Chapter Five electric power. Energy is defined as the property that must be transferred to an object in order to perform work, energy equals work. Consider these two weight lifters lifting the same amount of weight to the same height. Both do the same amount of work. Both expend the same amount of energy in terms of physics, work equals mass times acceleration or gravity times height, which equals 100 kilograms times 9.8 meters per second squared times two meters, which equals 1960 joules. However, the weightlifter on the left is slower than the weightlifter on the right.

Hence we say that the right weight lifter is more powerful than the left weightlifter. Power measures the rate at which work is done. Or that power equals work divided by time. So if power is equal to work divided by time and we know that the same amount of work is done by each weightlifter because they've lifted the same weight through the same distance, and that is equal to 1960 joules. But the weightlifter on the left completes his lifts in three seconds. So the power delivered by that weight lifter is 653 joules per second.

The weightlifter on the right however lifts his weight in one Second, so that the power delivered by the right weightlifter is 1960 joules per second. Because work is measured in joules, power is measured in joules per second, and this measurement is defined as watts such that the work of one Joule completed in one second is equal to one watt. If we watch the lift, we see that the speed of the lift is not consistent. Regardless if the lift is completed in three or one seconds, some of the lift is completed faster or slower than the other parts, which means the power delivered will vary. So if we use the total time for the lift, in our equation, a one or the three seconds we define that power power delivered as average power, if we break the whole lift up into smaller time increments, such as such that the power over that small increment is consistent.

We will define that as instantaneous power, which is consistent over that small time increment. In terms of electrical power, the work done or electrical energy is the movement of charges caused by the push of emf. In other words, it is the energy required to move an electric charge of q q looms over a potential difference of the volts and is expressed as V times q electron volts. By definition one electron volt is the amount of energy gained or lost by the charge of a single electron moving across an electric button. Difference of one volt, one kilo is equal to 6250 followed by 15 zeros electrons. And one electron is equal to one over q, which is equal to 1.6 times 10 to the negative 19 kilo ohms.

Electric power p is the rate at which electric energy is transferred by an electric circuit. The power p is the energy dissipated over time t, but the energy E is equal to the voltage times a charge being passed. Therefore, p is equal to the product of the voltage times the charge all over T or voltage times q divided by T and because Q divided by T is I or current, where one amp is equal to one coulomb of electrons passing by in one second of time. P is given by the voltage times the current, for a constant voltage and constant current or DC values for voltage and current, or it might be considered the product of the instantaneous voltage times the instantaneous current. electrical power is measured in watts. A watt, sometimes symbolized by the capital letter W, is a derived unit of power in the International System of Units si.

This is a chart of the International System of Units for power. The most common ones that are used are the milli watt which is point 000001 of a lot or 10 to the minus six watts, or a kilowatt, which is 1000 watts or 10 to the third watts or a megawatt, which is 1 million watts, or 10 to the six watts, or a gigawatt, which is 1000 megawatts, or 10 to the ninth watts. Now, the power of a circuit and the voltage and the current are related by these equation, which is the same equation just written three different ways and you can memorize their relationship by the above triangle, you can see that P is always over either V or I, depending on what you're looking for if you're looking for V voltage than its power over AI. If you're looking for current then it's p Oliver V. The most common one used is the first one, of course, power's equal to the voltage times the current.

But AI is used is equal to P over V is used all the time in calculating the current draw on electrical circuits and electricians use this when contemplating or designing the circuits for house wiring say. However, I'm getting a little bit ahead of myself now because house wiring is AC or alternating current, whereas we're just talking about DC circuits right now. However, the thing I've said up to this point is applies to DC circuits. And they will somewhat as you'll see, when we get into AC circuits in the, in another course, that these formulas in these symbols still apply, but you have to take them into consideration due to the fact that we are Using AC circuits however, let's get back to talking about DC circuits. Okay let's do a calculation of power in a in an electric circuit. In this circuit, according to ohms law, if we have a voltage of 18 volts pumping into a lamp of three ohms the current that will flow in that circuit given by ohms law is six amps.

So, the lamp would be consuming the current times the voltage which is six amps times 18 volts would be equal to 108 watts. In this example, we have a 100 watt light bulb connected to 120 volt battery according to the power equation Power is given by the current times the voltage that is to say that the current times 120 volts is equal to 100 watts. Therefore, the current is equal to 100 divided by 120 which is equal 2.833 amps. So the current flowing to that light bulb would be point 833 amps. Also the resistance by the way would be according to ohms law voltage over the current which is equal to 120 divided by point 833 which is equal to 144 ohms. So, power can be expressed in terms of voltage and current in this case, we can say that the power is equal to i the current times IE the voltage which stands for electromotive force.

We know from ohms law that the current is equal to e L over R. So, we can describe power consumption in a circuit in terms of the voltage across the resistance and the resistance only. So the power consumed by a resistive load would be E squared, all over r where r is the resistance of the load and e is the voltage drop across that resistance. We can also describe the power consumption in a circuit in terms of the current in that state. circuit, the current through the resistor, and that is given by i squared R. In other words, if we only know current and voltage, then we can calculate the power. If we only know resistance in voltage, we can still calculate the power. And if we only know resistance in current, we can still calculate power by using one of these three equations.

This ends chapter five