[MUSIC] Welcome to Linear Circuits. I'm Dr. Harris. As an overview of this module, by the end of it, you should be able to analyze simple electronic circuits. You'll recognize the circuit symbols. You'll be able to calculate and describe current and voltage, as well as resistance, power and energy. In this lesson, we'll discuss current and charge. By the end of this lesson you should be able to describe and quantify electric charge and current. So let's start with Electric Current. Consider a substance with charge particles flowing through it. Electric Current is the rate at which electric charge flows through that substance. Therefore, in order to understand electric current, we have to examine electric charge. Specifically, what is electric charge and why does electric charge flow? So, starting with the first question What is electric charge? First of all it's a physical property of all matter. It's quantized into integer multiples of e, which is written here as 1.602 times 10 to the minus 19. And there are two types of charge, positive Which is a value of positive e and negative, which has a value of negative e. The unit per charge is coulombs, and the variables will use a lower case q or an upper case Q. So now, why exactly does electric charge flow? Well, let's consider these charges that are shown here. The positive and the negative charge. Because they have a charge, they exert a force on other charged particles and this force per unit charge is called an electric field. The electric field points away from positive charges, which are show here in the lines pointing away from the positive. An electric field points towards negative charge, so in other others, if you remember from physics, an electic field pushes a positive charge. Therefore, charge flows because their electric fields exert forces that push each other. Hence, we can use this charge flow to define current. Electric current is the quantity of charge that passes through a given area in a specified amount of time. The current therefore as a function of time can be written as the first derivative dQ, dt. However if the constant current, if the current is constant, then you can simply write it as Q over t. We'll use variables i and I, and units coulombs per second or amperes. Now that we understand the definition of electric current, let's try calculating electric current with these two quizzes. Attempt quiz one and quiz two on your own. By pressing the pause button, and when you're done, let's work through the problem together. First quiz says, a one millimeter, cross section of copper wire. Is isolated in 50 C of charge flow through it in two seconds. How much current flow through the wire, in that time? So let's solve this. We know that the current in this case is charge over time, or 50 C of time in two seconds, which is 25 amps of current. The second quiz says, for that same one millimeter cross section of wire that's isolated, the charge that flows through the cross section is a function now Q(t)=4t^2+5. How much current flow through the wire in 6 seconds? So now in this case, I is a function of time, so we need to differentiate Q, with respect to time, to get 8t and at six seconds, our current is eight times six, or 48 amps. Now current can actually flow in any direction depending on the source we're using. So what are the reference directions for current flow? You see here, two pieces of material, and in one of the pieces current is flowing from the + to the -, and in the other the current is flowing from the- to the +. Positive current flows from high to low. In other words, from plus to minus. Negative flows from low to high, or in other words, from minus to plus. That's why I1 which is flowing from the plus to the minus is a positive one amp. I2 which is flowing from minus to plus is a negative one amp. So the key concepts from this lesson are first, electric current is a quantity of charge that passes through a given area in a specified amount of time. The current can be represented as the first derivative of the charge with respect to time or simply Q over t. Positive current flows from positive to negative or from high to low and negative current flows from minus to plus or from low to high. [SOUND]
