Physics of Electricity (High School Physics and AP Physics)

Capacitor is a device that stores energy and the measure of energy that a capacitor can store is its capacitance. Capacitance formula for various type of capacitors can be found using the capacitance formula that is ratio of charge on capacitor to the potential difference between the plates. This lesson includes capacitors working principle and how to calculate capacitance of various types of capacitor. Units for capacitance are farad and often used values of capacitance are pico farad or micro farad Various capacitors types include spherical capacitors, parallel plate capacitors, cylindrical capacitors etc. Often to enhance the capacitor capacitance a dielectric is put between the plates with a certain dielectric constant.

Capacitors in series and parallel are a combination of capacitors placed in network. In such a network, some capacitors have same voltage across and some same charge. Try to understand the derivation for equivalent capacitance of capacitors in series and in parallel. This will help you use the formula for capacitors in series and parallel a lot more easier. This lesson has derivation of equivalent capacitance for capacitors in series and parallel. The lesson also has a solved example that finds equivalent capacitance of a network of capacitors.

Energy stored in a capacitor is a result of accumulation of charge on the parallel plates of a capacitor. This charge Q builds up due to movement of electrons from one plate to the other. The charge flow is on account of potential difference between the plates due to say a battery with voltage V. So when we charge a capacitor and say the final charge on it is Q and the potential difference is V, we have a formula that connects V and Q through the equation V = Q/C or Q = CV. The energy stored in the capacitor is 1/2 (C V square)

Dielectric in a capacitor increase the potential energy of the capacitor. This happens due to polarization of the dielectric material. The polarized dielectric helps the capacitor to store more charge and therefore increase the electric field value across the capacitor plates. High electric field means more voltage and more voltage across plates increases the stored potential energy. Dielectric material have dielectric strength that determines the factor by which the capacitors energy can be increased. Dielectric constant is a number that is used as a measure of this strength. Higher dielectric strength means more energy storage on capacitor. E.g water dielectric constant is 78.2 while that of mylar is 3.1. Then if dielectric constant of water is 78.2 but still it is not used as a dielectric. The reason for this is that the dielectric constant of water changes with motion of the water. In other words, the capacitance of the capacitor will constantly vary with motion of water. Thus even though dielectric constant of water is 78.2, it is a bad dielectric medium

I this lesson we will cover what is electric current. We will also study how it is different from current density. Electric current is like a river, well almost. The definition of electric current is the rate of flow of electric charge through a conducting material. It can be written as dq/dt where dq is the charge passing through a point in time dt. Thus any forward motion of charge constitutes current. To move charges, we apply a potential difference or voltage between the ends of a conducting material and produce an electric field. This electric field makes the charges move forward and create electric current. The speed at which electrons move forward is called the drift velocity. The direction of conventional current is always opposite to that of the direction of electrons. The units of electric current are Ampere. Current density is the value of current divided by the area through which it flows. Current density is denoted by the symbol J and the formula for current density is I/A or current by the area of cross section.

Topics covered: 00:18 What is electric current? 01:10 When does current flow? 01:51 Direction of motion of charges 02:20 Force on a charge 03:27 What is drift velocity 05:40 What is direction of conventional current 06:36 Definition of electric current 08:11 Derivation of drift velocity 10:20 What is charge density?

Resistance of a conductor measures the opposition to the flow of current in an electrical circuit. Resistance is measured in ohms and the symbol used to represent it is the Greek letter omega (Ω). Resistance of a wire is directly proportional to the length of the wire and inversely proportional to the area of cross section. It is also directly proportional to resistivity of the wire. Resistivity of a wire a a material characteristic. It is defined as the resistance offered by a wire having unit length and area of cross section.

Topics covered:  00:15 Comparison of electric current with flowing water 01:56 Electric current through wires of same shape but different material 02:30 Definition of resistance 03:35 Relationship between current with resistance 03:55 Change in resistance with length and area of cross section of wire 05:07 What is resistivity? Resistance Vs. resistivity 08:56 Perfect conductors and perfect insulators 10:13 How does resistivity change with temperature 11:20 Resistivity and conductivity

Ohm's law states that the electrical current flowing through a resistor is directly proportional to the applied voltage and inversely proportional to the resistance. This relationship between the Voltage, Current and Resistance is what Ohm's law establishes. If the values of two variables out of Voltage, Current or Resistance is known, Ohms Law can be used to find the third variable. Electrical Power in a circuit is the rate at which energy is produced in a circuit. Often a source of energy such as a battery will produce the voltage and a connected device would take the power from the electrons moving in the circuit. Examples of such devices are bulbs, heaters, motors etc. The symbol used to denote power is P and is the product of voltage and current. The unit of measurement is Watt ( W ). Formulas for finding power P = V x I where P is in watts, V in volts and I in amps P = V square by R where R is in ohm (Ω) P = I square x R

Electromotive force or emf of a battery causes the potential difference across the battery terminals. This potential difference called the voltage or terminal voltage is what generates the current. The unit of emf or the electromotive fore are volt or joules per coulomb. The difference between electromotive force and voltage is that emf is generated by the chemical energy in the battery. This helps move positive charge from the negative terminal of a battery to the positive terminal. The gain in potential difference is what helps the charge to move in the circuit from the positive terminal to the negative terminal. Most sources of emf have internal resistance that reduces the potential difference between the terminals. This happens because potential is lost to this resistance as the charge moves across this internal resistance.

