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## How does a battery create potential difference?

How battery creates potential difference??? …plz give me a detailed description of it.

Posted by Arti.

Potential difference means “Difference in electrical potential. In an electrochemical cell, the potential difference is created by chemical reactions. In a cell there are two electrodes and they are maintained at different potentials by the chemical reactions taking place.

## Collection of Problems received from Class 12 Physics

These problems were posted by Geena. Hope that we will be able to post the answers to these questions soon; each in a separate post. By the time visitors can attempt to post their answers as comments to this post. (Only selected posts will be published)

Please note that the answers are not published until the Physics teachers are not satisfied with the answers. So, please be patient to get the correct answers rather than approximations in a haste.

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1. A series battery of 10 lead accumulators each of emf 2 V and internal resistance 0.25 ohm is charged by a 220 V D.C. mains to limit the charging current a resistance of 47.5 ohm is used in series in the charging circuit. What is a) the power supplied by the mains and b) power dissipated as heat? Account for the difference of powers is (a) and (b).
2. A potentiometer wire has a length L and a resistance Ro. It is connected to a battery and
a resistance combination as shown. Obtain an expression for the potential drop per unit length of this potentiometer wire. What is the maximum emf of a ‘test cell’ for which one can get a ‘balance point’ on this potentiometer wire? What precaution should one take, while connecting this ‘test cell’ , in the circuit?
3. A cell, of emf 4 V and .internal resistance 0.5 Ω, is connected across a load of resistance (i) 7.5 Ω, (ii) 11.5 Ω. Calculate (i) the ratio of the differences in the emf of the cell and the potential drop across the load, and (ii) the ratio of the currents in the two cases.
4. In the figure shown, calculate the total flux of the electrostatic field through the spheres S1 and S2 The wire, AB, shown here, has a linear charge density, λ given by λ = kx where x is the distance measured along the wire, from the end A.
5. A straight wire, of length L, carrying a current I, stays suspended horizontally in mid air in a region where there is a uniform magnetic field B . The linear mass density of the wire is λ. Obtain the magnitude and direction of this magnetic field.
6. Two cells of same emf E, but different internal resistance rl and r2 are connected to an external resistance R as shown. in the figure. The voltmeter V reads zero. Obtain an expression for R in terms of rl and r2. Calculate the voltage across the cell of internal resistance r2. (Assume that the voltmeter V is of infinite resistance).
7. A galvanometer with a coil of resistance 120 ohm shows full scale deflection for a current of 2.5 mA How will you convert the galvanometer into an ammeter of range 0 to 7.5 A ? Determine the net resistance of the ammeter. When an ammeter is put in a circuit, does it read slightly less or more than the actual current in the original circuit? Justify your answer.
8. Figure shows a bar magnet M falling under gravity through an air cored coil C. Plot a graph
showing the variation of induced e.m.f (E) with time (t). What does the area enclosed by the
E – t curve depict ?
9. The electron in the hydrogen atom circles around the proton with a speed of 2.18 x 106 m/s in an orbit of radius 5.13 x 10 -11 m. What magnetic field does it produce at the centre?
10. A proton moves with a speed of 7.45 x 105 m/s directly towards a free proton originally at rest. Find the distance of the closest approach for the two protons. (Given: mass of proton = 1.67 x 10–27 kg and e = 1.6 x 10 –19 C)
11. Figure (a), (b) and (c) show three a.c. circuits in which equal currents are flowing. If the frequency of emf be increased, how will the current be affected in these circuits? Give reason for your answer.

## Questions normally not answered by Students in Viva

Here are some questions which are not properly answered by students during Viva.

