Resistance (R) This is the ratio of the p.d. across a conductor to the current flowing through it. i.e.R = V I Unit: Ohm http://micro.magnet.fsu.e du/electromag/java/filam entresistance/
Ohm’s Law This states that for certain conductors (mainly metals) the current flowing through them is directly proportional to the p.d. across them at a constant temperature. i.e.V I or V = IR http://micro.magnet.fsu.edu/ele ctromag/java/ohmslaw/
Resistors in Series and Parallel In Series the total resistance is R = R 1 + R 2 + R 3 R1R1 R2R2 R3R3 R1R1 R2R2 R3R3 In Parallel the total resistance is 1 = 1 + 1 + 1 R R 1 R 2 R 3 http://lectureonline.cl.msu.edu/~mmp/kap20/RR506a.htm
Factors affecting Resistance of a conductor Resistance depends on Temperature Material of conductor Length Cross-sectional area Temperature The resistance of a metallic conductor increases as the temperature increases e.g. copper The resistance of a semiconductor/insulator decreases as the temperature increases e.g. thermistor.
Length Resistance of a uniform conductor is directly proportional to its length. i.e. R L Factors affecting Resistance of a conductor Cross-sectional area Resistance of a uniform conductor is inversely proportional to its cross- sectional area. i.e.R 1 A
Factors affecting Resistance of a conductor Material The material also affects the resistance of a conductor by a fixed amount for different materials. This is known as resistivity ( ). R = L = constant of proportionality A Unit: ohm meter m = R d 2 (For a wire with circular cross-sectional area) 4L
Wheatstone Bridge Uses Temperature control Fail-Safe Device (switch circuit off) Measure an unknown resistance R 1 = R 3 (When it’s balanced) R 2 R 4 Metre Bridge R 1 = R 2 (|AB|) |BC| www.dwiarda.com/scientific/Bridge.html www.dwiarda.com/scientific/Bridge.html I r 1 r 2 r 4 r 3 ACBD
Effects of an Electric Current 1. Heat 2. Chemical 3. Magnetic Joule’s Law States that the rate at which heat produced in a conductor is directly proportional to the square of the current provided its resistance is constant i.e. P = I 2 R In order to prevent power lines from overheating, electricity is transmitted at a very high voltage (EHT: Extra High Tension). From Joule’s law the larger the current the more heat produced hence a transformer is used to increase voltage and lower current i.e. P = V I
Effects of an Electric Current Electrolysis is the chemical effect of an electric current Voltameter consists of electrodes, an electrolyte and a container Inactive electrodes are electrodes that don’t take part in the chemical reaction e.g. platinum in H 2 SO 4 Active electrodes are electrodes that take part in the chemical reaction e.g. copper in CuSO 4
Effects of an Electric Current Ion is an atom or molecule that has lost or gained 1 or more electrons Charge Carriers in an electrolyte are + and – carriers Uses Electroplating to make metal look better, prevent corrosion Purifying metals Making electrolytic capacitors
Relationship between V and I for conductors Metallic conductor Negative electrons are the charge carriers I V I V I V Filament bulb Negative electrons are the charge carriers Semiconductor Negative electrons and positive holes are the charge carriers
Relationship between V and I for conductors Active electrodes Positive and negative ions are the charge carriers I V I V I V I V Inactive (Inert) electrodes Positive and negative ions are the charge carriers Gas Positive and negative ions and electrons are the charge carriers Vacuum Electrons are the charge carriers
Domestic Electric Circuits Electricity entering the home is supplied at 230V a.c. 2 wires enter the house from the mains: Live + neutral and pass through the meter box These 2 wires pass into a distribution box with fuses
Domestic Electric Circuits Radial circuit are used for appliances that take a large current. Each closed circuit has its own live wire, neutral wire and fuse e.g. cooker, electric shower Ring circuit are used for connections to sockets. Live terminals are connected together as are the neutral terminals Lights are connected in parallel and a number of them are connected to the same fuse
Domestic Electric Circuits Safety in house circuits Switch: should always be connected in the live wire Fuse: piece of wire that will melt when a current of a certain size passes though it. Connected to the live wire.
