Transcript Slide 1
Electrical Resistance Electrical resistance is a measure of how difficult it is for electricity (electrons) to flow through a material. The units of resistance are ohms and the symbol is Ω. Measuring Resistance The ammeter measures the current flowing through the bulb. It is connected in series with the bulb. A V The voltmeter measures the potential difference (p.d.) across the lamp. It is connected in parallel with the lamp. We can calculate the resistance of a component if we have measured the current and the voltage Resistance = Potential difference (volts) Current (amps) (ohms) Using symbols eg R= R= V I 4.5V 0.5A = 9Ω Ohm’s law The current through a conductor is directly proportional to the potential difference across it provided the temperature remains constant. A 1 Amp A 2 Amp *If the voltage doubles then the current doubles” V= I R Eg 1 Calculate the voltage across the resistor : 2 Amp 4 ohm V V= I R V= 2x4 V = 8 Volt Resistance Practical Set up the circuit below. Measure the current through and the voltage across the resistor. Record your results. Calculate the resistance of the resistor. Component Current A A V Repeat the experiment with different components (amps) Potential difference (volts) Resistance (ohms) resistor bulb Resistance Resistance is a measure of how difficult it is for electrons to flow through something. More bulbs, more resistance, less current The high resistance filament converts more energy into heat and light than the low resistance cable. Circuit Y has twice the resistance of circuit X. However, there is no extra voltage and so the current is halved. atoms block the paths of the electrons electrons long thin wires have a high resistance Long thin wires have a high resistance so less current flows Wide wires have many more routes for the electrons so have LOWER RESISTANCE electrons The moving electrons give the atoms some extra kinetic energy, so the wire heats up. This is used in kettles, hair dryers toasters, irons. Doubling the voltage doubles the current ! Ohm’s law The current flowing in a conductor is directly proportional to the potential difference across its ends, provided that the temperature remains constant. • Potential difference, current and resistance are related by the equation: potential difference current resistance (volt, V) (ampere, A) (ohm, Ω )