1. Name the Physical quantities to be kept constant for Ohm’s law to be true.
2. State the Principle of a potentiometer. (The students say that potential drop is proportional to length but the constant quantities are not mentioned)
3. How can we increase the sensitivity of a potentiometer?
4. Define figure of merit of a galvanometer.
5. Which has more resistance – a galvanometer or a milliammeter?
6. How does an LED emit light?
7. What is the difference between an ordinary diode and an LED?
8. Define principal axis of a convex lens?
9. What happens to the focal length of a concave mirror if it is immersed in water?
10. What are the factors affecting the intrernal resistance of a cell?
11. What are the difference between primary and secondary cell?
12. Why can’t we use a dry cell for starting a car?

1. length, area of cross section,temperature
2. The potential drop across any length of a conductor of uniform cross section and composition carrying a constant current is directly proportional to the length.
3. increasing the length of potentiometer wire, decreasing the current, decreasing the potential gradient
4. Current for unit deflection
5. galvanometer
6. The energy released during recombination of electrons and holes across the junction is responsible for the release of light by LED
7. In ordinary diode the energy emitted during recombination of electrons and holes is in the invisible region of the em spectrum but in the case of LED, the energy is in the visible region.
8. Straight line joining the centres of curvature of the lens..
9. The focal length of mirror does not change by changing the medium. Their is a pure geometrical relationship between the radius of curvature and focal length and the relation does not include any term depending on refractive index.
10. the nature of electrolyte, the concentration of electrolyte, temperature, distance between electrodes, area of electrodes
11. Primary cell cannot be recharged, secondary cells can be recharged; secondary cells have less internal resistance than primary cells.
12. Due to the high internal resistance of a dry cell, it won’t be able to provide the current sufficient to start the car

## Life Live and Waves

Is lifeline actually a wave?

First of I have to clarify the question.

The word LifeLine has got more than one meaning.

See the link below for various reference to LifeLine

http://en.wikipedia.org/wiki/Lifeline

I hope your reference is towards ECG (If not, please respond to this post through the commenting system)

“The ECG works mostly by detecting and amplifying the tiny electrical changes on the skin that are caused when the heart muscle “depolarises” during each heart beat. At rest, each heart muscle cell has a charge across its outer wall, or cell membrane. Reducing this charge towards zero is called de-polarization, which activates the mechanisms in the cell that cause it to contract. During each heartbeat a healthy heart will have an orderly progression of a wave of depolarisation that is triggered by the cells in the sinoatrial node, spreads out through the atrium, passes through “intrinsic conduction pathways” and then spreads all over the ventricles. This is detected as tiny rises and falls in the voltage between two electrodes placed either side of the heart which is displayed as a wavy line either on a screen or on paper. This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle.” (Wikipedia)

http://www.ecglibrary.com/ecghist.html

http://en.wikipedia.org/wiki/Electrocardiography

WAVE

A wave is the mechanism of transfer of energy through vibrations. In an ECG a waveform is generated corresponding to the pulses produced by heartbeats.

As such ECG cannot be treated as a wave. But the term wave is used to talk about the screen or print pattern obtained because of the graphical similarity with a wave pattern.

(Click the image on left for an animation of heart beats and the corresponding ECG obtained)

(I don’t think this as a complete answer. Anyone with better knowledge can add to this through comments)

## Difference between emf and pd

EMF stands for electromotive force.

EMF of a cell is equal to the potential difference between the two terminals of a cell when no current is drawn from it.

Pd (potential difference) between any two points on a circuit is defined as the work done per unit charge in carrying a positive test charge from one point to the other. With reference to a cell, the potential difference between the terminals of a cell when a current is drawn from it is called terminal voltage.

## How can the electric current in a circuit be kept continuous?

1. What is Current?
2. How can electric current in a circuit be kept continuous?

Ans:

The word current means flow.

Electric Current refers to charges in motion. The most common way of producing current is by applying a potential difference across the terminals of a conductor. Then the free electrons in the conductor drifts towards the positive terminal.

The current in the circuit can be kept continuous by providing a constant potential difference between two points of the circuit. This is usually done by connecting the ends of the circuit to the terminals of an electrochemical cell (which converts chemical energy to electrical energy) or a dynamo (which converts mechanical energy to electrical energy)

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