Module 4.3 - Electricity: Electrical Circuits Flashcards
What is Kirchhoff’s first law?
The sum of currents ENTERING a junction in a circuit is EQUAL to the sum of currents LEAVING the junction - a form of conservation of charge.
What is Kirchhoff’s second law?
The sum of e.m.f. is EQUAL to the sum of the P.D. in a closed loop - a form of conservation of energy.
∑e.m.f = ∑IR
Describe Series circuits
> Current SAME at all points
e.m.f. is equal to the sum of p.d. across the resistors
Total resistance is equal to the sum of individual resistors
How do you work out the total p.d. in a series circuit?
Vt = V1 + V2 + V3 + …
How do you work out the total resistance in a series circuit?
Rt = R1 + R2 + R3 + …
Describe Parallel circuits
> Total current is the sum of the individuals branch currents
The potential difference across the resistors in each branch is the same and equal to the e.m.f.
How do you work out the total current in parallel circuits?
It = I1 + I2 + I3 + …
How do you work out the e.m.f. of parallel circuits?
e.m.f = V1 = V2 = V3 = …
How do you work out the total resistance of parallel circuit?
1/Rt = 1/R1 + 1/R2 + 1/R3 + …
How do you find the output voltage, Vout, of the resistor, R2?
Vout = Vin x ( R1 / R1 + R2)
When is the output voltage of R2 the largest?
When Vin and R2 are large, and also when R1 is smaller than R2
Define Internal Resistance, r
The resistance of a SOURCE of e.m.f
Why is some energy lost when charge carriers flow through a source of e.m.f?
Internal resistance causes the charge carriers to do work against them, therefore losing energy.
What is terminal p.d.?
The actual p.d. across the terminals of an e.m.f source when it is connected to a circuit
What happens to e.m.f. and terminal p.d. when no current is flowing across a cell?
They are EQUAL
What happens to the terminal p.d. and e.m.f. when current is flowing across a cell?
Terminal p.d. will be lower than the e.m.f.
How is the terminal p.d. lower than the e.m.f. when current is flowing? Why?
This is because the energy transferred to charge carriers (Eq) and energy transferred to the rest of the circuit (Ec) is different.
Eq > Ec
e.m.f = Eq ÷ Q
T p.d. = Ec ÷ Q
Therefore e.m.f > T p.d.
This is because energy is lost by charge carriers doing work against the internal resistance in a source of e.m.f.
What is Lost Volts?
The difference between e.m.f. and terminal p.d.
e.m.f. - T p.d.
How do you work out internal resistance?
Lost Volts ÷ Current
How do you work out terminal p.d.?
= e.m.f. - [LOST VOLTS]
= e.m.f - [current x internal resistance]
3 equation to work out e.m.f using internal resistance?
= T p.d. + [current x internal resistance]
= IR + [current x internal resistance]
= I (R + internal resistance)
How do you deal with more than one e.m.f.?
Add them together, unless polarities are OPPOSITE then subtract
