Summary Grade 12 IEB Physical Sciences - Electric Circuits Notes (Physics Section E)

Current electricity


Voltage or potential difference
= work done per unit positive charge
(Work done - energy transfer)

𝑊
V= 𝑄

V - voltage (volts V)
W - energy (joules J)
Q - charge (coulomb C)

Voltage across a battery
= energy given to charges by a battery

Voltage across a resistor
= energy the resistor is transferring from the charges

EMF
= total energy supplied per coulomb of charge by the cell
(This voltage is taken across the battery when the battery is not delivering a current
i.e. the switch is open)

EMF = Vext + Vint



Vload or Terminal voltage
= The value that is measured across the battery when the current is flowing
i.e. the switch is closed

EMF = I(Rext + r)
EMF = IRext + Ir
EMF = Vext + Ir

, Electric current
= the rate of flow of charge
𝒒
I = ∆𝒕

I - current (ampere A)
t - time (seconds s)
q - charge (coulomb C)

Quantitation of charge
Q = n.qe

Q - charge (coulomb C)
n – integer number
qe – 1.6 x 10-19 C




Resistance
= a material’s opposition to the flow of charge

𝑽
R= 𝑰

R - resistance (ohm Ω)
V - voltage (volts V)
I - current (ampere A)

Resistance depends on
1. Type of material
2. Length of conductor
(length increases, resistance increases)

3. Diameter of conductor
(diameter increases, resistance decreases)

4. Temperature
(temperature increases, resistance increases)