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Chapter 13 Current Electricity Physics 12th Class Punjab Short Questions

Question No. 1 - A potential difference is applied across the ends of a copper wire, What is the effect on the drift velocity of free electrons by decreasing the length and temperature of the wire?

Answer No. 1 - By decreasing the length and temperature of wire the value of resistance in the wire also decrease which cause an increase in the value of current. Hence the drift velocity of free electrons also increase.

Question No. 2 - Differentiate between resistance and resistivity. Give their unit.

Answer No. 2 - Resistance: The opposition against the flow of current is known as resistance. The SI unit of resistance is Ohm.
R=V/I

Question No. 3 - State Ohms law and write its formula.

Answer No. 3 - It states that "the current flowing through a conductor is directly proportional to the applied potential difference provided that the physical state remains same"
V=IR
Resistance: The opposition against the flow of current is known as resistance.The SI unit of resistance is Ohm
R=V/I

Question No. 4 - Does the bends in wire effect its electrical resistance?

Answer No. 4 - No bend in wire do not affect its electrical resistance.

Question No. 5 - Why does no current pass through Galvanometer in a balanced wheat stone bridge although the keys in the circuit are closed ?

Answer No. 5 - No current pass through the galvanometer when wheat stone bridge is balanced. Because at this stage, both the terminals of the galvanometer are at the same potential. Hence no current will flow through it.

Question No. 6 - Define resistivity and electrolysis.

Answer No. 6 - The resistance of a meter cube of a material is called resistivity. Certain liquids such dilute sulphuric acid or copper sulphate solution conduct electricity due to some chemical reactions that take place within them. The study of this process is known as electrolysis.

Question No. 7 - Write about any two sources of Current.

Answer No. 7 - 1. Electric generators convert mechanical energy into the
electrical energy.
2. Solar cells convert sunlight directly into electrical
energy.
3. Thermoplastic convert heat energy into electrical
energy.

Question No. 8 - Why does the resistance of a conductor rise with a temperature?

Answer No. 8 - As the temperature of the conductor rises, the amplitude of vibration of atoms increase and hence the probability of their collision with free electrons also increases which results in increase of resistance of conductor.

Question No. 9 - What is the conventional current? How does it differs from electric current?

Answer No. 9 - The current flow due to positive charges from a point at higher potential to a point at lower potential is called conventional current. It is due to positive charges while electric current is due to negative charges i.e electrons.

Question No. 10 - Name any four source of current.

Answer No. 10 - Source of current are:
1. Cells
2. Electric generators
3. Thermoplastic
4. Solar cells

Question No. 11 - Give two substance having negative temperature coefficient. Also define the temperature coefficient.

Answer No. 11 - The temperature coefficient of a resistance is defined as fractional change in resistance per kelvin rise in temperature. Its unit is K-1.Substance like Ge and Si have negative temperature coefficient.

Question No. 12 - Name some effects of current.

Answer No. 12 - Some effects are :
1. Heating effects
2. Magnetic effect
3. Chemical effect

Question No. 13 - A voltmeter can not read the exact EMF of the cell. Why?

Answer No. 13 - When a voltmeter is connected across a cell, it will draw some current from the cell and small potential takes place due current flowing through the internal resistance of the cell. As a result the actual emf of the cell decrease and the voltmeter cannot read exact value.

Question No. 14 - What is short circuit and open circuit mean to you?

Answer No. 14 - When switched is close and current is passing through the circuit. It is called closed circuit and in this situation resistance is zero whereas the circuit is said to be open if it has infinite resistance and no current is passing through it.

Question No. 15 - What is wheat stone bridge?

Answer No. 15 - It is an electrical circuit which can be used to find the unknown resistance of a wire. It consists of four resistance connected in the form of mesh. Galvanometer, battery and switch.

Question No. 16 - Briefly describe the current through a metallic conductor and drift velocity.

Answer No. 16 - In a metallic conductor,free electron are in random motion with the speed of several hundred km/s at the room temperature. If the ends of the wire are connected to the battery the free electrons are experience a force and directed to move in the electric field direction. The accelerating electrons keep on colliding with atoms of the conductors and transfer their energy to the lattice with the result that the electrons acquire an average velocity called drift velocity. The drift velocity is of the order of 10-3. A steady current is establish in the wire.

Question No. 17 - What are non ohmic devices? Give two examples.

Answer No. 17 - Those devices which dont obey the Ohms law are called non-ohmic devices. Their current-voltage graphic is not a straight line. For example, filament bulb and semiconductor diodes.

