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science world 1 – chapter 10 how things work ELECTRIC CIRCUITS When electrons are forced to travel along metal wires, an electric current is produced. The flow of electric current is called electricity. Electrons must move around a pathway called an electric circuit. For current to flow, it must have a complete pathway, or circuit, to follow. If there is a break in the circuit, the electrons stop moving and no current flows. The cell or battery supplies the energy to push the current around the circuit. The amount of energy supplied by the cell is measured in volts. The voltage is a measure of the electrical potential energy of the cell. Materials through which electric current can move easily are called conductors (e.g. metals). Materials through which electric current cannot easily move are called insulators (e.g. rubber, plastic, glass, wood and paper). CIRCUIT DIAGRAMS Drawing electric circuits can be confusing and takes time. So electricians have decided on a simple symbol for each of the parts in a circuit. Just like when drawing scientific diagrams, circuit diagrams should be drawn in lead pencil and using a ruler. Students to copy “Electrical Symbols” from p 192. 1 SERIES & PARALLEL CIRCUITS Two different circuit arrangements can be used to connect a number of light globes to a single power supply. In a series circuit, the electric current is made to pass through every globe, one after the other. In a parallel circuit, the electric current splits up and portions go through each globe. (Draw Figures 10 and 12 on p 192 and 193.) Many circuits are quite complicated and contain some devices connected in series and some connected in parallel. Such circuits are called combination circuits. SIMPLE MACHINES Simple machines are things such as levers, pulleys, ramps, screws and gears. Complex machines such as clocks and bicycles are built up of many simple machines. Machines are devices that help you do things more easily. They do this in three different ways. 1. 2. 3. Machines magnify the force you use (e.g. using a crowbar to lift a heavy object). Machines change the direction of the force (e.g. using a pulley to lift the sails up on a boat). Machines make things go faster (e.g. using the gears on your bicycle). It is impossible to get more energy out of a machine than you put in. Machines transfer energy from the effort to the load. Due to friction there is always some energy wasted as it is converted to heat and sound. 2 LEVERS A crowbar is a type of simple machine called a lever. A lever is a long bar or stick which moves around a fixed point called the pivot or fulcrum. The object that you want to move is called the load and the force you apply to move the load is called the effort. A lever makes work easier by reducing the force needed to move something. However, in order to use less force, the lever must be moved through a greater distance. There are many types of levers around you. Pliers have the pivot between the load and the effort. Wheelbarrows have the load between the pivot and the effort. effort load load effort pivot pivot You can get a measure of how useful a simple machine is by calculating its mechanical advantage using the following formula: Mechanical Advantage = Load_ Effort INCLINED PLANES & WHEELS & AXLES A ramp is an example of a machine called an inclined plane. Rolling a barrel up a ramp is much easier than lifting it into place. Inclined planes have a force advantage – you apply a small effort over a large distance to lift or move a heavy load over a small distance. 3 A screw is an inclined place in a spiral shape – it is essentially an inclined plane wrapped around a cylinder. Some objects are actually a double inclined plane which changes the direction of a force e.g. axes and wedges. A steering wheel is an example of a wheel-and-axle. The axle is the central rod or column and the steering wheel is attached to this axle. A steering wheel enables a small force to be applied and the axle will turn. Doorknobs and screwdrivers are also examples of a wheel-and-axle. PULLEYS & GEARS A pulley is a grooved wheel with a rope over it. A single pulley does not magnify your force, but it does change the direction of the force. To magnify your force you need to use more than one pulley. This lets you lift heavy loads by using only a small effort. Gear wheels are wheels with teeth on them. The teeth of one gear fit into the teeth of another. Gears are used to transfer the force from one wheel to another. In a bicycle, the gear wheel attached to the pedals is called the driving gear and the gear wheel on the back wheel is called the driven gear. Gears can speed things up or slow things down. To speed things up the driving gear has more teeth (is larger) than the driven gear (see Figure 53 on p 206). To slow things down the driving gear has less teeth (is smaller) than the driven gear (see Figure 54 on p 206). 4 SPELLING WORDS No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 EASY current globe series voltage battery switch simple machine force pivot planes gears pulleys crowbar effort wedge screw engine load effort HARD electricity circuit conductor insulator ammeter parallel combination appliance complex mechanical magnify direction advantage axle arrangement leverage fulcrum inclined rotating bicycle 5