Electrical Quantities e.m.f and p.d Lecture

IGCSE Question: 4.2 Electrical Quantities e.m.f and p.d Markscheme: Energy Transfers and p.d. Energy ((an electric current)) (Energy store ex: a battery) lamp) Energy (Energy into the sorroundings ex: a Mains lamp with big potential difference (230 V) gives much more light and heat than the car lamp with V Evidently the12p.d. across a device affects the rate at which it transfers energy. This gives us a way of defining the unit of potential Model of a Circuit Imagine that: • The current in a circuit is formed by ‘drops’ of electricity, each having a charge of 1 coulomb and carrying equal-sized bundles of electrical energy. Mr Coulomb represents one such drop. • Mr Coulomb travels round the circuit and unloads energy as he goes, most of it in the lamp. He receiving a fresh bundle every time he passes through the battery, which suggests he must be travelling very fast. The greater the e.m.f. of a supply, the larger is the bundle of energy given to each coulomb and the greater is the rate at which energy is transferred from a lamp. The Volt How to find e.m.f or E (with the unit is volts) • If W (joules) is the energy transferred (i.e. the work done) when charge Q (coulombs) moves around a complete circuit, the e.m.f. E (volts) of the supply is given by Or How to find p.d or V (with unit is volts) • If W (joules) is the work done when charge Q (coulombs) passes between two points, the p.d.V (volts) between the points is given by or Remember that so that IGCSE Question: 4.2 Electrical Quantities e.m.f and p.d Markscheme: Voltmeter Voltmeter A voltmeter is used to measure potential differences; it should always be placed in parallel with the component across which the p.d. is to be measured. The positive terminal on the voltmeter should be connected to the side of the component into which current flows. • When making a measurement on either an analogue or digital voltmeter a suitable range must first be chosen. For example, if a voltage of a few millivolts is expected, the 10 mV range might be selected and the value of the voltage (in mV) read from the display; • If the reading is off-scale, the sensitivity should be reduced by changing to the higher, perhaps 100 mV, range. IGCSE Question: 4.2 Electrical Quantities e.m.f and p.d Markscheme: 4.2.4 Resistance Resistor Resistance is the opposition of a conductor to current. A good conductor has a low resistance and a poor conductor has a high resistance (see slide 4). If the current in a conductor is I when the voltage across it is V its resistance R is defined by Conductors intended to have resistance are called resistors and are made either from wires of special alloys or from carbon. Those used in radio and television sets have values from a few ohms up to millions of ohms. Circuit symbol for a resistor Fixed resistor Resistor Variabe resistor (potentiometer) These consist of a coil of constantan wire (an alloy of 60% copper, 40% nickel) wound on a tube with a sliding contact on a metal bar above the tube. There are two ways of using such a variable resistor. 1. As a rheostat for changing the current in a circuit; In Figure 4.2.26a moving the sliding contact to the left reduces the resistance and increases the current. 2. As a potential divider for changing the p.d. applied to a device; In Figure 4.2.26b any fraction from the total p.d. of the battery to Resistance of a metal wire The resistance of a metallic wire will • increases as its length increases • increases as its cross-sectional decreases • depends on the material area Resistance of a metal wire I–V graphs: Ohm’s law The variation of current with voltage is shown for various conductors in Figure I–V graphs: Ohm’s law Metallic conductors They are called ohmic or linear conductors and since I ∝ V, it follows that V/I = a constant (obtained from the slope of the I–V graph). The resistance of an ohmic conductor therefore does not changelaw, when the p.d. does. Conductors obey Ohm’s stated as follows: “The current in a metallic conductor is directly proportional to the p.d. across its ends if the temperature and other conditions are constant.” Semiconductor diode Figure shows that current passes when the p.d. is applied in one direction but is almost zero when the p.d. is applied in the opposite direction. It conducts in one direction only and is a nonohmic conductor. I–V graphs: Ohm’s law Filament lamp A filament lamp is a nonohmic conductor at high temperatures. For a filament lamp the I–V graph curve flattens as V and I increase. That is, the resistance (V/I) increases as I increases and makes the filament hotter. Thermistor The resistance of semiconductor thermistors decreases if their temperature rises, i.e. their I–V graph bends upwards, as in Figure 4.2.29d. 4.2.5 Electrical working Electric power is the rate of transfer of electrical energy within a circuit. Its SI unit is the Watt. Worked Example From the definition of p.d. that (see slide 20) so, You can use Ohm’s Law formula of power or to find another If a lamp on a 240 V supply has a current of 0.25 A in it, its power This means that 60 J of energy are transferred to the lamp each second. Joulemeter To measure the electrical energy transferred to an appliance, a joulemeter can be used to measure it directly in joules. The circuit connections are shown Paying for Electricity Electricity supply companies charge for the amount of energy they supply. The kilowatthour (kWh), is used. A kilowatt-hour is the electrical energy used by a 1 kW appliance in 1 hour. How to calculate electricity 1. Calculate the power P of an electrical appliance 2. Convert unit of power become kW (kilowatt) 3. Calculate energy used by appliance in hour Unit of 4. Calculate total costs (see WB pg 60; number 17) Electricity meters, which are joulemeters, are marked in kWh: the latest have digital readouts like the one in Figure