Chapter 13 - DC Circuits

Question 1

What are the current rules?

Answer 1

At any junction in a circuit, the total current leaving the junction is equal to the total current entering the junction.
Current total = Current 1 + Current 2
where component 1 and 2 are in parallel.
The current entering a component is the same as the current leaving it.
For components in SERIES, the current in them is the same.

Question 2

What are the potential difference rules?

Answer 2

For components in SERIES, the total pd across all the components is equal to the sum of pds across each component.
Vtotal = V1 + V2 + V3
The pd across components in PARALLEL is the same.
For any complete loop of a circuit, Σℰ = ΣV

Question 3

What is the total resistance of resistors in series?

Answer 3

Resistors in series pass the same current.

Rtotal = R1 + R2 + R3 +…

Question 4

What is the total resistance of resistors in parallel?

Answer 4

Resistors in parallel have the same pd. The current through a parallel combination of resistors is equal to the sum of the individual currents.

1/Rtotal = 1/R1 + 1/R2 + 1/R3

Question 6

What is the electromotive force of a source?

Answer 6

The electrical energy per unit charge produced by the source.

Question 6

What is the internal resistance of a source?

Answer 6

The loss of potential difference per unit current in the source when current passes through the source.

Question 6

How does the terminal pd compare to the emf?

Answer 6

The terminal pd is less than the emf whenever current passes through the source.

The difference is due to the internal resistance of the source.

Question 7

What assumptions should be made about a silicon diode in circuits?

Answer 7

Forward pd of 0.6V whenever a current passes through it.
- Infinite resistance in reverse direction or at pds less than 0.6V in the forward direction.

Question 8

What is a potential divider used for?

Answer 8

A potential divider is used to supply a constant or variable potential difference from a power supply.

Question 9

What is the advantage of using a potential divider instead of a variable resistor?

Answer 9

The potential divider can vary the pd across a component from zero, therefore it can stop any current from flowing.

The variable resistor can only reduce the current to a minimum (non-zero) value.

Question 10

What is the electromotive force (emf), ε, of a source of electricity?

Answer 10

The amount of energy it transfers from other forms of energy to electrical energy per unit charge
(i.e. the energy transferred per unit charge).

When charge carriers travel through a battery, the battery does work on them and so they gain energy.

Chemical energy in battery → Electrical energy

Question 11

What is Kirchhoff’s Second Law?

Answer 11

Σℰ = ΣV
Energy is conserved in a circuit.
The total energy transferred to the charges in a circuit is equal to the total energy transferred from the charges to the components.

Question 12

What is the relationship between pd and resistance in a potential divider circuit?

Answer 12

V1/V2 = R1/R2

Question 13

What is a potential divider?

Answer 13

Two or more resistors in series connected to a source of pd.

Question 14

How can a potential divider be used to supply a variable pd?

Answer 14

Replace one of the resistor with a variable resistor
- Uniform resistance wire + sliding contact

Question 15

What is terminal pd?

Answer 15

The actual pd across the terminals of an emf source when it is connected in a circuit.

Terminal pd < ℰ
due to the loss of energy from internal resistance of source.

Question 16

What is the equation for terminal pd?

Answer 16

Terminal pd, V = ℰ - Ir (lost volts)

Question 17

What is the equation for internal resistance involving lost volts?

Answer 17

r = lost volts ÷ I

Question 18

Maximum power delivered to a load occurs when…

Answer 18

…R = r.

Question 19

What is load resistance?

Answer 19

The total resistance of all the components in the external circuit (i.e. not including internal resistance of battery).