Current Electricity: About Voltage
Starting off with this interesting question that has always been in my mind!
Benjamin Franklin in 1752, with his famous kite experiment, proved that lightning is not some magical force. He made a kite with silk not paper so it wouldn’t get ruined in the storm. To the top of it, he attached a wire to conduct electricity. To the bottom of the kite he attached a hemp string, and to that he attached a silk string. The hemp, wetted by the rain, would conduct an electrical charge quickly. The silk string, held by Franklin in the doorway of a shed, wouldn’t as it was dry. Franklin attached a metal key to the hemp string, and with his son’s help, got the kite aloft. Franklin moved his finger near the key, and as the negative charges in the metal piece were attracted to the positive charges in his hand, he felt a spark. Franklin thus proved that lightning in nature and electricity produced in laboratories are the same.
But what kind of electricity is produced in the clouds? Static electricity!
Static electricity is the imbalance between the positive and negative charges in the body. The most common cause for this is the rubbing of two bodies, causing transfer of electrons. The charges build up in the bodies and are stationary (stay there) till they find a way to be released.
Lightning is the flow of static charge that builds up in the clouds. This charge is polarized - positive and negative charges are separated, but how? Water from the ground evaporates and forms droplets which collide with water droplets within the clouds, causing loosely attached electrons to rip off the rising droplets. Thus, they become positively charged and are carried to the upper part of the cloud and the negatively charged electrons are carried to the lower parts of the cloud.
The cloud being charged creates an electric field around it that influences even the earth. This electric field is nothing but a change in the space around the cloud. The charge of the cloud influences any charged body in that space. The electrons at the bottom of the cloud repel electrons on the earth, creating a build up of positive static charge. This occurs in trees, buildings, etc. on the surface.
As the static charge buildup in a storm cloud increases, its electric field becomes stronger. It is capable of ionising the surrounding air and making it more conductive. The ionization involves the removal of electrons from the outer shells of air molecules. Thus, positive ions and free electrons are produced. This makes charge transfer (in the form of a lightning bolt) from the cloud to the ground possible.
The electrons at the bottom of the cloud are now able to move towards the ground. As they do, they cause stronger repulsion in the electrons on the earth’s surface, causing the positive charge to build up even more. This charge begins to rise and meets the descending electrons somewhere above the earth’s surface. Then, lightning begins as the path is complete.
The flow of charge per unit time is known as current. When we talk about a circuit, it’s the electrons or negative charge that flows. The metal wires have many loosely bound electrons that keep jumping from one atom to another. Thus, they are almost free to move.
There is always some work (or energy) involved in moving a charge closer to another charge due to the influence of their electric field. If they are like charges, the repulsive force between them needs to be overcome by doing some work. If they are unlike charges, the attractive force between them does the work to bring them closer.
The potential at a point in the vicinity of other charges is the work done to bring a charge of small value (so that it does not change positions of other charges by attracting or repelling them strongly) from a point at infinite separation to that point. At such a far distance, there is neither an attractive force to do work nor a repulsive force to do work against. Therefore, potential is zero and this point is taken to calculate total potential (from zero to any particular point).
In the flow of current in a circuit, we don’t really need to know the potential at a point - how much energy was needed (or work was done) to bring a unit positive charge (charge can be negative too, the work done will then be negative) to a point in the wire.
We do need to know the potential difference (or voltage) - energy needed to move a unit positive charge from one point to another in the wire. It is equal to the difference in the potential at the two points, that is the difference of the two energies required to bring a charge to the two points (from infinity).
It is significant as it determines current. The more energy available for flow of charge, the more the charges will flow. Thus, more will be the current. This explains the direct proportion between current and potential difference which is the statement of Ohm’s law.
Resistance is the constant of proportionality in Ohm’s law. Like friction is the opposition in motion, resistance is the obstruction in the flow of current by the wire. The cause of resistance is the collisions of flowing electrons with the positive ions that are not free to move.
