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Emf and Internal Resistance

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Emf and Internal Resistance

The electromotive force, known as emf, is the terminal potential difference of a source when there is no current flow. Internal resistance is the resistance to current flow inside the source itself. But, importantly, how do we calculate these values? Let’s find out.

What is emf in electric circuits?

All voltage sources create a potential difference, providing current when connected to a circuit with resistance. This potential difference produces an electric field that acts on charges as a force, causing current to flow.

Despite its name, emf is not exactly a force. In fact, it is a unique kind of potential difference and is measured in volts (V).

Emf is the potential difference of the source when there is no current flowing through it.

We can also define emf as the work W done per unit charge Q, which gives us the following equation:

Emf and internal resistance Solar panels StudySmarterAll voltage sources produce an emf, Unsplash

Think of a battery.

  • If the battery is supplying current, the voltage across the battery’s terminals is less than the emf. As the battery depletes, this voltage level begins to decrease.
  • When the battery is fully depleted and hence not supplying current, the voltage across the batterys terminals will equal the emf.

How do we calculate emf?

We can also calculate emf (ε) with the equation below:

E stands for electrical energy in joules (J), and Q is the charge in coulombs (C).

In this equation, the potential difference is called the terminal potential difference. It will be equal to the emf if there is no internal resistance. However, this is not the case with real power supplies because there is always an internal resistance. Lost volts refer to the energy spent per coulomb while overcoming the internal resistance.

We know that the conservation of energy is apparent in electric circuits, and it is valid for the cases where there is internal resistance as well.

Emf and internal resistance Conservation of energy in internal resistance StudySmarterThe equation for conservation of energy with internal resistance, Oğulcan Tezcan – StudySmarter Originals

Lost volts is the name given to the energy spent per coulomb while overcoming the internal resistance. Also, be sure to check out our explanation on Energy Conservation.

What is internal resistance in electric circuits?

We already know that the load resistance (also known as external resistance) is the total resistance of the components in an external electric circuit. On the other hand, internal resistance is the resistance within the power source that resists current flow. It usually causes the power source to generate heat.

  • Load resistance = the total resistance of the components in an external electric circuit.
  • Internal resistance = the resistance within the power source that resists current flow.

Measuring the internal resistance

Ohms law

From Ohms law, we know that

where V is the voltage in volts, I is the current in amperes, and R is the external resistance in ohms.

Internal resistance

If we include the internal resistance, the total resistance will be R+r where internal resistance is shown by r, and the voltage can be expressed as emf (ε).

If you expand the brackets, you will get

where I⋅R is the terminal potential difference in volts, and I⋅r is the lost volts (also measured in volts).

Now we can rearrange the equation as

where VR is the terminal potential difference and Vr is the lost volts.

Relationship between terminal potential difference and lost volts

Here is the relationship between terminal potential difference and lost volts. You can see from the equation that if there is no internal resistance (so no lost volts), the terminal resistance will be equal to the emf.

Emf and Internal Resistance Internal and load resistance diagram StudySmarterA circuit diagram that shows the internal and load resistances, Oğulcan Tezcan – StudySmarter Originals

Internal resistance (r) has complex behaviour. Lets look at our battery example again. As the battery depletes, its internal resistance rises. But what else affects the internal resistance? Here are some factors:

  • The size of the voltage source.
  • How much and how long it has been used for.
  • The magnitude and direction of the current through the voltage source.

What are some examples of calculating emf and internal resistance?

Calculating the internal resistance of a source is an important factor in achieving optimum efficiency and getting the source to provide maximum power to the electric circuit. Here are some examples of calculating different quantities with internal resistance.

Remember that R is for load resistance and r is for internal resistance.

A battery has an emf of 0.28V and an internal resistance of 0.65Ω. Calculate the terminal potential difference when the current flowing through the battery is 7.8mA.

Solution

Emf (ε), internal resistance (r), and the current (I) flowing through the battery are given in the question. Lets put these into the terminal potential difference (VR) equation.

A cell has 0.45A flowing through it with an internal resistance of 0.25Ω. Find the energy wasted per second on the internal resistance in joules.

