Physics OCR Chapter 10 (Electrical Circuits)

• Current cannot be created or destroyed

Kirchoff's First Law

• Sum of current into a point = Sum of current out of that point (i.e. 10A = 5A + 5A)
• Based on the conservation of charge, no. of charge carriers into point must = number leaving that point

Kirchoff's Second Law

• Around a closed loop, sum of E.M.F = sum of P.D

Ohms Law

• For a metallic conductor kept at a constant temperature the current is directly proportional to p.d across its ends

Series Circuits

• A single loop, (1 pathway for current to travel)
• Current is the same in every position of circuit
• Rate of flow of charge remains the same, charge is not used up

• The E.M.F is shared across the components
  • If resistance is equal share of e.m.f is equal
  • If resistance of one is higher, has larger share of e.m.f

• Circuits with more than one source of e.m.f, add total e.m.f value, (i.e. 3V + 6V = 9V)

• If cell opposes another cell e.m.f's subtract, (i.e. 6V - 3V = 3V)

Parallel Circuits

• Provides multiple pathways, branches don't affect one another
• Amount of charge flow down a pathway depends on its resistance

• Current into junction = current out of the same junction
• Greater the resistance of branch the lower the current

• The p.d of every branch = must = the e.m.f of the power supply
  • Due to Kirchoff's second law as each branch forms its own loop

Internal Resistance 

• All power sources have internal resistance, though some have higher than others

• Work has to be done for charge carriers to move through power source
• Resulting in some energy transferred 'lost'
• Not all energy transferred to charge is available for the circuit
• The p.d. at terminals (terminal p.d.) is less than e.m.f of the power source

• e.m.f = terminal p.d. + lost volts
• e.m.f = V + lost volts --> e.m.f = V + Ir --> e.m.f = I (R + r)

• Increased current = more lost volts = lower terminal p.d.

Potential Dividers

• Two resistors connected in series, one resistor is Vout 
• Vout can be varied from a range of 0 to terminal p.d.
  • Kirchoff's second law total p.d of across resistors = terminal p.d. of power source

• Ratio between the two resistors --> if one has double the other receive 2/3 of total p.d

• V1/V2 = R1/R2