explains and gives examples and multiple choice questions and gives questions with answers on electricity and electric circuits ,electrolytic cells, electrodynamics, fertilizers, chemical industry , photo electric effect, and more
MOVING BEYOND THE LIMITS, SETTING THE STANDARD AND
LEAVING NO CHILD BEHIND
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MOVING BEYOND THE LIMITS, SETTING THE STANDARD AND
LEAVING NO CHILD BEHIND
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FORMULAE
V emf ( ε ) = I(R + r)
R
I
R s R1 R 2 ...
1 1 1 q It
...
R p R1 R 2
W = Vq W
P
t
W = VI t
P = VI
W = I2R t
P I2R
V 2 Δt
W= V2
R P
R
TERMINOLOGY
1.1 The relationship between current, potential difference (voltage) and
resistance at constant temperature
1.1.1 Resistance
Resistance is a measure of the opposing force which is applied to flow of charge in a circuit.
Resistance in a wire is the opposition of a wire to the flow of charge. It is caused by collisions
between the electrons and the atoms in the wire. Motors, light globes, and heating coils
are all examples. The symbol used for resistance is R and it is measured in ohm ().
The resistance of the conductor depends on
The type of material used
The length of the conductor – the longer the conductor, the greater the resistance
The thickness of the conductor – the thicker the conductor, the smaller the resistance
The temperature of the conductor – the higher the temperature, the greater the
resistance
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1.1.2 Ohm’s Law
The current (I) through a conductor is directly
proportional to the potential difference (V) across it,
provided the temperature remains constant.
V
The value is the resistance R of the conductor
I
and is a constant.
potential difference V
Resistance = In symbols: R =
current I
1.2 Ohmic and non-ohmic conductors
Materials that obey Ohm's Law are often called ohmic conductors or linear conductors.
A graph of current versus potential difference (voltage) of an ohmic conductor is a straight
1
line through the origin. The gradient ( ) is constant – thus the resistance is constant.
R
Metals and alloys obey Ohm's Law. Carbon is a non-metal that also obeys Ohm's Law. For
most other materials the resistance is not a constant and changes with the
applied potential difference (voltage). An example of a non-ohmic conductor
is a tungsten light bulb.
We can easily measure potential difference (voltage) and current and then
use the data to plot current versus potential difference (voltage) graphs.
We use a circuit represented by the accompanying circuit diagram.
The current versus potential difference graph for an Ohmic conductor is a straight line
through the origin. The straight line shows a constant ratio between current and potential
difference - Ohm’s Law is obeyed.
For a filament lamp the resistance does not remain constant, but increases as the filament
gets hotter, which is shown by the gradient getting steeper. A filament lamp does not
obey Ohm’s law.
Current (A)
A diode only allows current to flow in one direction and only then it will only let current pass
until a certain minimum voltage has been reached. However small increases in voltage after
the minimum voltage result in large increases in current.
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