4.3 Electrical Circuits

Question 1

What is Kirchhoff’s first law?

Answer 1

Current entering a junction is equal to the current output of that junction, charge is conserved.
ΣI in = ΣI out

Question 2

What is Kirchhoff’s second law?

Answer 2

In any circuit, the sum of electromotive force is equal to the sum of the p.ds around a closed loop, energy is conserved.

Question 3

Why is Kirchhoff’s second law the conservation of energy in circuits?

Answer 3

It states that total energy transferred to the charges in a circuit is always equal to the total energy transferred from a circuit, so energy is neither created nor destroyed.

Question 4

How is e.m.f of a circuit split between components in series?

Answer 4

E.m.f is shared between components in series, and it is always equal to the p.d of the components.

Question 5

How is are e.m.f and p.d related in a parallel circuit?

Answer 5

The p.d across each branch is equal to the total e.m.f from the power supply.

Question 6

How is the resistance of a number of resistors connected in series determined?

Answer 6

The total resistance of resistors in series is equal to the sum of individual resistances.

Question 7

How is the resistance of a number of resistors connected in parallel determined?

Answer 7

Resistance drops due to an alternate path for current being offered and is calculated by:
1/R = 1/R₁ + 1/R₂ …

Question 8

What are 4 key equations for analysing electrical circuits?

Answer 8

I=Q/t
V= W/Q
P = VI
V = IR

Question 9

What is internal resistance?

Answer 9

The resistance of a source of e.m.f due to its construction, which causes a loss in energy/voltage as the charge passes through the source.

Question 10

What is the reason for internal resistance in a battery?

Answer 10

They differ because of construction and are designed this way depending on the intended purpose.

Question 11

What is meant by ‘lost volts’?

Answer 11

The potential difference across the internal resistor of a source of e.m.f.

Question 12

What is meant by terminal p.d?

Answer 12

The p.d across an electrical power source when there is no current, this is equal to the e.m.f of a source, but if there is a current, it is equal to (e.m.f - lost volts)

Question 13

What is the relationship between e.m.f, terminal p.d and lost volts?

Answer 13

E.m.f = terminal p.d - lost volts.

Question 14

Why does the amount of lost volts increase when current is high?

Answer 14

Increasing current means more charge carriers travel through the cell each second, so more work is done by the charges and increases lost volts, reducing terminal p.d.

Question 15

What equation relates e.m.f, current, and internal resistance?

Answer 15

E.m.f = Current (Resistance + Internal Resistance)

Question 16

What is the experiment used to determine internal resistance and e.m.f of a cell?

Answer 16

• Set up an ammeter in a circuit, with a voltmeter in parallel with the cell, and a variable resistor.
• Find many values for I and V, and plot a graph V against I.
• Find the gradient, this is equal to internal resistance.
• Use ε = I (R+r) to find e.m.f.

Question 17

What is the function of a potential divider circuit?

Answer 17

You use potential dividers to divide the p.d to give any value up to the maximum supplied from the power source.

Question 18

In a potential divider circuit, how is the ratio of the resistance related to the ratio of p.d.

Answer 18

V₁/V ₂ = R₁/R₂

Question 19

State the potential divider equation.

Answer 19

V(out) = [R₂/(R₁ + R₂)] x V(in)

Question 20

What does it mean if a circuit is loaded?

Answer 20

A component is connected to P(out) which connects a component in parallel with R₂.

Question 21

Explain why the output p.d of a potential divider circuit drops when it is loaded.

Answer 21

This lowers the resistance of this part of the circuit which lowers the fraction of p.d and therefore lowers p.d/v out.

Question 22

Explain the function of a potentiometer.

Answer 22

A potentiometer is a variable resistor with 3 terminals and a sliding contact, which varies p.d between terminals and gives a variable V out.