Skip to content

Salam Academy

Sign in Create account

LearnEdexcelYPH112018 specificationWPH12 · Unit 2: Waves and ElectricityElectric circuits

WPH12 · Module

Electric circuits

6 subtopics≈ 205 min in total

Start the module: Electric current Open graphing calculator

Electric current
Current is the rate of flow of charge, $I=\dfrac{\Delta Q}{\Delta t}$ . Charge comes in whole multiples of the elementary charge $e=1.60\times10^{-19}$ C. In a conductor, current depends on the number density of charge carriers, their charge and their drift velocity: $I=Anvq$ . Electrons drift slowly, but the signal travels almost instantly.
≈ 30 min
E.m.f. and internal resistance
A real source has an e.m.f. $E$ and an internal resistance $r$ . When it drives a current $I$ through an external resistance $R$ , some energy is dissipated inside the source: $E=I(R+r)$ , so the terminal p.d. is $V=E-Ir$ . The "lost volts" $Ir$ grow with current. A graph of $V$ against $I$ is a straight line with intercept $E$ and gradient $-r$ .
≈ 35 min
Potential difference and power
The potential difference across a component is the energy transferred from electrical to other forms per unit charge: $V=\dfrac WQ$ , so $1$ V $=1$ J C⁻¹. Electrical power is $P=VI$ ; combining with $V=IR$ gives $P=I^2R=\dfrac{V^2}R$ . Choose whichever form uses the quantities you know.
≈ 30 min
Kirchhoff's laws
Kirchhoff's first law: the sum of currents into a junction equals the sum out (conservation of charge). Kirchhoff's second law: round any closed loop, the sum of e.m.f.s equals the sum of p.d.s (conservation of energy). From these: series $R=R_1+R_2+\dots$ ; parallel $\dfrac1R=\dfrac1{R_1}+\dfrac1{R_2}+\dots$ .
≈ 40 min
Resistance and resistivity
Resistance is $R=\dfrac VI$ . A metal at constant temperature obeys Ohm's law: current is proportional to p.d. A wire's resistance depends on its material, length and area: $R=\dfrac{\rho L}A$ , where the resistivity $\rho$ is a material property. Filament lamps, diodes, thermistors and LDRs have non-ohmic, changing resistances.
≈ 35 min
Potential dividers
Two resistors in series share the supply p.d. in proportion to their resistances: $V_{\text{out}}=\dfrac{R_2}{R_1+R_2}V_{\text{in}}$ . Replacing one resistor with a thermistor or LDR makes the output depend on temperature or light. A potentiometer is a uniform resistance wire whose p.d. per unit length lets you compare e.m.f.s by finding the balance length, where a galvanometer reads zero.
≈ 35 min