LECTURE - 2 : CLASS VIII : SCIENCE : CHAPTER 12 : FRICTION

CLASS VIII   |    SCIENCE    |    CHAPTER 12

      Notes prepared by Subhankar Karmakar

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• FACTORS AFFECTING FRICTION:

It has been found that the friction between two surfaces depends on two factors:

(i) the nature of the two surfaces (smoothness or roughness of the two surfaces)

(ii) the force with which two surfaces are pressed together.

• 1. Dependence of friction on the nature of two surfaces:

Friction depends on the smoothness or roughness of the two surfaces which are in contact with each other. When the two surfaces in contact are smooth, then the friction between them will be small. As the degree of roughness of the two surfaces in contact increases, the friction also increases. 

• 2. Dependence of friction on the forces with which two surfaces are placed together.

The friction between two surfaces depends on the force with which the two surfaces are pressed together. Greater the weight of an object which moves over another surface, more is the force with which the two surfaces are pressed together and greater will be the friction between them. 

• Types of friction:

There are three types of friction. 

• 1. Static friction
• 2. Sliding friction and
• 3. Rolling friction

• 1. Static friction: 

The maximum frictional force present between two any two objects when one object just tends to move or slip over the surface of the other object, is called static friction. In the case of static friction, the object is actually not moving or sliding over the other object, it only takes to move or slide. It is the maximum frictional force. 

• 2. Sliding friction:

the frictional force present when one object moves slowly or slides over the surface of another object is known as sliding friction. Is the sliding friction is smaller than the static friction, it is easier to keep an object moving which is already in motion than to move the same object from rest or stationary position. Sliding friction is smaller than the static friction. 

• Why does sliding friction is smaller than the static friction?

When an object has already started moving or sliding the irregularities on its surface do not get enough time to lock into the irregularities on the surface of the other object completely. Since the interlocking of the two surfaces is less when an object has already started moving, therefore, the sliding friction is smaller than the static friction. 

• 3. Rolling friction:

when an object like a wheel rolls over the surface of another object the resistance to its motion is called rolling friction. Therefore it is always easier to roll than to slide an object over another object. Show rolling friction is much less than the sliding friction. Rolling reduces friction.

heavy machines can be easily move from one place to another by placing round logs of wood under them and then pushing with the force of hands. 

• Maximum Friction : Static Friction 
• Minimum Friction : Rolling Friction

Static friction > sliding friction > rolling friction.

Lecture-4 : CLASS-X: SCIENCE : Chapter: Electricity

CLASS X  |    SCIENCE    |    ELECTRICITY

      notes prepared by subhankar Karmakar

                                                                         

Ohm's Law: 

According to Ohm's Law, at constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends. 

If Ｉis the current flowing through a conductor and V is the potential difference (voltage) across its ends, then according to Ohm's Law:

Ｉ ∝ V  (at constant temperature)

This can also be written as:  V ∝ Ｉ

or V = RＩ

where R is a constant called "resistance" of the conductor. The value of this constant depends on the nature, length, area of cross section and temperature of the conductor. 

V/Ｉ = R

The ratio of potential difference applied between the ends of a conductor and the current flowing through it is a constant quantity called resistance.

Therefore, 

(i) The current is directly proportional to potential difference, and

(ii) The current is inversely proportional to resistance. 

a. "Current is proportional to the potential difference" - it means if the potential difference across the ends of o conductor is doubled, the current flowing through it also gets doubled, and if the the potential difference is halved the current also gets halved.

b. "The current is inversely proportional to resistance" - it means if the resistance is doubled, the current gets halved, and if the resistance is halved the current gets doubled.

Resistance of a conductor:

The property of a conductor due to which it opposes the flow of current through it is called resistance.

The resistance of a conductor depends on length, thickness, nature of material and temperature of the conductor. 

SI unit of resistance:

The SI unit of resistance is ohm. It is generated by the symbol Omega, Ω. The unit of resistance can be defined as following. "1 ohm is the resistance of a conductor such that when a potential difference of 1 volt is applied to its ends a current of 1 ampere flows through the conductor". 

Numericals on Ohm's law:

1. Potential difference between two points of a wire carrying a 0.4 ampere current is 0.6 volt. Calculate the resistance between these points.

Soln: From Ohm's law we know that resistance of a conductor is equal to the ratio of the potential difference applied across the conductor and the current flowing through it. Therefore, R = V/Ｉ

or, resistance, R = 0.6/0.4 = 3/2 = 1.5 Ω

2. A simple electric circuit has a a 12 volt battery and a resistor of 60 ohms. What will be the current in the circuit? The resistance of the connecting wires is negligible. 

Soln. From Ohm's law we know that current flowing through the circuit, Ｉ is equal to the ratio of the potential difference across the ends of the conductor V and the resistance R attached to it. 

Therefore, Ｉ = V/R = 12/60 = 0.2 A (ampere).

3. An electric appliance draws a current of 2.2 ampere from a 220 volt supply line. What current will this electric iron draw when connected to 110 volt supply line?

Soln. From Ohm's law we know that resistance of a conductor is equal to the ratio of the potential difference applied across the conductor and the current flowing through it. As the resistance of the appliance is same.

R = V₁/Ｉ₁ = V₂/Ｉ₂

V₁ = 220 V, Ｉ₁ = 2.2 A,  V₂= 110 V, Ｉ₂ = ?

Ｉ₂ =  V₂Ｉ₁/V₁ = 110x2.2 /220 = 1.1 A

4. If 20 C of charge pass a point in a circuit in two second, what is the current flowing?

Soln. Charge passing through a conductor per unit time is called current. Therefore, current, Ｉ = q/t, q = charges passing through the conductor, t = time. 

Ｉ = q/t = 20 /2 = 10 A

5. If a potential difference of 20 volt causes a current of 1 ampere to flow for 2 minute how much energy is transferred?

Soln. Q = Heat generated, Ｉ= Current = 1 A;

          R = Resistance = V/Ｉ , t = time = 120 s

Q = Ｉ²Rt = ＩVt = 1 x 20 x 120 = 2400 Joules.