# What is Internal resistance: Definition and 157 Discussions

A practical electrical power source which is a linear electric circuit may, according to Thévenin's theorem, be represented as an ideal voltage source in series with an impedance. This impedance is termed the internal resistance of the source. When the power source delivers current, the measured voltage output is lower than the no-load voltage; the difference is the voltage drop (the product of current and resistance) caused by the internal resistance. The concept of internal resistance applies to all kinds of electrical sources and is useful for analyzing many types of electrical circuits.

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2. ### B Troubleshooting Battery Internal Resistance Measurements

Have been busy working with a student doing a Physics prac to determine the internal resistance of an AA 1.5 V (nominal) battery. The basic formula being investigated is: $$Vt=E_{cell}−Ir_i$$ where ##Vt## is the load (resistive only) voltage, ##E_{cell}## is the emf of the AA battery, ##I## is...
3. ### How to solve for EMF and internal resistance in a circuit?

I know the answers are 9 V and .5 ohms but I have no idea how to get there. Originally I solved for Delta V in the open circuit through (1.636)(5) which gave me 8.18V. Then, I solved for Delta V in the closed circuit through (1.565)(1/((1/5)+(1/10)) which gave me 5.51 V. I used these voltages...
4. ### Internal Resistance of the Battery - Ohmic Resistance

When current starts flowing in a circuit, voltage of the battery drops from EMF value. There are three sources of this however here I am only interested in ohmic resistance of the battery. EMF of the battery is created because of electrochemical processes on both electrodes which create...
5. ### Equation for the Internal Resistance of a Battery

Hello, i am currently working on a project where i have to calculate the internal resistance of a battery based on the voltage drop caused by placing a resistor over the battery. I am an EE, and i understand how i can calculate the IR by schematic, but i found this equation: ((Open circuit...
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### Putting internal resistance in series with a voltage source

Suppose I have a galvanic cell, where I've arbitrarily set the (-) anode to have a potential of zero volts and the (+) cathode to ##\epsilon## V. The electrodes are connected via the load, but also via the solutions and salt bridge in the centre. Edit: The two trailing wires are connected to a...
7. ### Parallel circuit with internal resistance

4 resistors R1= 10 Ohm R2= 7 Ohm R3= 6 Ohm R4 = 18 Ohm, internal resistance of source is 0,5 Ohm and supply voltage is 27V Total resistance in parallel resistors 1/R = 0.465 taking the inverse of that i get 2,15 Ohm. Add to that the internal 0,5 and the total comes out to 2,65 Ohm. Total...
8. ### I How do I calculate the expected Gain?

I am doing an experiment in electronics with AC RLC circuits, But there are a few things about what I need to do that I don't understand. First: While I know in the circuit diagram we include the internal resistance of the Inductor in series, but I don't know why we include it in series rather...
9. ### Engineering Finding Internal Resistance through Circuit Analysis

Homework Statement Given this following circuit: What is the internal resistance of the meter movement ( R_I). This is part of a project I'm doing and I know the equation that gives you the internal resistance in this circuit. $$R_I = \frac{R_sR_p}{R_s - R_p}$$. However, I have no idea how...
10. ### Voltage drop in the internal resistance of a voltage source

In this circuit,first there will be voltage drop in the internal resistance of the voltage source.Mostly,the voltage source is assumed ideal in circuits,however,here a practical voltage source is assumed. So,there will be voltage drop in the internal resistance of the voltage source which will...
11. ### Calculating current with and without internal resistance

Homework Statement Calculate the magnitudes and directions of the currents in each of the 3 resistors in the circuit above, if... (a) ...both batteries have zero internal resistance. (b) ...each battery has an internal resistance of 1.0 Ω. Homework Equations I1 = I2 + I3 (Node Law)The...
12. ### Batteries in series, parallel, and internal resistance

Homework Statement Three identical batteries are first connected in parallel to a resistor. The power dissipated by the resistor is measured to be P. After that, the batteries are connected to the same resistor in series and the dissipated power is measured to be 4P (four times larger than for...
13. ### Emf in a circuit (multiple choice question)

Homework Statement Correct answer is Homework Equations emf = I(R + r) The Attempt at a Solution When current flows the voltage across the battery is less than the emf so it's not A I really don't understand why the answer is C because B, C, D all seem correct to me. Please help me with...
14. ### Question about measuring the internal resistance of power supplies

In measuring the internal resistance of a power supply, why is a resistor with low resistance used?
15. ### Internal Resistance of a Car Battery

Homework Statement An automobile battery has a terminal voltage of 12.8 V with no load. When the starting motor (which draws 90A) is running, the terminal voltage drops to 11.0 V. What is the internal resistance of the battery? Homework Equations V= EMF- Ir The Attempt at a Solution I think...
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### EMF and internal resistance laboratory work

1. The problem statement I did a laboratory work in which I had to find power and internal resistance. I used an ordinary battery so it's closed circuit. I did all tasks except writing a hypothesis and conclusion I can't really think of it. I don't know what to write since I just measure using...
17. I

