Q&A for How to Calculate Total Resistance in Circuits

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How do I calculate the resistance of 2 resistors when I know the sum of the resistors?

Community Answer

Assuming you mean total resistance, you first need to determine if they are in series or parallel. In series the total resistance simply equals the sum of the resistors. In parallel, the inverse of the total resistance equals the sum of the inverse of each individual resistor. Therefore, you will not be able to calculate total resistance in a parallel circuit if you only know the sum.

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Question

If V = IR, how do I calculate if one cell = 2V and the resistor is 4 ohm?

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I = V/R . This is derived from the equation V =I R. In the question the value of potential difference (v) is mentioned as 2V, i.e, 2 volts. The value of resistance of the resistor is given as 4 ohms. Substitute these values in the first equation; i.e, l = V/R, so, I = 2/4. Therefore, I = 0.5 amps.

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Question

Can I use frequency to calculate resistance?

Community Answer

Resistance does not change with frequency. However, AC circuits do have a similar quality called reactance which does change with frequency. Learn more here.

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Question

How do I find total resistance in a series RL circuit with two resistors?

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The resistors are connected in series, so just add them together. You may also need to calculate impedance using the inductor's reactance.

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Question

There are four resistors connected in parallel. We know three resistor values, but how do we find the missing resistor value if no other info is given?

Community Answer

The problem is not possible to solve if this is the only info. If you know the circuit's total current and voltage, you can use Ohm's Law R = V/I to find the total resistance, then the formula for resistors in parallel to solve for the missing resistor value.

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Question

Is a battery's internal resistance included in total resistance?

Community Answer

Textbooks often use "perfect batteries" with zero resistance, but in the real world they do have a small amount of resistance. If this is relevant to the circuit diagram, the internal resistance is drawn in series with the voltage drop, with a box around them indicating that they are both part of the battery.

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Question

How do I find resistance in a skeleton cube?

Community Answer

Following the path of the current, draw the skeleton cube (12 wires joined into a hollow cube frame) as a two-dimensional circuit diagram. Use Kirchhoff's laws to write equations for each juncture and solve for unknown values. (Remember, if you know the voltage of the power source and can find the total current of the circuit, you can use Ohm's Law to solve for total resistance.)

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Question

How do I calculate resistance?

Community Answer

The resistance R in ohms (Ω) is equal to the voltage V in volts (V) divided by the current I in amps (A): Since the current is set by the values of the voltage and resistance, the Ohm's law formula can show that if you increase the voltage, the current will also increase.

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Question

How do I calculate a resistor if I know total resistance?

Community Answer

Rearrange the total resistance formula to solve for the unknown resistor. For example, if a series circuit has total resistance 10 ohms, one 7 ohm resistor, and one unknown resistor R, use the formula for total resistance in a series circuit: 10 = 7 + R.

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Question

What is the equivalent resistance between point A and point B in a parallel circuit?

Community Answer

Equivalent resistance in a parallel circuit formula is 1/eqR= 1/R1+1/R2. First, calculate 1/R1+1/R2 and then you will get a small decimal value. Then you do 1/ANS (1 over the answer) to find the value of the resistance.

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Question

If a rating of electrical heater is 1000W and 200V, calculate it resistance and the current it consumes when it is in use?

Community Answer

P=V/I , I = 230/1000, I = 0.23 Amps, V = I R, R= 0.23/230 = 0.001 Ohms.

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Question

How do I calculate the total resistance and current for each circuit?

Community Answer

It will depend on the configuration of each circuit, whether it's series or parallel.

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Question

How do I solve this: Given an incandescent light bulb rated at 75 watts and 240 volts, find the resistance of the light bulb?

Community Answer

You can use the formula Resistance (R) = Voltage (V) divided by Power (P). So that will mean 75 watts divided by 240 volts, which will give you a resistance of 3.2 ohms.

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Question

How can I calculate the internal resistance of a battery?

Community Answer

You can measure the voltage of the battery and then build a circuit using a fixed resistor. Measure the current in the circuit and the voltage of the battery in operation. Divide the difference in voltage readings by the current to get an estimate of the internal resistance. The resistance changes based on load current, though.

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Question

How do I find total current on a parallel circuit with resistors of 4ohm, 3ohm and 2ohm, and a voltage of 15v?

Jim Coombes

Community Answer

15v/4Ω + 15v/3Ω + 15v/2Ω = total current in amps. You are basically calculating the current through each resistor then adding them together.

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