ENERGY TRANSFER IN ELECTRICAL SYSTEMS What is electricity? CIRCUITS AND CURRENT ELECTRICITY What is a circuit? A system for transferring electrical energy. A complete conducting pathway for electricity. http://rghs-svrtwig.rghs.org.za/schools -english-uk/films/whatis-electricity-1540.html In order for a device (such as a lightbulb) in a circuit to work, the circuit must be closed. What does this mean? No breaks in the circuit. It must be complete. When electrical charge flows through a conductor, we have current electricity. How is current electricity different to static electricity? Current electricity flows, while static electricity does not flow. Fill in the table below by classifying the following materials as either conductors or insulators: INSULATORS Glass bottle Eraser Books / paper Plastic / lunch box CONDUCTORS Can Coin Paper clip 1 COMPONENTS OF A CIRCUIT Circuits A circuit is a system that transfers electricity through a complete conducting pathway. A minimum of three components are required or order to create a working circuit: non - http://rghs-svrtwig.rghs.org.za/schoolsenglish-uk/films/circuits1555.html 1. A source of electrical energy. Eg: Cell or battery 2. Conducting material. Eg: Wires 3. A device to show that the circuit is complete and current is flowing. Eg: A light bulb, heater (any appliance) essential A device / works device : light bulb / appliance Cells and batteries An expiray Agtr shell life : chemicals date : shews connection (acid ) leaks out will show if the circuit of cells Chemical potential energy and melts appliance . appliances positive , wires etc terminal . Chemical systems Store chemical substances (chemical potential energy) Source of energy (electrical energy) When in a complete circuit, the chemicals react and produce a potential difference ↑ Negative Terminal Chemical energy is converted to electrical energy An electric current is the flow of charges along a conductor The ends of the cell are called terminals. There is a positive terminal and a negative terminal. How do you tell the difference between the positive and negative terminals of a cell/battery? Positive terminal has a bump (pimple) and the negative / end has a dent (dimple). What is the difference between a cell and a battery? A battery is two or more cells connected together. 2 Energy conversion in electric circuits Chemical potential energy Electrical potential energy movement Kinetic energy Heat energy - Light energy _ Conducting wires Usually made of metal (copper). Silver is actually the best conductor but it is not used in electrical cables because it is much more expensive than copper. Provides a path along which charge can flow. Can carry electricity over long or short distances. Switch : where you break the circuit Convenient way of preventing or allowing the flow of current electricity. What other ways are there of preventing the flow of current electricity and why are these not advisable? Completely disconnect the battery. Could be dangerous. Switches usually have plastic over them so that you don’t come into contact with the conducting wires. 3 Resistors Don't stop the charge slow the flow The wires won't heat up less resistance - more as fast flow Made of materials that resist or oppose the flow of charge (current) in a circuit Resistance http://rghs-svrtwig.rghs.org.za/schoolsenglish-uk/films/resistance1556.html They influence the amount of current in a circuit. How is this different to a switch? A switch would completely prevent the flow of current while a resistor could change it gradually. Some resistors can restrict or vary the amount of current in an electrical circuit (e.g. variable resistors) What variable resistors might exist in your home? Dimmer lights, stove with varying temperatures, heater with varying temperatures, speakers More resistance less current Less resistance more current As current flows through a resistor, the resistor heats up. What are some advantages and disadvantages of this? Can utilize the heat (in a stove or heater). The heat could be dangerous and cause fires. It could also hamper the functioning of the appliance. Examples of resistors: Any appliance, light bulbs etc. 4 DRAWING CIRCUITS Drawing circuits Complete the following table by drawing in the symbols for the different devices: Device Name http://rghs-svr-twig.rghs.org.za/schoolsenglish-uk/films/factpack-how-to-draw-acircuit-1562.html Symbol Light bulb Cell / battery Switch closed Resistor Fuse Ammeter Voltmeter 5 open Activity 1 1. How does the brightness of a single bulb change if you increase the number of bulbs in a circuit without changing the number of cells? Draw at least 2 more different circuits and compare them. 2. How does the brightness of a single bulb change when you increase the number of cells in a circuit? Draw at least 3 different circuits and compare them. 3. Circuits can transform electrical energy into other types of energy. Identify the energy conversions in the following appliances: a. A fan – electrical energy is transformed into kinetic energy b. A stove – electrical energy is transformed into heat and light energy c. A hairdryer – electrical energy is transformed into heat and kinetic energy d. A light bulb – electrical energy is transformed into light (and heat) energy e. An electric car – electrical energy is transformed into kinetic energy 6 VOLTAGE (POTENTIAL DIFFERENCE) Voltage / potential difference causes a current to flow as it pushes the charge from a HIGH potential in the cell to a LOW potential in the circuit. Work has to be done on the charge to cause it to move through the circuit. The cell provides the energy for that work to be done. Voltage is a measure of how strongly the charges are ‘pushed’ along the conductor. Voltage / potential difference is measured by a device called a voltmeter. CURRENT Current is the rate of flow of charge in a conductor. Conventional current flows from the positive terminal to the negative terminal. Electrons flow from the negative terminal to the positive terminal of a cell. This is called electron current. Current is measured by a device called an ammeter. 7
