Grade 12 summary presentation for physics: electric circuits

1. 1
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10. Electric circuits
Physics
Grade 12

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Revision
Ohm’s Law governs the relationship between current and potential difference for a circuit
element at constant temperature. Mathematically we write:
Conductors that obey Ohm’s Law are called ohmic conductors; those that do not are called non-
ohmic conductors.
Ohm’s Law can be applied to a single circuit element or the circuit as a whole (if the components
are ohmic).
I=
V
R

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Series and parallel circuits
The equivalent resistance of resistors in series (Rs
) can be calculated as follows:
The potential difference and current for series
circuits is given by:
The equivalent resistance of resistors in parallel (Rp
) can be calculated as follows:
The potential difference and current for parallel
circuits is given by:
Rs=R1+R2+R3+ … +Rn
VT=V 1+V2+V 3+ … +V n
IT=I1=I2=I3= … =In
1
Rp
=
1
R1
+
1
R2
+
1
R3
+ … +
1
Rn
IT=I1+I2+I3+ … +In
VT=V 1=V2=V3= … =V n

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Internal resistance
Real batteries have an internal resistance.
The potential difference V of the battery is related to its emf Ɛ and internal resistance r by:
The external resistance (R) in the circuit is referred to as the load.
ℇ=V +Ir

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