### Electric power and internal resistance

Homework Statement Today I did a laboratory work in which I had to find power and internal resistance. I used an ordinary battery so it's closed circuit. I did all tasks except writing a hypothesis and conclusion I can't really think of it. I don't know what to write since I just measure using...
18. ### Unraveling the Mystery of EMF Internal Resistance

I'm having a difficult time understanding why the internal resistance of the EMF source is disregarded in this problem. 1. Homework Statement You are asked to determine the resistance per meter of a long piece of wire. You have a battery, a voltmeter, and an ammeter. You put the leads from the...
19. ### Finding the internal resistance of battery

Homework Statement I measured a battery rated at 1.5V(ε) and get 0.9V out and want to calculate the internal resistance (r). So, I model the battery as an ideal voltage source and a resistor in parallel. I know the voltmeter has a resistance of 1kΩ (R). How do I calculate the internal...
20. C

### Internal resistance of a battery cell

if a cell is running down, there would be a constant drop in p.d. across the cell, does the internal resistance of the cell increase or decrease?
21. ### Potentiometer and EMFs with internal resistance

Homework Statement In a potentiometer 99cm of wire of resistance 99ohms is connected with a battery of 50V and 1ohm internal resistance.Find emf of battery giving zero deflection for a length of 13cm. Homework Equations Sadly,the formula I know is only for ideal emfs,which states that: If two...
22. ### Potential difference of an emf with internal resistance

Homework Statement A battery of emf 2V and internal resistance 0.1ohm is being charged with a current of 5 ampere. In what direction will the current flow inside the battery?What is the potential difference between the two terminals of the battery? Homework Equations If a battery of emf E and...
23. ### Finding internal resistance & EMF of a battery in a circuit

Homework Statement When an external resistor of resistance R 1 = 14 Ω is connected to the terminals of a battery, a current of 6.0 A flows through the resistor. When an external resistor of resistance R2 = 64.4 Ω is connected instead, the current is 2.0 A. Calculate the emf and the internal...
24. ### Internal resistance of a cell varying with power drawn

If we continue to draw a specific amount of power from a cell (re-chargable), how will the internal resistance of the cell vary? Is there any particular mathematical relation between them? Please help me understand
25. ### Calculating EMF and Internal Resistance: A Practical Guide

Homework Statement I need to calculate EMF and internal resistance from the given resistance and voltage. R = 3 ohms U = 3,6 V EMF- ? I0 - ? r - ? Homework Equations I = U/R EMF = A/q I = EMF/R+r I0 = EMF/r The Attempt at a Solution I = 3,6/3 = 1,2 A I = EMF/R+r 1,2= EMF/3+r ^ I hit a wall...
26. ### Calculating Internal Resistance: Methods A and B

Homework Statement [/B] Calculate the internal resistance of V & A based on methods A & B below: Method A Voltmeter = 1.46 V Ammeter = 0.24 A E (emf of battery) = 1.48 V Method B Voltmeter = 1.48 V Ammeter = 0.24 A E (emf of battery) = 1.48 VHomework Equations Ohm's Law: V = I*R The...
27. ### Internal resistance of each cell+energy dissipated in 1 min

Homework Statement A bulb is used in a torch which is powered by two identical cells in series each of EMF 1.5 V. The bulb then dissipates power at the rate of 625 mW and the PD across the bulb is 2.5 V. Calculate (i) the internal reistance of each cell and (ii) the energy dissipated in each...
28. ### Internal resistance of the battery and energy transformed

Homework Statement The battery in the circuit below has EMF 5.4 V and drives a current of 0.30 A through a lamp. The voltmeter reading is 4.8 V. Explain why the voltmeter reading is less than the EMF of the cell. Calculate values for (a) the internal resistance of the battery, and (b) the...
29. ### Finding Internal Resistance of a Circuit

Homework Statement See image attached. I am currently stuck on part b. Homework Equations (Rt) (Rint) / Rt + (Rint) The Attempt at a Solution I thought that Rt would be 26.7 kohms, but my answer is not matching up with the answer in the book. I am unsure why since the resistors are in...
30. ### Find Internal Resistance From 2 Circuits w/ The Same Battery

Homework Statement A battery with an emf of 60 V and an internal resistance (r) is connected to a 10 ohm external resistance. The power lost inside the battery is 50w. The same battery is then connected to a 4.0 ohm resistance. The power loss is 200w. What is the internal resistance of the...
31. ### Finding Internal Resistance of a Battery

Homework Statement A real battery can be modeled as an ideal voltage source in series with a resistor. A voltmeter with input resistance of ##1000\Omega## measures the voltage of a 1.5V worn out battery as 0.9V. What is the internal resistance of the battery. Homework Equations ##V=IR## The...
32. ### Internal Resistance in a Battery

Homework Statement Three resistors are connected in series to a battery. The resistances are R1 = 15 Ω ,R2 = 25 Ω , and R3 = 30 Ω and the current through the 15-Ω, resistor is 2.7 A . What is the potential difference across the battery terminals if the battery is ideal? What is the potential...
33. ### Do EMF Sources Combine Conservative and Non-Conservative Electric Fields?

