# KPK Board 12th class Physics Ch 12 Current Electricity short questions answers

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Why it is not possible to measure the drift speed of electron by time timing their travel along a conductor?

Drift speed:

The average speed gained by electron inside a conductor when connected to a voltage source is called drift speed.

Explanation: changes speed is the distance travelled by a body in unit time. But in case of drift speed of electron in this formula is not accurate, because drift speed of electron is very low. When a conductor is connected to a voltage source, the randomly moving electrons start moving in a proper direction due to electric field created by the source. These electrons collide with one another and also with the atoms of conductor. Due to this collision the electrons does not move on a straight path rather it move on a zigzag path with increasing and decreasing speed. So the electrons do not cover appreciable distance in this time. This is the reason that we cannot measure the Drift velocity for electrons by timing their travel along a conductor.

The relationship R=V/I tells us that the resistance of a conductor is directly proportional to the potential difference applied to it. What do you think of this proportion?

The relation R=V/I is a definition of resistance, showing that it is the ratio of potential difference V across the conductor to the current I flowing through it.

Explanation: relation is the mathematical form of Ohm’s Law in which R is the resistance and is constant of proportionality subjected to the condition that physical state of system remains the same while current I is proportional to the potential difference V. when potential difference change the current I also changed proportionality and their ratio R=V/I remain the same. So R is not proportional to potential difference V.

A heavy duty battery of truck maintains a current of 3A 24 hours. How much charge flow from the battery during this time?

The charge flowing from the battery carrying 3A current for 24 hours will be 259200 C.

Current I = 3A

Time t = 24 hours = 24x60x60= 86400s

By the definition of current,

I = ΔQ/Δt

Putting values

I = 3Ax86400s = 259200 C.

While analyzing a circuit the internal resistance of EMF source is ignored why?

In most cases the load resistance of circuit is very high as compared to the internal resistance of the battery. Thus EMF of battery is nearly equal to the terminal potential difference V. this difference along with the small internal resistance is usually ignored.

Explanation: the opposition offered to the flow of charges inside and EMF source is called internal resistance r of the source. This internal resistance is a very small therefore it is ignored. when some load resistance R is connected across the terminals of the battery, the internal resistance r of the battery and load resistance are in series, therefore for series combination of resistance the EMF is given as

Since the internal resistance is a very some also the internal voltage is a usually taken nearly equal to EMF internal resistance is usually ignored.

Under what circumstances can the terminal P. D. of a battery exceed its EMF?

The Terminal potential difference exceeds its EMF when source are connected in series with EMF in opposition.

Explanation: whenever a source of EMF is being charged, its terminal potential difference V exceeds the EMF. In a closed circuit, having some source of EMF connected, terminal potential difference V is lesser than EMF its due to potential drop across internal resistor of EMF source. The equation for terminal potential difference is

Now if an EMF source in this state is charged, then its current reverse in the direction making terminal potential difference greater than the EMF of the circuit.

Thus, equation shows that during charging of an EMF source, its terminal potential difference exceeds the EMF.

What is the difference between an EMF and a potential difference?

EMF the work done in moving a positive unit charge from lower potential to higher potential inside a battery is regarded as EMF of the battery.

Inside a battery work has to be done to bring the positive charge from lower potential to higher potential. The influence that makes current flow from lower to higher potential inside the battery is called electromotive force EMF. if w is the work done by battery in taking a charge Q from negative terminal to positive terminal, then work done by battery Per unit charge is

Voltage or potential difference

voltage is simply the potential energy Per unit charge the electric potential difference between any two points I and F is in the electric field is equal to difference in electric potential energy Per unit charge between the same two points

The EMF is generated by conversion of one form of energy to other form, where as the potential difference is the difference in potential energy per unit charge between two points provided by the EMF source.

Actually inside the source the potential difference between the terminals is called EMF while potential difference between any two points in the outside the circuit is simply called potential difference.

The law of rule is based on the conservation of energy principle and junction rule on conservation of charge principal. Explain just how these are based on the principles.

Kirchoff’s loop rule: this law is based on the principle of conservation of energy and state that “in any closed electrical circuit the algebraic sum of all the electromotive force and the voltage drop in resistors is equal to zero”

ΣE – ΣIR = 0

ΣE = ΣIR

1. The left hand side of this equation gives the total energy supplies by the source to do work on charge.
2. The right hand side of the equation gives the dissipation of energy when the charges flow through resistors.

Therefore

Energy lost = Energy gain.

Kirchoff’s junction rule: this law is based on the conservation of charge and state that, the algebraic sum of all the current flowing towards a Junction is equal to the sum of all the current flowing away from the junction.

ΣI = 0

Why rise in temperature of a conductor is accompanied by a rise in the resistance?

The opposition offered by a conductor to the flow of electric current is known as resistance. the resistance offered by a conductor to the flow of electric current in due to the collision of free electron with atoms present in the lattice of the conductor, in their way. With the rise of temperature, these atom gain energy and vibrate with large amplitude so there rate of collision with free electron increase. As a result the resistance of the conductor increases. If R is the resistance of conductor at 0C° then the resistance of the conductor at temperature t is

Rt – R̥ = αRT

Rt = R̥ (1+t)

Hence α Alpha is a constant and is known as the temperature coefficient of resistance.

Does the direction of EMF provided by a battery depend on the direction of current flow through the battery?

Yes the direction of EMF provided by a battery depends on the direction of current flow through the battery.

Explanation:

To determine the direction of current flow through the battery is adopted in which rise in the potential is taken as positive and drop is potential as negative. The same principle is used in a Kirchoff’s rule to determine the direction of current flow through the battery.

When the current flow through the battery from negative to positive terminal there is a rise in potential and is taken as positive but when the current flows through the battery from positive to negative terminal there is a drop in potential and is taken as Negative.

Thus the direction of EMF provided by the battery can be used to determine the direction of current through the battery.

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