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Electric circuits What is the job of an electrical circuit? • Electrical energy can be transferred through it • This electrical energy comes from a battery or other power supply • The energy can be transferred to components in the circuit • The unit of energy is the joule (J) What is a current? • Current is a flow of electrons round the circuit • Current will only flow through a component in a circuit if there is a voltage across that component What is direct current? • Direct current is travels in one direction only • Cells and batteries supply direct current • Direct current is also known as d.c. current or d.c. supply Direct current • With d.c. current the voltage is constant voltage • The cathode ray oscilloscope (CRO) trace will show d.c. supply as a horizontal line What is alternating current? • Alternating current is also known as a.c. current of a.c supply • This current is always changing direction, so the voltage goes up and down all the time too Alternating current • a.c. supply will produce a wave on the cathode ray oscilloscope • The voltage with a.c. supply is constantly going up and down Types of circuits: • • • • SERIES These circuits go in one continuous ‘loop’ The current is the same anywhere in the circuit An ammeter can be placed anywhere in the circuit If we get one gap in the circuit the current stops flowing The other type of circuit: PARALLEL • These circuits have two or more routes for the current to flow round • If we get a gap in the circuit not all components will go off • Parallel circuits are used for house and car lights • The current leaving the power supply will split up to go through different parts of the circuit • The current returning to the battery is the same as what had left it Circuit symbols: Measuring current • An ammeter is used to measure current • Current is measured in amperes or amps (A) for short • In physics electrical current is shown as (I) Heating effect • When electrical charge flows through a resistor it will generate heat • This heat is useful in the following: • Hairdryers • Immersion heaters • Light bulbs Voltage • A voltmeter is used to measure voltage • Voltage is also known as potential difference • The unit of voltage is volts (V) Resistance • Resistance tells us how easy or difficult it is for electrical charge to flow through a conductor • Resistance (R) is measured in ohms () • We can use the following equation to find out the resistance of a wire or a component: • Voltage = current x resistance • V=IR What does a variable resistor do? • You change change the resistance in a circuit by twisting a knob or moving a slider on an oldfashioned variable resistor • We can use a variable resistor to change the current flowing through a circuit What is a light dependant resistor? • In bright light the resistance will drop in an LDR • When it is dark the resistance is highest • This makes an LDR useful in automatic circuits such as automatic night lights and burglar detectors What do thermistors do? • A thermistor is a temperature dependant resistor • The resistance drops when it is hot • When it is cold the resistance goes up • Thermistors are useful temperature detectors, e.g. in car engines and electronic thermostats What else can affect the resistance in a circuit? • Thick wires have a low resistance as it is easier for current to flow through • Thin wires have a high resistance as it is harder for current to flow through • Long wires in a circuit have a higher resistance than short wires as the current has further to travel What else affects resistance? • Some metals are better conductors than others • Therefore some metals give a higher resistance than others • Copper is a good conductor of electricity • Copper has a low resistance Temperature and resistance • If a wire gets warmer, the resistance increases • The atoms in the wire start to vibrate more when they have more energy • This makes it harder for the flow of electrons to flow through the wire. This powerpoint was kindly donated to www.worldofteaching.com http://www.worldofteaching.com is home to over a thousand powerpoints submitted by teachers. This is a completely free site and requires no registration. Please visit and I hope it will help in your teaching.