Question of the Day 25 Nov 2017

Neha's father was reading the manual of a camera which was in very small print. He was not able to read comfortably. Neha had two convex lens A and B of focal length 25 cm and 40 cm respectively. She gave the better of the two lenses to her father. He then read the manual comfortably without any strain in his eyes.

Now answer the following question.
(a) Which one of the two convex lenses did she gave to her father and why?
(b) What is the power of the lens A and B?
(c) What is the range of the distance that his father can keep the manual from the lens for comfortable reading?
(d) Unfortunately while handling the better of the two lenses, it got slipped from her father hand and broke into two piece. Will her father still be able to use the broken pieces for magnifying the small prints? 

Question of the day 15 Nov 2107

Question of the day 15 Nov 2017

A student has two resistors A and B of resistance 4 ohm and 6 ohm respectively.  The student was challenged by the Physics teacher to connect them to a 12 V battery so as to draw maximum current from the voltage source.

How would you advice the student to connect the resistors and what would be the current obtained?

Question of the day 14/11/2017

Question of the day - 14 Nov 2017

The resistance of an electric heating device is 20 ohms. If an electric current of 5 A flows through the device for 30 seconds. Determine

(i) Potential difference applied across the device.

(ii) heat produced in the device in 30 seconds.

The Story of Electricity

THE STORY OF ELECTRICITY

Imagine a world without any electricity. Imagine your home now… no televisions, no air conditioners, no light, no refrigerator, no machines, no mobile phones, no electronic gadgets to assist you, no apps, no facebook, no twitter… The only form of electricity available to us would be that of lightning.

Ohh.. what a boring life…

But life would be very close to nature. Sun and fire will be the only source of light. Watching birds, animals would be your favorite pass time. Crude handmade gadgets like magnetic compass, hammer, knife would be your apps. Pigeon would serve as twitter, FB…

So my dear friend, electricity is an integral part of our modern life. The study of electricity is necessary so that such a vast knowledge is not lost overtime. It must be passed to the next generation.

I am doing my duty as I am passing the story to electricity to you..

Take a piece of copper metal. If you view the copper at the microscopic level you would find a lot of empty space with copper atoms suspended in space in the cloud of electrons. Most of the electrons in the cloud are orbiting to some copper atoms or the other. Some electrons in the cloud are wandering from one atom to the other, colliding with other atoms and electrons. At this microscopic level, everything happens very fast. Electrons are moving at 10⁶m/s, atoms are vibrating very rapidly with a frequency of ……..

Suddenly, in the macroscopic, outer world, a human connected the metal plate to a battery.

In the inner atomic world, there is a change in the scene now. The cloud of wandering electrons  start moving towards positive terminal of the battery with a very slow speed of 1mm/s. The number of wandering electrons are however enormous. About 10²² in few grams of copper.

We call this flow of electrons as electric current.

When such moving electrons are made of pass through a filament of electric bulb, it heats up to approx 2700°C and starts glowing. 

This is how we use electricity of light a bulb.

Watch the video from YouTube to understand electricity in a more detail.

Ohm's Law

Ohm's Law is a relationship between potential difference applied across a conductor to the electric current flowing in it.

You must have experienced the following situations in daily life:

(1) Fluctuations in voltage which leads to dimming or brightening of electric bulb. (This is the reason why we use stabilizers for ACs - to prevent any damage due to voltage fluctuations ) 
(2)  While playing with LEDs, torch bulb, electric cells and connecting wires you must have observed that by increasing the number of cells in the circuit the LEDs glows brighter.
(3) A 12V battery provides a large amount of current than a 3V battery.

This suggests that there must be a relation between voltage (potential difference) and electric current. This relationship is known as Ohm's Law.

Ohm's law was given by George Simon Ohm in the year 1827. This is a relation between potential difference (V) applied across the ends of a conductor to the electric current (I) flowing in the conductor

According the Ohm's Law, the electric current flowing in a conductor is directly proportional to the potential difference (V) applied across the ends of a conductor provided the temperature of the wire remains the constant.

Mathematically,
                              I    ∝    V
or it can also be written as

                            V  ∝    I
removing the proportionality sign and introducing a constant

                            V   =  R I

Where R is a constant known as the resistance of the given conductor.

Graph:

The V–I graph is a straight line that passes through the origin of the graph, as shown in Figure. Thus,
\[\frac{V}{I}\]
is a constant ratio.

Resistance

Physically, resistance is defined as a property of a conductor to resist the flow of charge through it.

The SI unit of resistance is ohm (Ω)  (Greek symbol omega)

A conductor is said to have a resistance of 1Ω when a potential difference of 1V is applied across the ends of a conductor and a current of 1A flows through it.
\[R=\frac{V}{I}\]
\[1Ω=\frac{1V}{1A}\]

A fan regulator is actually a variable resistor. It changes the resistance of the circuit and thereby we can control the speed of the fan.

To test your knowledge on Ohm's Law, attempt the following test.

you will get your score immediately. 

Click the link below to attempt the test:

Online Test on Ohm's Law

Some more questions on Ohm's Law:

1. Name and state the law which relates the current in a conductor to the potential difference across a conductor and the current flowing through it.

2. Let the resistance of an electrical component remains constant while the potential difference across the two ends of the component decreases to half of its former value. What change will occur in the current through it?

3. When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in the circuit. Find the value of the resistance of the resistor.


4. (a) How much current will an electric bulb draw from a 220 V source, if the resistance of the bulb filament is 1200 Ω? (b) How much current will an electric heater coil draw from a 220 V source, if the resistance of the heater coil is 100 Ω?

5. The potential difference between the terminals of an electric heater is 60 V when it draws a current of 4A from the source. What current will the heater draw if the potential difference is increased to 120 V?

6. The values of current I flowing in a given resistor for the corresponding values of potential difference V across the resistor are given below –
I (amperes) 0.5    1.0     2.0      3.0       4.0
V (volts)     1.6    3.4     6.7     10.2     13.2
Plot a graph between V and I and calculate the resistance of that resistor.

7. Keeping the potential difference constant, the resistance of a circuit is doubled. By how much does the current change?