Grade 12 IEB Physical Science (Physics) Electric circuits (Section E) Summary & notes

Electric Circuits:
Voltage: Work done per unit positive charge OR Energy transferred per unit positive charge
W • V : Voltage (Volts, V)
V = • W : Energy (Joule, J)
Q
• Q : Charge (Coulomb, C)

• What happens to electron in resistor: Electron gives o heat energy & loses kinetic energy

• Energy conversion in resistor: Ekinetic ! Ethermal
• Voltage across BATTERY = energy given TO charges by battery

• Voltage across RESISTOR = energy taken FROM charges by resistor



EMF:Total energy supplied per coulomb of charge by cell.

• Taken with a voltmeter across battery when battery is NOT delivering current to circuit.


• Vext: Vload measured across battery when current is owing
• emf = Vext + Vint • Vint: Inside battery



r = internal resistance of battery
Electric current: Rate of ow of charge

q • I : Current (Ampere, A)
I= • q : Charge (Coulomb, C)
¢t • t : Time (Seconds, s)

Resistance: A material's opposition to the ow of electric current

• R (Ohms, « )



• Resistance depends on:

- Type of material

- Length of conductor (Increased length = decreased resistance)

- Diameter of conductor (Increased diameter = decreased resistance)

- Temperature




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, Ohm's Law: Current through conductor is directly proportional to potential di erence across the

conductor at a constant temperature
V
R=
I

Rheostat: Variable resistor


or




Ohmic conductors: Have constant resistance no matter what the voltage across or current through it

is.

Non-ohmic conductors: Have varying resistance that depends on current through it



Series circuit:

• Resistance in series: Rseries = R1 + R2:::
• Why Vext decreases when resistor in series is removed:



Rext decreases (De nition of R)

I increases (De nition of R)



Vint increases (Vint = I.r) (r is constant)

emf = Vext + Vint

Vext = emf - Vint (emf constant)

,Therefore Vext decreases



Parallel circuit:
1 1 1
• Resistance in parallel: = + :::
Rp R1 R2




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