Electrical Circuits

Question 1

State Kirchhoff’s second law

Answer 1

In any closed loop in any circuit of steady currents, the sum of the e.m.f’s acting is equal to the sum of the p.d’s

Question 2

How would you calculate the total resistance of two or more resistors in series?

Answer 2

Rᴛ = R1 + R2 + R3 etc

Question 3

How would you calculate the total resistance of two or more resistors in parallel?

Answer 3

1/Rᴛ = 1/R1 + 1/R2 + 1/R3 etc

Question 4

How would you calculate the total resistance if the circuit involves resistors in both series and parallel?

Answer 4

First combine the resistors in series, then calculate the total resistance for parallel

Question 5

Explain the term ‘lost volts’

Answer 5

The difference between the theoretical e.m.f. of the cell and the terminal p.d. is known as the ‘lost volts’

Question 6

Derive the equation ε = I(R + r)

Answer 6

E.m.f. = terminal p.d. + ‘lost volts’
ε = V + v
Since V = IR and v = Ir
ε = IR + Ir
ε = I(R + r)

Question 7

Define internal resistance

Answer 7

The ratio of the lost volts to the current flowing through the source is called internal resistance
r = (ε - V)/I
= v/I, where v is the ‘lost volts’

Question 8

State the two equations for e.m.f.

Answer 8

ε = I(R + r)

ε = V + Ir

Question 9

State two potential divider equations

Answer 9

Vout = R2/(R1 + R2) x Vin

V1/V2 = R1/R2

Question 10

How do potential divider circuits usually appear?

Answer 10

Potential divider circuits are often drawn using two rails, rather than showing the power supply. They are often composed of either two resistors or a rheostat.

Question 11

How can LDRs and thermistors be used in potential divider circuits?

Answer 11

• A light dependant resistor can be used in a potential divider to provide an output p.d. which varies with light intensity
• a thermistor can be used in a potential divider to provide an output p.d. which varies with temperature