• Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

Sources of e.m.f. – internal resistance.

Extracts from this document...

Introduction

Sources of e.m.f. – internal resistance.

A source of e.m.f. always has some resistance to electric current within it, called its internal resistance. The internal resistance of a source of e.m.f. has two effects:

  1. It results in a voltage across the terminals of the source dropping as a current is drawn from it.
  2. It results in the source being less than 100% efficient as energy is dissipated in the internal resistance as current flows through it.

The voltage quoted on the label of a source of e.m.f. such as battery is the voltage measured when no current is being drawn from it, often called the open-circuit voltage.

The internal resistance of a source of e.m.f.

...read more.

Middle

image00.png

On the graph above, when a source of e.m.f. delivers power to a load (resistance), there is a value of R for which the maximum power is transferred, as the graph shows. Calculation shows that this value is equal to r, the internal resistance of the source of e.m.f. So to calculate the internal resistance from this graph, we have to look at the maximum point on the line, which is 0.18W and the internal resistance for that is 23 ohms.

Consider a source of e.m.f. E with an internal resistance r, delivering a current I into a load resistance R.

The current I is given by:

I=E/R + r

The power P dissipated in the load is given by:

P=I2

...read more.

Conclusion

P occurs where dP/dR = 0, we can set this expression equal to zero. dP/dR = 0 when r – R = 0, that is, when R = r. Therefore the condition for matching source and load is that the resistance of the load must be equal to the internal resistance of the source.

Electromotive force (e.m.f.) is the difference in electric potential, or voltage, between the terminals of a source of electricity, e.g., a battery from which no current is being drawn. When current is drawn, the potential difference drops below the e.m.f. value. Electromotive force is usually measured in volts.

image01.png

To work out the electromotive force, we need to look at the current against voltage graph and look at the line and carry it on until it cuts the x-axes, which represent the voltage. So we estimate it at 3.3 volts.

...read more.

This student written piece of work is one of many that can be found in our AS and A Level Electrical & Thermal Physics section.

Found what you're looking for?

  • Start learning 29% faster today
  • 150,000+ documents available
  • Just £6.99 a month

Not the one? Search for your essay title...
  • Join over 1.2 million students every month
  • Accelerate your learning by 29%
  • Unlimited access from just £6.99 per month

See related essaysSee related essays

Related AS and A Level Electrical & Thermal Physics essays

  1. In this experiment, we will measure the e.m.f. and the internal resistance of a ...

    The experimental errors can be divided into two errors, systematic errors and random errors. The zero errors of the ammeter and voltmeter, the heating effect on resistance, the internal resistance of the connecting wires and unclear scales of the ammeter and voltmeter are belonged to the systematic errors.

  2. Investigating the E.m.f and Internal Resistance of 2 cells on different circuit Structures.

    the same, which doesn't help with the investigation when plotting the graphs or comparing readings, therefore milliamps which is a more accurate degree of accuracy is used. The resistor value used is also noted. I will also consider the percentage error of the resistor and the extra resistance during the

  1. Measuring the e.m.f. And Internal Resistance of a Cell

    This was 0.03 Volts out from the other, but this difference is negligible. I am happy with the values that I have obtained from this experiment; I found no major discrepancies between the expected results and the experimental evidence. I feel that the results and conclusions are accurate to the

  2. Design and Carry Out an experiment to determine the EMF and Internal Resistance of ...

    (NOTE: the voltage on the power pack must remain the same for the whole process) 5. To further the experiment, repeat the entire process using a different voltage on the power pack. RESULTS Voltage Current Resistance Internal Resistance E=IR + Ir V = E-Ir (Volts) (Amps (Ohms) (Ohms) (Volts) (Volts)

  1. The potato - a source of EMF

    = EMF V = Terminal p.d. I = Current r = Internal resistance R = Load resistance There are two cases where there is no difference between EMF and V, so that EMF = V, these are: 1. When there is no internal resistance at all.

  2. The aim of the experiment is to verify the maximum power theorem and investigate ...

    From the formula, we can know that the magnitude of the kinetic friction is direct proportional to the normal reaction force as the coefficient of kinetic friction is constant. The factors that can affect the kinetic friction are same as that of the static friction.

  1. To find out the internal resistance and EMF of a given power supply

    Skill 5 Conclusions and Evaluations Conclusions Several things could be found out from the data processing graph. Using the equation E = V + Ir, (where E is the EMF (V), V is the useful volts across the load (V), I is the current (A)

  2. Aim: To find out the internal resistance and EMF of a given power supply.

    I (Current) V (Volts) 1 2 1.97 2.2 1 2.41 3.9 0.7 2.63 4.7 0.5 2.68 10 0.3 2.85 15 0.2 2.91 22 0.1 2.96 33 0.1 3.01 47 0.05 3.04 100 0 3.1 150 0 3.11 . Skill 5 Conclusions and Evaluations Conclusions The internal resistances are given from the slopes of the graphs.

  • Over 160,000 pieces
    of student written work
  • Annotated by
    experienced teachers
  • Ideas and feedback to
    improve your own work