Physics question?

Question

1. A length of wire is cut into five equal pieces. The five pieces are then connected in parallel, with the resulting reistance being 2.00 ohms. WHat was the resistance of the original lenght of wor before it was cut up.

2. An 18.0 ohms, 9.00 ohms and 6.00 ohms resistor are connected in parallel to an emf source. A current of 4.00 A is in the 9.00 ohms resistor.

a. Calculate the equivalnet reistance of the ciruit.
b. What is the potential difference across the source
c. Calculate the current in the other resistors.

Answer 1

1. For parallel circuit. total resistance is
1/Rt = 1/r1+1/R2 +...+1/Rn where R1, R2,...Rn are teh values of the n resistors connected in parallel. Now if al the resistors are equal in values:

1/Rt = n/R or Rt = R/n

If Rt = 2 ohms and n =5 then R = 5*2 ohms = 10 ohms

2. a. Use the above formula

1/Rt = 1/6+1/9+1/18 = 3/18+2/18+1/18 = 6/18 = 1/3

So Rt = 3 Ohms

b. The voltage drop across each branch is equal to the source. So
V = 4 A * 9 ohms = 36 volts

c. SInce V = IR and V = 36 V

Current through 6 ohms I = 36/6 = 6 Amps

Current through 18 ohms I = 36/18 = 2 AMps

Note total current is 2+4+6 =12 Amps and V/Rt = 36/3 =12 Amps as it has to be.

Answer 2

1. Use this formula: 1/R1 + 1/R2 + 1/R3 +1/R4 +1/R5 = 1/2, where R1 = R2,3,4, and 5 (this is the rule for resistances in a parallel branch). Then solve for R1 and multiply the answer by five.
2.a Use the formula given above, to find Re. I get three ohms.
b Voltage drops are constant in a parallel branch so using Kirchhoff's law (V=IR) find the voltage drop of the 9 ohm branch. I get 36 volts.
c Use the same law as above. I get 2 amps for the 18 ohm branch, and 6 amps for the 6 ohm branch.

Answer 3

1. Assuming the resistances of the 5 pieces of wire are equal, with them being arranged in parallel with a net resistance of 2 ohms, there's 2 ways to figure it out.

The first is 1/Rt = 1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5 which can be used if the resistors in parallel are of unequal resistance. The other trick is if they're all of equal resistance is to divide the number of resistors by the value of each one and that'll get your total resistance.

So if we're assuming they're all equal resistance it becomes an equation like this.

Rt = value of resistor / # of resistors

so to find the value of each resistor (wire length) we multiply Rt by # of resistors in the circuit.

Rt * # of resistors = value of resistor

2 ohms * 5 resistors = value of resistor

10 ohms = value of resistor.

Now that we know the value of the resistors in the circuit we can simply add them up to get the resistance of the wire they were cut from.

10 ohms/resistor * 5 resistor = 50 ohms for the wire.

50 ohms Final answer.



2. Ok, to solve this problem we're going to have to use a couple of electronics equations.

a. To calculate the total resistance of a parallel circuit you will need to use the equation 1/Rt = 1/R1 + 1/R2...this is because the resistors are of different values.

1/Rt = 1/18 + 1/9 + 1/6

1/Rt = 1/27 + 1/6

1/Rt = 1/33

Rt = .03 ohms Final answer.


b. For this part of the problem you would use the equation V = IR (Voltage = Current * Resistance). You're given 2 out of the three variables you need. In a parallel circuit the voltage across each leg of the circuit is the same as the source.

So in this case the same amount of voltage is being dropped across all three resistors in this circuit, the only difference is the current across each leg. That's a function of the resistance value of each leg.

Vt = IR

Vt = 4.00A * 9 Ohms

Vt = 36 V Final answer.


c. Now that we know the voltage of the source we can calculate the current across the other resistors.


For the 18 ohm resistor

I = V/R

I = 36 v / 18 ohms

I = 2 A Final answer

For the 6 ohm resistor

I = V/R

I = 36 / 6

I = 6 A Final answer


I hope this helps you.

Answer 4

Let R = 5r
then
1/r + 1/r + 1/r + 1/r + 1/r = 1/2
5/r = 1/2
r = 10 Ω
R = 50 Ω

2.
a.
1/Req = 1/18 + 1/9 + 1/6
1/Req = (1/3)(1/6 + 1/3 + 1/2)
1/Req = (1/3)
Req = 1/3 Ω
b.
V = 4*9 = 36 V.
c.
36/18 = 2 A.
36/6 = 6 A.

Answer 5

AP Physics is tough. If your math grades are at least a B then I would consider taking AP Physics. It will be tough for you unless you are really strong in math.