Topics covered: 00:12 Can a capacitor generate electric current? 00:38 What is an emf device? 01:00 How does a battery work? 01:23 The ski resort analogy 02:10 What happens inside a battery? 03:57 Derivation of emf equation 06:00 What are units of emf? 08:55 What is internal resistance of a battery 09:06 What is terminal voltage? 10:20 Summary of the lesson

Kirchoff's law helps us work around electric circuits and find various unknowns like current, voltage and resistance value distributions. There are basically two rules to follow- The first one is called the resistance rule that says that if you are moving through a resistance in the direction of the current, the change in potential is –iR but if you are moving in the opposite direction it will be +iR…and this is exaclty what we did earlier

The 2nd rule is if you are moving from the negative terminal of a battery to the positive that is also the direction of emf, the change in potential is positive. On the other hand if you are moving from positive to negative terminal of the battery or against the direction of emf, the potential will drop and therefore we take a negative value of emf

When you attempt problems around circuits, you need to find the potential difference between two points. This lesson explains the steps you can use to calculate the potential difference between points a and b. At a basic level, we start at point “a” where the potential is Va, and as we move up towards b, we add up all gains and losses in potential. Also in this lesson, power from batteries. Batteries form part of most circuits. So when a battery establishes current by doing work on a charge carrier what is happening is that the chemical energy of the battery is getting transferred to the charge that enables it to move in the circuit and create current. And since any emf device, like a battery here, would have an internal resistance, some energy would get wasted there. Well we know that the useful power a battery can generate is iV where V is the terminal PD and I is the current in the circuit. The terminal potential difference is the emf value less the potential drop across the internal resistance of the battery.

Resistors in a parallel circuit have the same voltage across the ends. Circuits that have resistors in a parallel often make use of Kirchhoff's current law (KCL). This law is also termed as Kirchhoff's junction rule. Kirchhoff's law states that if you take sum of the currents entering a junction then it should equal the sum of current exiting the junction. The law makes use of conservation of charge. This means in steady state no charge is added or lost at a junction.

RC Circuits are circuits that have capacitors in them. These circuits have current that varies with time. Charge accumulation on capacitors in RC circuits varies with time and so does the voltage. Equation derivation of time dependent variables like current and voltage helps us understand the way RC circuits function. RC circuits can have charging and discharging cycles. In both cycles there are equations that can be used to explain the current, charge and voltage on a capacitor. RC circuits also have a term called capacitive time constant. This has units of time and is equal to product of resistor value into capacitor value

Bulbs can be put in series or parallel circuits. The question is which bulb glows brighter when connected in series and parallel & Why? So, in this video you will see how are bulbs in series different from parallel. However, the more important question I think is why one set would glow brighter than the other? Bulbs are nothing but resistors that are glowing. If we can find current through these resistors, we can find the power delivered by using the formula P is equal to I square r.

The second question that should come to your mind is that if you are given 2 bulbs then is it not better to put them in parallel than in series. Simply because you can get 4 times power for the same bulb. It makes sense to put the bulbs in parallel to get more light and therefore illumination but you should also remember that the energy consumption will also go up by 4 times. So if you connect these bulbs in parallel with a battery, the energy from the battery will get drained 4 times faster than it would have been if the bulbs were connected in series. Therefore, it is 4 times costlier to put bulbs in parallel than in series. But of-course you also get 4 times the power.

Another question is what happens when one light bulb goes out in a series or a parallel circuit. Does the other light bulbs continues to glow? The answer is that if the bulb in series burns out, the current becomes zero because the charge carriers that make the current have no path to travel to the other side. In absence of current, the other bulb will also not function. However if one of the bulbs in parallel burns out, the other bulb will continue to function or deliver power because the current will continue flowing through the path in which it is present.

Main ideas:

00:00 Which set of bulbs will glow brighter?

00:17 Bulbs are resistors that are glowing

00:52 When resistors are connected in series, the current through each resistor is the same

01:32 For two identical resistors in series, the supply voltage divides equally across them

02:12 Power in bulbs in series

03:10 In parallel, both bulbs have the same voltage across them

04:10 Power delivered to bulbs in parallel

04:35 The electrical power in a resistor becomes 4 times when current and voltage is doubled

05:20 Energy consumption in parallel goes up by four times

06:00 What happens when a bulb in series or parallel burns out?

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