NCEA L2 D.C. ELECTRICITY
WHAT IS ELECTRICAL CURRENT?
ANS: Electric current is the flow (movement) of electric charge.
Symbol : I
Units: Amperes (A)
I = q/t
Where I = current in amps (A)
q = charge in Coulombs (C)
t = time (s)
DO
EXERCISES
PAGE 156
RUTTER
Current is measured with an ammeter placed in series in the
circuit
Conventional current is where electrons travel from +ve to –
ve. Natural current is where it travels from a build up of –ve
charge to an area deficient in –ve charge namely and area of
+ve charge
WHAT IS ELECTRICAL ENERGY
ANS: Voltage (sometimes also called electric or electrical
tension) is the difference of electric potential between
two points of an electrical or electronic circuit,
expressed in volts
Symbol : V
Units: Volts (V)
V = I/q
Where V = voltage in volts (V)
I = current in amps (A)
q = charge in Coulombs (C)
Often thought of as the change in energy as the charge moves
between two points.
Thus:
E = Vq
Where E = change in energy (J)
V = voltage in volts (V)
q = charge in Coulombs (C)
F = Eq
Where E = electric field strength
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EXERCISES
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RUTTER
F = force on the charge (N)
q = charge in Coulombs (C)
The voltage between two points, distance apart ‘d’, in an
electric field ‘E’
V = Ed
WHAT IS ELECTRICAL POWER?
ANS: is defined as the rate at which electrical
energy is transferred by an electric
circuit.
Symbol : P
Units: Watts (W)
P = IV
Where P = power in watts (W)
I = current in amps (A)
V = voltage in volts (V)
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EXERCISES
PAGE 158 - 9
RUTTER
WHAT IS RESISTANCE?
ANS: is defined as a measure of the degree to which an object
opposes an electric current through it
Symbol : R
Units: Ohms (Ω)
V = IR
Where R = resistance in ohms (Ω)
I = current in amps (A)
V = voltage in volts (V)
Resistance occurs in all conductors. A good conductor has low
resistance and vice versa.
The current in a conductor depends on the resistance and the
voltage applied across it, Ohm’s Law).
Resistance often very large kΩ or MΩ
RESISTANCE IN SERIES & PARALLEL
V
V
I
A
A
R1
V1
I
I
R2
V2
SERIES
I1
A1
I2
R2
A2
R3
A3
I3
PARALLEL
• The current, I, measured by each
ammeter has the same value in
each part of the circuit.
• The currents in the parallel
components add up to the current
from the supply,
ie I1 = I2 + I3.
• The voltages across the series
resistors add up to the supply
voltage Vs = V1 + V2.
• The voltage is the same across all
branches in parallel
• The equivalent resistance, Rs, of
the whole circuit is given by: Rs = R1
+ R2.
• The equivalent resistance, Rs, of
the whole circuit is given by: 1/Rp =
1/R2 + 1/R3.
READ INFORMATION PAGE 162 - 163
COMPLETE RELEVANT
EXERCISES FROM RUTTER