Domestic Electric Circuits Safety in house circuits MCBs: miniature circuit breakers are found in the distribution box. They are bimetallic strips(for small currents) and electromagnets (for large currents). Can be reset when the switch trips, faster than fuse. RCDs: residual current devices protect sockets and people against electrocution by detecting a difference between current in live and neutral wire (30 mA).
Domestic Electric Circuits Safety in house circuits Bonding: All metal taps, pipes, water tanks etc are connected to the earth Earthing: Earth wire prevents electrocution from touching metal parts of appliances by providing a path of least resistance when faults occur.
E.S.B Kilowatt-hour kWh This is the amount of energy used by a 1000 W appliance in one hour The ESB charge bills based on the no. of units, kWh, used in the home
Credits Slide 2: Slide 2: Resistors image www.sffej.net/educational/resistor_Colour.htm www.sffej.net/educational/resistor_Colour.htm Resistor colour codes www.radiodaze.com/rescarbcomp.htm www.radiodaze.com/rescarbcomp.htm Slide 3: Slide 3: George Ohm image~ www.past.dk/artefacts/photos/53/photo-1113908435-89551-5995.tkl?owww.past.dk/artefacts/photos/53/photo-1113908435-89551-5995.tkl?o Slide 4: Slide 4: None (Note: Use P, for previous and N, for next on key board to go back and forth between photos if no remote control available. Both circuits are connected to a 12V power supply and can be compared in terms of how bright the 3 bulbs are) Slide 5: Slide 5: None Slide 6: Slide 6: Temperature and resistance animation ~ Science Joy Wagon (www.sciencejoywagon.com)www.sciencejoywagon.com www.regentsprep.org/Regents/physics/phys03/bresit/default.htm www.regentsprep.org/Regents/physics/phys03/bresit/default.htm Slide 7: Slide 7: Cross sectional area and resistance animation ~ Science Joy Wagon (www.sciencejoywagon.com)www.sciencejoywagon.com www.regentsprep.org/Regents/physics/phys03/bresit/default.htm www.regentsprep.org/Regents/physics/phys03/bresit/default.htm Slide 8: Slide 8: Resistors image http://homepages.nildram.co.uk/~vwlowen/radio/alarm/how2.htm http://homepages.nildram.co.uk/~vwlowen/radio/alarm/how2.htm Slide 9: Slide 9: Sir Charles Wheatstone image ~ from the BT Connected Earth Collection. See www.connected-earth.com www.connected-earth.com Slide 10: Slide 10: Sunset Power Lines www.tonyboon.co.uk/imgs/pages/powerlines.htm www.tonyboon.co.uk/imgs/pages/powerlines.htm Slide 11: Slide 11: Hoffman Voltameter image www.dalefield.com/earth/hydrogen1.html www.dalefield.com/earth/hydrogen1.html Slide 12: Slide 12: Electroplating image ~ www.finishing.com/faqs/howworks.htmlwww.finishing.com/faqs/howworks.html Slide 13: Slide 13: None Slide 14: Slide 14: None Slide 15: Slide 15: Circuit Breaker image ~ Edfenergy www.edfenergy.com/powerup/keystage3/in/page2.html www.edfenergy.com/powerup/keystage3/in/page2.html Slide 16: Slide 16: Circuit Breaker image ~ Edfenergy as above Light Circuit image ~ www.buzzybee.org/diy/projects/electrical/lighting/wiring.htmlwww.buzzybee.org/diy/projects/electrical/lighting/wiring.html Slide 17: Slide 17: None Slide 18: Slide 18: None Slide 19: Slide 19: None Slide 20: Slide 20: None