Solution

We know that

where P is the power in watts, I is the current in amperes, and R is the resistance in ohms.

Since the question asks for the energy wasted per second, we use the power equation because power is energy per second. We can also put the internal resistance r for resistance in the equation.

A battery has an emf of 0.35V. The current flowing through the battery is 0.03A, and the load resistance is 1.2Ω. Find the internal resistance of the battery.

Solution

The emf value (ε) of the battery, the current (I) flowing through the battery, and the load resistance (R) are all given in the question. This is the right equation to use to find the internal resistance (r):

Lets put the given variables into the equation:

If we solve the equation for r, we will get

Emf and Internal Resistance - Key takeaways

  • Electromotive force is not exactly a force: it is a unique kind of potential difference and is measured in volts.
  • If there is no current, the voltage across the terminals of the voltage source will be equal to the emf.
  • Lost volts is the given name for the energy spent per coulomb while overcoming the internal resistance.
  • Internal resistance is the resistance within the power source that resists current flow and generally causes the power source to generate heat.
  • The internal resistance of a voltage source depends on a variety of conditions, including how much it has been used, the size of the voltage source, the magnitude, and the direction of the current flowing through the voltage source.

Frequently Asked Questions about Emf and Internal Resistance

By using the following equation, you can determine the emf and internal resistance of an electrical cell. The equation that describes the relation between emf, terminal voltage, and internal resistance is ε = V+ Vr, where ε is emf in volts, VR is the terminal voltage in volts, I is current in amperes, and r is the internal resistance in ohms. 

Calculating the internal resistance of a source is an important factor in achieving optimum efficiency and getting the source to provide maximum power to the electric circuit. By using the following equation, you can calculate efficiency with emf and internal resistance. The equation that describes the relation between emf, terminal voltage, and internal resistance is ε = VR + Vr, where ε is emf in volts, VR is the terminal voltage in volts, I is current in amperes, and r is the internal resistance in ohms. 

If you draw a graph that has the terminal potential difference on the y-axis and the circuit's current on the x-axis, you will obtain a straight line that has a negative gradient. The emf is then the intercept on the y-axis and the gradient represents r, the internal resistance. 

Emf is the potential difference of the source when there is no current flowing through it, and internal resistance is the resistance within the power source that resists current flow.

It is important to know the emf and internal resistance values of a source in order to determine how to get the source to provide maximum power to an electric circuit.

Final Emf and Internal Resistance Quiz

Question

A battery has an emf of 0.45V. The current flowing through the battery is 0.07A and the load resistance is 1.3Ω. Find the internal resistance of the battery.

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Answer

5.13Ω

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Question

A cell has 0.40A flowing through it with an internal resistance of 0.35Ω. Find the energy wasted per second on the internal resistance in joules.

Show answer

Answer

0.056J

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Question

A battery has an emf of 0.42V and an internal resistance of 0.30Ω. Calculate the terminal potential difference when the current flowing through the battery is 6.7mA.


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Answer

0.418V

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Question

As a battery depletes, which of the following happens?


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Answer

Its internal resistance increases.

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Question

What is the symbol for internal resistance?


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Answer

The symbol for internal resistance is r.

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Question

What is the symbol for electromotive force?


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Answer

The symbol for electromotive force is ε.

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Question

Which unit is the electrical force measured in?


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Answer

Volts.

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Question

If there is no internal resistance, thus no lost volts, the terminal resistance will be equal to the emf. True or false?


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Answer

True.

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Question

What is the symbol for load resistance?


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Answer

The symbol for load resistance is R.

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Question

Electromotive force is a force, not a potential difference. True or false?


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Answer

False.

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Question

In real power supplies, there is always internal resistance. True or false?


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Answer

True.

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Question

Is the energy conservation law valid in electric circuits when there is internal resistance?


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Answer

Yes, the energy conservation law is valid in electric circuits when there is internal resistance.

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Question

We know that internal resistance wastes electrical energy. We also know that energy is conserved in electric circuits. Into which form of energy does this wasted energy convert?


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Answer

Thermal energy.

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