# ELECTRICAL CIRCUITS

## Transcription

ELECTRICAL CIRCUITS

5/5/2015 ELECTRICAL CIRCUITS The CELL The cell stores chemical energy and transfers it to electrical energy when a circuit is connected. When two or more cells are connected together we call this a Battery. The cells chemical energy is used up pushing a current round a circuit. S.MORRIS 2006 More free powerpoints at www.worldofteaching.com What is an electric current? simple circuits An electric current is a flow of microscopic particles Here is a simple electric circuit. It has a cell, a lamp called electrons flowing through wires and and a switch. components. + cell wires switch lamp - In which direction does the current flow? from the Negative terminal to the Positive terminal of a cell. To make the circuit, these components are connected together with metal connecting wires. simple circuits When the switch is closed, the lamp lights up. This is because there is a continuous path of metal for the electric current to flow around. If there were any breaks in the circuit, the current could not flow. circuit diagram Scientists usually draw electric circuits using symbols; cell lamp switch wires 1 5/5/2015 Circuit Symbols: Battery Resistor Light-bulb Three general types of circuits: Closed Circuit - There is a complete loop with wires going from one side of the battery through a resistor(s) to the other side of the battery. Switch Open Circuit - There is not a complete loop. Wire Short Circuit - There is a complete loop, but it does not contain any resistors. Chapter 22: Current and Resistance Only Working Circuit CURRENT •Most modern day appliances require electricity to operate. •The electricity generates a current which runs through the appliance. •The current is made out of charged particles •Current is defined as the rate at which charge flows through a surface area perpendicular to the direction of flow. •For example: if the charges are flowing through a wire, the area is the cross-sectional area of the wire I Q t Where: I = current (amps, A) Q = change in charge or how much charge flows (Coulombs, C) t = time (seconds) Example: The amount of charge that passes through the filament of a certain lightbulb in 2.00 sec is 1.67 C. Find (a) the current in the bulb and (b) the number of electrons that pass through the filament in 5.00 sec. I Q 1.67C 0.835A t 2.00s (b) First find the total charge in 5.00 sec. Then find how many electrons that is (remember, 1 electron = 1.6x10-19C) Q 5.00s 1electron Q 4.175C 2.611019 electrons 1.6 10 19 C 0.835A Electric Current Flow In real circuits it is electrons (-charges) that are flowing. We call this kind of current “electron current”. However, for simplicity we assume that all charge carriers are positive. We draw current arrows in the direction that such charges would move….that is, positive to negative. 2 5/5/2015 I was positive I was right about this. Historically this is because Ben Franklin guessed wrong about the charge carriers in a metal being positive. For an electric current to flow between two points, two conditions must be met. 1. There must be a conducting path between the points along which the charges can move. WHY? The conducting material allows the charges to move. 2. There must be a difference of electric potential (volts) between the two points. WHY? The potential difference makes the charges want to move. What does potential have to do with it? But tip the pipe, increase the potential of one end so there is a difference in potential across the ends of the pipe, and water will flow. And so it is with electric current flow. Water doesn’t flow in the pipe when both ends are at the same height. That is to say, water will not flow when both ends of the pipe are at the same potential (height). You get current to flow through a conductor by applying a voltage across it. This voltage creates an electric field within the conductor. This electric field in turn creates a force on all the charges within the conductor. If charges are free to move, as in a conductor, they do. In solids these charges are always electrons. An ampere is a fairly large amount of current: 0.1 A flowing between your hands across your heart will kill you. (Fortunately, your body has fairly high resistance so it takes a substantial voltage to drive that much current.) Current flow is the measure of electrons moving within a conductor. Amperes, or amps, measures the amount of current that flows. Amps is named after a 19th-century French physicist, AndreMarie Ampere, and abbreviated A. - + 3