Current, Grade 10 Physics 1/ Flow of Charge We have been talking about moving charge. We need to be able to deal with numbers, how much charge is moving, how fast is it moving? The concept that gives us this information is called current. Current allows us to quantify the movement of charge. When we talk about current we talk about how much charge moves past a fixed point in circuit in one second. Think of charges being pushed around the circuit by the battery, there are charges in the wires but unless there is a battery they won’t move. When one charge moves the charges next to it also move. They keep their spacing. If you had a tube of marbles like in this picture. If you push one marble into the tube one must come out the other side. If you look at any point in the tube and push one marble into the tube, one marble will move past the point you are looking at. This is similar to charges in the wires of a circuit. If a charge moves they all move and the same number move at every point in the circuit. 2/ Current Now that we’ve thought about the moving charges and visualised what is happening we need to get back to quantifying moving charge. I’ve already told you that we use current but we still need to define it. Definition: Current Current is the rate at which charges moves past a fixed point in a circuit. We use the symbol I to show current and it is measured in amperes (A). One ampere is one coulomb of charge moving in one second. \[I = \dfrac{Q}{\Delta t}\] When current flows in a circuit we show this on a diagram by adding arrows. The arrows show the direction of flow in a circuit. By convention we say that charge flows from the positive terminal on a battery to the negative terminal. 3/ Series Circuits In a series circuit, the charge has a single path from the battery, returning to the battery. Important: A cell does not produce the same amount of current no matter what is connected to it. While the voltage produced by a cell is constant, the amount of current supplied depends on what is in the circuit. How does the current through the battery in a circuit with several resistors in series compare to the current in a circuit with a single resistor? Activity :: Experiment : Current in Series Circuits Aim: To determine the effect of multiple resistors on current in a circuit Apparatus: Battery Resistors Light bulb Wires Method: Construct the following circuits Rank the three circuits in terms of the brightness of the bulb. Conclusions: The brightness of the bulb is an indicator of how much current is flowing. If the bulb gets brighter because of a change then more current is flowing. If the bulb gets dimmer less current is flowing. You will find that the more resistors you have the dimmer the bulb. 4/ Parallel Circuits How does the current through the battery in a circuit with several resistors in parallel compare to the current in a circuit with a single resistor? Activity :: Experiment : Current in Series Circuits Aim: To determine the effect of multiple resistors on current in a circuit Apparatus: Battery Resistors Light bulb Wires Method: Construct the following circuits Rank the three circuits in terms of the brightness of the bulb. Conclusions: The brightness of the bulb is an indicator of how much current is flowing. If the bulb gets brighter because of a change then more current is flowing. If the bulb gets dimmer less current is flowing. You will find that the more resistors you have the brighter the bulb. Why is this the case? Why do more resistors make it easier for charge to flow in the circuit? It is because they are in parallel so there are more paths for charge to take to move. You can think of it like a highway with more lanes, or the tube of marbles splitting into multiple parallel tubes. The more branches there are, the easier it is for charge to flow. You will learn more about the total resistance of parallel resistors later but always remember that more resistors in parallel mean more pathways. In series the pathways come one after the other so it does not make it easier for charge to flow. Xem thêm: Physics 10.IX Electric Circuits High School Students Studying the Sciences Physics