Twenty flashcards covering DC circuit analysis and introductory electronics. Includes Ohm's law, resistivity, series and parallel resistor/capacitor combinations, Kirchhoff's current and voltage laws, electrical power, EMF and internal resistance, capacitor ch...
Twenty flashcards covering DC circuit analysis and introductory electronics. Includes Ohm's law, resistivity, series and parallel resistor/capacitor combinations, Kirchhoff's current and voltage laws, electrical power, EMF and internal resistance, capacitor charge/discharge, and the basics of diodes and transistors. Formula-focused revision for GCSE, A-Level and AP Physics 1/C.
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V=IR — voltage across a conductor equals current multiplied by resistance, for a conductor at constant temperature.
𝑅
=
𝜌
𝐿
𝐴
R=
A
ρL
, where
𝜌
ρ is resistivity,
𝐿
L is conductor length and
𝐴
A is cross-sectional area.
Series:
𝑅
𝑇
=
𝑅
1
+
𝑅
2
+
…
R
T
=R
1
+R
2
+…. Parallel:
1
𝑅
𝑇
=
1
𝑅
1
+
1
𝑅
2
+
…
R
T
1
=
R
1
1
+
R
2
1
+…
The total current entering a junction equals the total current leaving it — a consequence of conservation of charge.
The sum of EMFs around any closed loop equals the sum of potential drops around that loop — a consequence of conservation of energy.
𝑃
=
𝐼
𝑉
P=IV,
𝑃
=
𝐼
2
𝑅
P=I
2
R,
𝑃
=
𝑉
2
𝑅
P=
R
V
2
— all equivalent forms derived by substituting Ohm's law.
The energy converted from another form (chemical, mechanical) into electrical energy per unit charge driven around a complete circuit, measured in volts.
The resistance within a cell itself. Terminal voltage
𝑉
=
𝜀
−
𝐼
𝑟
V=ε−Ir, so terminal voltage drops below EMF
𝜀
ε as current
𝐼
I increases.
Series:
1
𝐶
𝑇
=
1
𝐶
1
+
1
𝐶
2
C
T
1
=
C
1
1
+
C
2
1
. Parallel:
𝐶
𝑇
=
𝐶
1
+
𝐶
2
+
…
C
T
=C
1
+C
2
+… — opposite pattern to resistors.
Q=CV — charge equals capacitance multiplied by potential difference across it.
Charge and voltage rise following
𝑉
=
𝑉
0
(
1
−
𝑒
−
𝑡
/
𝑅
𝐶
)
V=V
0
(1−e
−t/RC
), approaching the supply voltage asymptotically; the product
𝑅
𝐶
RC is the time constant.
τ=RC — the time for charge/voltage to fall to
1
/
𝑒
1/e (about 37%) of its initial value during discharge, or to rise to 63% during charging.
Conductors have very low resistivity (metals); insulators very high (rubber, glass); semiconductors intermediate, with resistivity that decreases as temperature rises (e.g. silicon).
It allows current to flow easily in one direction (forward bias) while blocking it in the reverse direction, used for rectification and protection.
Approximately 0.7 V. Germanium diodes have a lower drop of around 0.3 V, and LEDs typically drop 1.8–3.3 V depending on colour.
A diode that emits light when forward-biased, as electrons and holes recombine across the junction and release energy as photons.
It acts as an electronically controlled switch — a small base/gate current or voltage controls a much larger current flow between the other two terminals, enabling amplification and logic gates.
An ammeter should have near-zero resistance (connected in series, so it doesn't reduce circuit current). A voltmeter should have near-infinite resistance (connected in parallel, so it draws negligible current).
For two resistors in series across a supply,
𝑉
𝑜
𝑢
𝑡
=
𝑉
𝑖
𝑛
×
𝑅
2
𝑅
1
+
𝑅
2
V
out
=V
in
×
R
1
+R
2
R
2
— used to obtain a fraction of a supply voltage.
