {R_2} + {R_3} + .......... + {R_n}

Substitute 25Ω25\Omega for RBD{R_{BD}} and 50Ω50\Omega for RA{R_A} as:

RABD=RBD+RA=25Ω+50Ω=75Ω\begin{array}{c}\\{R_{ABD}} = {R_{BD}} + {R_A}\\\\ = 25\Omega + 50\Omega \\\\ = 75\Omega \\\end{array}

Therefore the net resistance of the circuit when the switch is closed is 75Ω75\Omega .

From the ohm’s law determine the current from the circuit as:

I=VRI = \frac{V}{R}

Substitute the value of voltage V=83.33VV = 83.33{\rm{V}} and the net resistance of the circuit R=75ΩR = 75\Omega as:

I=VR=83.33V75Ω=1.11A\begin{array}{c}\\I = \frac{V}{R}\\\\ = \frac{{83.33{\rm{V}}}}{{75\Omega }}\\\\ = 1.11{\rm{A}}\\\end{array}

Therefore again from the ohm’s law the voltage across the resistance RA{R_A} is as:

V=IRV = IR

Substitute the value of current I=1.11AI = 1.11{\rm{A}} and the value of resistance A RA=50Ω{R_A} = 50\Omega as:

VA=IRA=(1.11A)(50Ω)=55.50V\begin{array}{c}\\{V_A} = I{R_A}\\\\ = \left( {1.11{\rm{A}}} \right)\left( {50\Omega } \right)\\\\ = 55.50{\rm{V}}\\\end{array}

Therefore the voltage in the bulb A has increased from 50V50{\rm{V}} to 55.50V55.50{\rm{V}} when the switch is closed.

Ans:

The potential difference across A increases as the voltage in the bulb A has increased from 50V50{\rm{V}} to 55.50V55.50{\rm{V}} when the switch is closed.


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