Resistance (R)
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Current Electricity 2
Copyright © Maire Duffy, Clonkeen College
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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
R1
R2
In Series the total resistance is
R = R1 + R2 + R3
R3
In Parallel the total resistance is
1=1+1+1
R1
R2
R3
R
R1 R2 R3
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 crosssectional area.
i.e.
R1
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
= constant of proportionality
= Rd 2
4L
(For a wire with circular cross-sectional area)
Unit: ohm meter m
Wheatstone Bridge
Uses
Temperature control
Fail-Safe Device (switch circuit off)
Measure an unknown resistance
R1 = R3
R2 R4
B
A
C
(When it’s balanced)
Metre Bridge
R1 = R2 (|AB|)
|BC|
www.dwiarda.com/scientific/Bridge.html
I
D
Effects of an Electric Current
1.
2.
3.
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 2R
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 H2SO4
Active electrodes are electrodes
that take part in the chemical reaction
e.g. copper in CuSO4
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
Filament bulb
Negative electrons are the charge
carriers
Semiconductor
Negative electrons and positive
holes are the charge carriers
I
V
I
V
I
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
Vacuum
Electrons are the charge carriers
I
V
I
V
I
V
I
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 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: 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)
Slide 8: Resistors image
www.regentsprep.org/Regents/physics/phys03/bresit/default.htm
http://homepages.nildram.co.uk/~vwlowen/radio/alarm/how2.htm
See www.connected-earth.com
Slide 9: Sir Charles Wheatstone image ~ from the BT Connected Earth Collection.
Slide 10: Sunset Power Lines
Slide 11: Hoffman Voltameter image
www.dalefield.com/earth/hydrogen1.html
Slide 12:
Slide 13:
Slide 14:
Slide 15:
www.tonyboon.co.uk/imgs/pages/powerlines.htm
Electroplating image ~ www.finishing.com/faqs/howworks.html
None
None
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