As a potential difference is applied (battery is connected), electrons gain kinetic energy and thus their speed increases. They start to accelerate towards the positive terminal of the battery due to attraction between unlike charges. But the collisions slow them down as they lose some kinetic energy. Still they don’t give up, actually it’s the potential difference that doesn’t allow them to give up and they begin moving towards the positive terminal again. We can say the positive ions are like speed breakers in the path of electrons, slowing them down.
If resistance is too high, electrons won’t be able to flow. No flow of charge means no current. So, resistance is the obstruction to current.
The potential difference (or energy) required to push charges is provided by the battery due to its E.M.F. It is the work done per unit positive charge in making it flow around the complete circuit (across the wires and in the cell). A battery has an E.M.F because of a chemical reaction. Due to the reaction, charges develop on the electrodes thus enabling forces of attraction and repulsion that can do work on the charges and move them.
E.M.F refers to the potential difference across the terminals of the cell when it’s not connected in circuit (written on the battery). It doesn’t account for resistance in the cell offered by the electrolyte - internal resistance. It just tells the energy that the battery can provide. Terminal voltage on the other hand refers to the potential difference across the terminals of the cell when it’s connected in circuit. That's when internal resistance causes heat to be radiated (just like friction), so some of the battery’s energy is lost to the surroundings. This reduces the work the battery can do or we can say potential difference reduces. Hence terminal voltage is less than E.M.F. It is the work done per unit positive charge in carrying it around the circuit connected across the cells.
The reduction in potential difference is only due to internal resistance. Thus it is equivalent to the work done in carrying a unit positive charge through the electrolyte in the cell (against internal resistance). This energy is lost as heat and not useful to us. Therefore it is called voltage drop.
That’s it for this blog. Current definitely feels like a tough topic with concepts like potential, potential difference, resistance, E.M.F, terminal voltage and voltage drop. But with understanding and practice it can be made less tough, so here are some questions to check your understanding.
If an electrical circuit were analogous to a water circuit at a water park, then the battery voltage would be comparable to _____.
a. the rate at which water flows through the circuit
b. the speed at which water flows through the circuit
c. the distance that water flows through the circuit
d. the water pressure between the top and bottom of the circuit
e. the hindrance caused by obstacles in the path of the moving water
If a battery provides a high voltage, it can ____.
a. do a lot of work over the course of its lifetime
b. do a lot of work on each charge it encounters
c. push a lot of charge through a circuit
d. last a long time
Answers to questions in the previous blog-:
Q The natural frequency of an oscillating object is 10 Hz. How often in seconds should you push the object to make the oscillations bigger?
To make oscillations bigger means to increase amplitude which occurs in resonance. For resonance the external periodic force applied should be at the natural frequency of the body so we should push the object at a frequency of 10 Hz. We have to push the object after 1 time period (one oscillation). Time period = 1/frequency = 1/10 = 0.1 s. So we will push the object every 0.1s to make the oscillations bigger.
Q How can someone break a glass just with their voice?
Matching the natural frequency of the glass (pitch of the sound on tapping it) with your voice can lead to resonance. The amplitude of vibrations of the glass will increase to such an extent that it will break.
Q An engine is approaching a tunnel surmounted by a cliff, and emits a short whistle when 1 km away. The echo reaches the engine after 5 seconds. Calculate the speed of the engine assuming the velocity of sound to be 340m/s.
Let the engine be travelling with speed x m/s.
Sound first travels a distance of 1km = 1000m from engine to cliff.
It reaches back to the engine after 5s. In this time the train travels a distance of 5x m (distance=speed x time). So now the distance of the engine from the cliff = 1000-5x m.
Thus the total distance traveled by sound = 1000+1000-5x m = 2000-5x m. The time taken to cover this distance is 5s (echo is heard after 5s) and speed of sound is 340 m/s.
Speed = distance/time
340=2000-5x5
1700=2000-5x
5x=2000-1700
5x=300
x=60m/s
Wow 👻👻👻
ReplyDeleteAmazing 😎
ReplyDeleteLightning is super interesting to understand!
ReplyDelete