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Electric current
• Electric current is a flow of charge
• In metallic conductors, the charge is
carried by electrons
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• Where do these electrons come from?
• Electrons are in the circuit
• Insulators do not have free electrons
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• Chemical reactions in the battery
supply the energy to move the
electrons
• All electrons in the circuit feel the effect
of the battery – action at a distance
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• Electrons are free in the conductor
• Thermal motion – 1000’s meters per
second
• Drift velocity caused by battery –
millimeters per second
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• Electrons collide with atoms and ions energy is converted into other forms
• The amount of energy converted
depends on the nature of the conductor
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• The drift of electrons is the electric
current
• Convention – electric current is from
plus to minus
• Ions carry charge in cells and solutions
of electrolytes
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Voltage
• Voltage = Potential difference
• Voltage is a measure of ENERGY
• Voltage of battery is the energy
available for each unit of charge.
6 V means that there are 6 J of energy
for each unit of charge
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• Voltage across a resistor is the energy
being converted by each unit of charge
in the resistor
• The greater the resistance at a point,
the greater the fraction of the total
energy that is converted at that point.
• Principle of Conservation of Energy
applies.
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6V
6 joules of energy are transferred to each
coulomb from the battery
20 Ω
4V
4 joules of energy are
converted for each coulomb
10 Ω
2V
2 joules of energy are
converted for each coulomb
If there was only one component, e.g. a bulb, in the
circuit, all 6 joules would be converted there.
6V
6 joules of energy are transferred to each
coulomb from the battery
6V
Each coulomb is giving up 6 joules of energy
• The more items that are in a series
circuit, the less energy will be
converted in each, as there are only
6 joules to be shared between them
• The energy is converted in direct
proportion to the size of the
resistance
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• In a parallel circuit, all the components
convert the same amount of energy
from each unit of charge that passes
through them i.e. the amount it receives
from the 6 V battery - 6 J
• The potential difference (voltage)
between the ends of each component
is the same - 6 V
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• In a parallel circuit, there will be
different currents flowing through
the components if they have
different resistances.
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6 joules of energy are converted for each unit of charge
I2
6V
6 joules of energy are converted for each unit of charge
I1
6V
6 joules of energy are transferred from the battery
to each unit of charge
I
6V
I = I1 + I 2
Remember
For different value resistors
• in series
– same current, different voltage
• in parallel
– same voltage, different current
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Resistance
Opposition to current in a metallic conductor
depends on
•
length
•
cross-sectional area
•
material
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R must not be in a working circuit
R
Ω
Ohmmeter
The Multimeter
DISPLAY
V
OFF
V
A
10A
10A DC
Ω
VΩmA
COM
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