Question No. 18 - What are the difficulties in testing weather the filament of lighted bulb obeys ohms law?

Answer No. 18 - In case of light bulb,the temperature of the filament increase with the passage of current through it. Hence the Ohms law cant be applied to filament bulb.

Question No. 19 - What is ohmic and non-ohmic devices with example.

Answer No. 19 - The devices which obey Ohms law are called ohmic devices and devices which do not obey Ohms law are called non-ohmic law. For example copper, silver and gold are ohmic whereas diodes and tungsten filaments are non ohmic.

Question No. 20 - What is meant by Tolerance? Also give one example.

Answer No. 20 - Tolerance mean the possible variation in the value of resistance of a carbon resistors from a marked value.In case of silver and gold ban its value is +-10% and +-5%.

Question No. 21 - How the heating effects produce when current flows through the conductor?

Answer No. 21 - During their motion free electrons collide frequently with the atoms of metal, on each collision they transfer some of their kinetic energy to the a time with which they collide. And these collisions produce heating effect in the wire.

Question No. 22 - What is meant by tolerance of resistors?

Answer No. 22 - Tolerance is the measure of possible variation from the marked value. A gold band has +-5% tolerance, Silver has +-10% and no band at all wound mean a +-20 tolerance.

Question No. 23 - State Ohms law and basic principle of electroplating.

Answer No. 23 - Current passing through a wire is directly proportional to the potential difference applied across its ends provided that the physical state conductors remains same.
Basic principle of electroplating is a process of coating a thin layer of sum expensive metal of gold, silver, etc on a article of some cheap metal.

Question No. 24 - How a Wheatstone bridge is used to determine an unknown resistance?

Answer No. 24 - Wheatstone bridge is an especially designed electrically circuit used to calculate the accurate value of any unknown resistance.It consists of four resistance, a galvanometer, a battery and a switch connected. When the switch is closed current pas through the galvanometer and then the three known resistance R1, R2 and R3 are adjusted in such a way that galvanometer show no deflection. In this balanced condition the fourth unknown resistance R4 can be calculated by using this relation. R1/R2= R3/R4.

Question No. 25 - What is meant by tolerance?

Answer No. 25 - Tolerance means the possible variation from the marked value.

Question No. 26 - What is the negative coefficient of temperature?

Answer No. 26 - If the resistance of a substance decrease with an increase in temperature, then it is termed as negative coefficient of temperature. For example, silicon and germanium have negative coefficient of temperature.

Question No. 27 - What is thermistor? Gives its two applications.

Answer No. 27 - Thermistors are heat sensitive resistors. Thermistors with positive temperature coefficient of resistance as well as negative temperature coefficient of resistance are available. They are used for accurate measuring of temperature up to 10K. They are used as temperature sensors.

Question No. 28 - Do two long and parallel current carrying wires attract each other?

Answer No. 28 - Yes, If the direction of current is same in two long parallel current carrying wires. Because the opposite pole of electromagnet comes in front of each other and attracts.

Question No. 29 - Why potential meter is accurate measuring meter?

Answer No. 29 - The voltage measured using potentiometer is the voltage across the terminal of the cell when current is not following through it. This voltage is exactly the emf of the cell. Further the accuracy of a potentiometer can be increased to a great extent by increasing the length of the "potentiometer wire".

Question No. 30 - Differentiate between conventional and non-conventional current.

Answer No. 30 - Conventional Current: The current flow die to positive charges from a point at higher potential to a point at lower potential is called conventional current.

Question No. 31 - How can rheostat be used as potential divider?

Answer No. 31 - By adjusting the sliding contact resistance of the rheostat can be altered which in turn would regulate the potential offered by the cell E to the main circuit. And thus a rheostat can be used as potential divider.

Question No. 32 - What is the effect on drift velocity of free electrons by increasing potential difference?

Answer No. 32 - By an increase in potential difference, drift velocity also increase. Because by increasing potential difference, the current also increase.

Question No. 33 - What does the equation H=I2Rt show?

Answer No. 33 - This equation show the heating effect, During their motion free electrons collide frequently with the atom of metal. On each collision they transfer some of their kinetic energy to the atom with which they collide.And these collisions produce heating effect in the wire.

Question No. 34 - Define temperature coefficient of resistivity.

Answer No. 34 - The temperature of coefficient of resistivity is defined as fractional change in resistivity per kelvin rise in temperature. Its unit is K-1.

Question No. 35 - Define Electromotive force and Terminal potential difference.

Answer No. 35 - Electromotive force: The energy supplied to a unit charge in moving it from negative to positive electrode inside the source is called electromotive force.

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