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Transcript
ELECTRICAL CIRCUITS SNC 1D An Introduction • The energy that results from moving electrons is called electrical energy. • When electrons move from one place to another they make an electric current. • The amount of energy that each electron delivers is called the voltage. • The path that the electrons follow is referred to as an electric circuit. Simple Circuits Here is a simple electric circuit. It has a source, a load and a switch to control the flow of electrons. source control connectors load To make the circuit, these components are connected together with metal connecting wires. Functions of the Parts • Source – where the electrons come from. (Commonly called the cell or battery) • Load – a device that converts electrical energy into another form. (Lightbulb, motor, buzzer, toaster, heater…) • Control – a device to start and stop the flow of electrons. (Turn load on and off) • Connectors – conducting wires to provide a path for the electrons to follow. 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. This is called a closed circuit. If there were any breaks in the circuit, the current could not flow. This is called an open circuit. 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. Types of Cells • Primary Cells – electric cells that can not be recharged because the chemical reaction is not reversible. • Secondary Cells – electric cells that can be recharged by forcing the chemical reaction in the reverse direction. • Fuel Cells – electric cells that require a continuous supply of chemicals to be pumped in. What is an electric current? An electric current is a flow of microscopic particles called electrons flowing through wires and components. + - In which direction does the current flow? from the Negative terminal to the Positive terminal of a cell. Circuit Diagram Scientists usually draw electric circuits using symbols; cell lamp switch wires Circuit Diagrams In circuit diagrams components are represented by the following symbols; cell ammeter battery voltmeter switch motor lamp buzzer resistor variable resistor Current • Made up of moving electrons • Measure of how fast the electrons are moving; how many move past a given point in a certain amount of time • Symbol for current is I • SI unit used to measure current is the Ampere (A) or Amps Measuring Current Electric current is measured in amps (A) using an ammeter connected in series in the circuit. A Measuring Current This is how we draw an ammeter in a circuit. A Connected right into the circuit Measuring Voltage The ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter V Measuring Voltage This is how we draw a voltmeter in a circuit. V Connected around the load and/or source Measuring Voltage Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current. Unlike an ammeter a voltmeter is connected across the components (in parallel). Scientist usually use the term Potential Difference (E) when they talk about voltage. Resistance • A measure of the opposition to the movement of electrons as they flow through a circuit. • SI unit used to measure resistance is ohms (Ω) • Measured using a ohmmeter • All materials have some internal resistance. The greater the resistance the lower the current and the warmer the material becomes. Factors Affecting Resistance • • • • Type of Material Cross-Sectional Area Length Temperature Relating Current, Voltage, and Resistance Gizmo Time! Ohm’s Law! Ohm’s Law As resistance increases, current decreases As voltage increases, current increases Ohm’s Law V=IR where V = voltage (V) I = current (A) R = resistance (R) Ohm’s Law Ohm’s Law Ohm’s Law Types of Circuits There are two types of electrical circuits; SERIES CIRCUITS PARALLEL CIRCUITS SERIES CIRCUITS The components are connected end-to-end, one after the other. They make a simple loop for the current to flow round. If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out. PARALLEL CIRCUITS The components are connected side by side. The current has a choice of routes. If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it stays alight. Measuring Current How does current differ in a series and parallel circuit? A A SERIES CIRCUIT PARALLEL CIRCUIT Measuring Current SERIES CIRCUIT • current is the same at all points in the circuit. 2A 2A 2A PARALLEL CIRCUIT • current is shared between the components 2A 2A 1A 1A Measuring Voltage How does voltage differ in a series and parallel circuit? V SERIES CIRCUIT V PARALLEL CIRCUIT Series Circuit • voltage is shared between the components 3V 1.5V 1.5V Parallel Circuit • voltage is the same in all parts of the circuit. 3V 3V 3V Measuring Current & Voltage Copy the following circuits on the next two slides. Complete the missing current and voltage readings. Remember the rules for current and voltage in series and parallel circuits. Using Ohm’s Law, calculate the resistance of each load in the circuits. measuring current & voltage a) 6V 4A A V V A measuring current & voltage b) 4A 6V A V A V A answers a) b) 4A 6V 6V 4A 6V 4A 4A 3V 2A 3V 4A 6V 2A