Transcript Resistance (R)
Current Electricity 2
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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/
Series Vs Parallel
+ _ Bulb + _
Resistors in Series and Parallel
R
1 In Series the total resistance is R = R 1 + R 2 + R 3
R
2
R
3
R
1
R
2
R
3 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.
Factors affecting Resistance of a conductor
Length
Resistance of a uniform conductor is directly proportional to its length.
i.e. R L
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 A
= R
d 4 L 2
= constant of proportionality Unit: ohm meter m (For a wire with circular cross-sectional area)
I A
Wheatstone Bridge
B Uses
Temperature control Fail-Safe Device (switch circuit off) Measure an unknown resistance
R 1 = R 3 R 2 R 4
(When it’s balanced)
C Metre Bridge R 1 = R 2 (| AB |) | BC |
www.dwiarda.com/scientific/Bridge.html
D
1.
2.
3.
Effects of an Electric Current
Heat Chemical 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
Filament bulb
Negative electrons are the charge carriers I
Semiconductor
Negative electrons and positive holes are the charge carriers I V V V
Relationship between V and I for conductors
Active electrodes
Positive and negative ions are the charge carriers
Inactive (Inert) electrodes
Positive and negative ions are the charge carriers
Gas
Positive and negative ions and electrons are the charge carriers I I I
Vacuum
Electrons are the charge carriers I V V V V
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
to sockets. are used for connections 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: Resistors image www.sffej.net/educational/resistor_Colour.htm
Resistor colour codes www.radiodaze.com/rescarbcomp.htm
Slide 3: George Ohm image~ www.past.dk/artefacts/photos/53/photo-1113908435-89551-5995.tkl?o
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: None Slide 6: Temperature and resistance animation ~ Science Joy Wagon ( www.sciencejoywagon.com
) www.regentsprep.org/Regents/physics/phys03/bresit/default.htm
Slide 7: Cross sectional area and resistance animation ~ Science Joy Wagon ( www.sciencejoywagon.com
) www.regentsprep.org/Regents/physics/phys03/bresit/default.htm
Slide 8: Resistors image
http://homepages.nildram.co.uk/~vwlowen/radio/alarm/how2.htm
Slide 9: Sir Charles Wheatstone image ~ from the BT Connected Earth Collection. See www.connected-earth.com
Slide 10: Sunset Power Lines www.tonyboon.co.uk/imgs/pages/powerlines.htm
Slide 11: Hoffman Voltameter image www.dalefield.com/earth/hydrogen1.html
Slide 12: Electroplating image ~ www.finishing.com/faqs/howworks.html
Slide 13: None Slide 14: None Slide 15: Circuit Breaker image ~ Edfenergy www.edfenergy.com/powerup/keystage3/in/page2.html
Slide 16: Circuit Breaker image ~ Edfenergy as above Light Circuit image ~ www.buzzybee.org/diy/projects/electrical/lighting/wiring.html
Slide 17: None Slide 18: None Slide 19: None Slide 20: None