Hello, We know that the electric field generated by a changing magnetic field is not conservative. But, for example let' say we have an EMF source that has an internal resistor, the voltage drop across the source is equal to EMF-rI where r is the internal resistance. My question is, inside the...
34. ### Exploring the Relationship Between Wire Length and Voltage in a Circuit

Homework Statement We conducted an investigation into wire length, current and resistance but kept track of the voltage rather than relying on the power pack setting. We noticed the voltmeter (measuring across the power pack) reading increased as the length of the wire increased though by only...
35. ### Find both the internal resistance of a battery and EMF

Homework Statement When a nonideal battery is connected to a 3.0-Ω resistor in a circuit, the current in the circuit is 2.0A . When the same battery is connected to a 1.0-Ω resistor in a circuit, the current in the circuit is 3.0 A. What is the internal resistance of the battery? What is the...
36. ### Internal resistance proportional to current?

The terminal voltage and current are measured as the resistance is decresed through a cell with emf and internal resistance. Graph of V (y axis) I (x axis) shows straight line decreasing gradient. 1. Explain why terminal pd decreases as the current increases. is terminal pd the same as...
37. ### Does internal resistance affect balance point?

Homework Statement if a potentiometer has driver cell and wire AB ,then an unknown emf (emf<driver cell) is balanced at some length L. does the balance point change if internal resistance of driver cell is changed? Homework Equations V= kl K= potential gradient The Attempt at a...
38. ### Is it really ok to assume that internal resistance of a cell is 0?

is it really ok to assume that internal resistance of a cell is absent in a circuit ?
39. ### Internal Resistance of a Battery homework

Homework Statement Students are given some resistors, a battery with internal resistance, and an ammeter. They are asked to determine the emf e and internal resistance of the battery using just this equipment. Working with the circuit shown at right, they insert each resistor into the circuit...
40. ### Power wasted/delivered to a resistor

Homework Statement two 8 ohm resistors and a battery of emf 12.0V and internal resistance of 8ohms are connected in series. Calculate the power delivered to each external resistor and then calculate the power wasted due to internal resistance. I keep coming up with the answer 18 but apparently...
41. ### What's the point of a voltmeter having a high internal resistance?

Homework Statement I understand that voltmeters are supposed to have high internal resistances so that they won't draw much current. However, they are being attached parallel to the resistor anyways and according to Kirchhoff' Law that means the voltage through both the resistor for which we...
42. ### Why formula for internal resistance for charging battery v = e + Ir

Homework Statement Why is the formula for internal resistance for a charging battery v = e + Ir? When the battery is supplying I to the circuit I understand why the terminal voltage, V, equals emf - IR (voltage lost by internal resistance), but if a battery is charging, why is the...
43. ### Question involving EMF and internal resistance

A cell supplies a current of 2 Ampere, when it is connected to 5 ohm resistance and supply a current of 1.2 Ampere. If it is connected to resistance of 9 ohm, find EMF(electro motive force) and Internal Resistance
44. ### Calculating E.M.F and Internal Resistance of a Cell in Series Circuit

Homework Statement A cell of unknown e.m.f. , ε , and internal resistance, r, is connected in series with a variable resistor. A voltmeter is then connected across the terminals of the resistor. When the resistor has a value of 16.0 Ω the voltmeter reads 1.20 V. When the resistance is...
45. ### Ideal Batteries' Internal Resistance

Homework Statement A circuit is constructed with five resistors and one real battery as shown above right. We model. The real battery as an ideal emf V = 12 V in series with an internal resistance r as shown above left. The values for the resistors are: R1 = R3 = 59 Ω, R4 = R5 = 79 Ω and R2 =...
46. ### Does a flashlight battery have internal resistance?

Homework Statement Does a flashlight battery have internal resistance? Explain. Homework Equations V = ε - Ir The Attempt at a Solution Yes, they do have internal resistance. This results in the open circuit ε being different than the voltage measured across the terminals if a current is...
47. ### Internal Resistance of a battery

Homework Statement When a 10 ohm load is placed across the terminals of a battery, the terminal voltage is 11.0V. When a 100 ohm load is used instead, the terminal voltage is 11.9V. What is the internal resistance of the battery? Homework Equations V=IR The Attempt at a Solution...
48. ### Internal resistance of the battery and power dissipation

I got this question ( i won't include a lot of details because this is not the homework forum) where i need to calculate the power dissipation due to internal resistance. What i did is , i calculated total V and total R and I , then i treated the battery as a normal resistance ( eg. a 1kohm...
49. ### Internal resistance seen by the voltage source, Vx?

Hi everyone, I have attached an image of the op-amp configuration (all op-amps are ideal). Basically, the question asks to calculate the resistance seen through the voltage source. The answer is 3 kΩ but I just don't understand how this is found. If someone could assist me, I would really...
50. ### Lead Acid Battery Internal Resistance and Temperature

Lead Acid Battery "Internal Resistance" and Temperature Hello, I've reached my wit's end trying to understand the theory behind charging lead-acid batteries. I believe I have read over twenty different articles, maybe six hours worth of studying. According to what I've read: Charging...