3.6 DC Circuits

Question 2

What does Ohm’s law describe?

Answer 2

The relationship between current and voltage for most conductors

Question 3

Who established Ohm’s law and when?

Answer 3

Georg Simon Ohm in 1827

Question 4

What is the formula for Ohm’s law?

Answer 4

V = I * R

Question 5

What do the symbols V, I, and R represent in Ohm’s law?

Answer 5

• V: voltage in volts (V)
• I: current in amperes (A)
• R: resistance in ohms (Ω)

Question 6

What is resistance?

Answer 6

The quotient of voltage to current, measured in ohms (Ω)

Question 7

What is the characteristic of materials obeying Ohm’s law regarding resistance?

Answer 7

Resistance does not change over enormous ranges of voltage and current at constant temperature

Question 8

What is Kirchhoff’s 1st Law?

Answer 8

At any junction in an electrical circuit, the sum of currents flowing towards that point equals the sum of currents flowing away from it

Question 9

What can Kirchhoff’s 1st Law be used to calculate?

Answer 9

Unknown currents at a branching point in an electrical circuit

Question 10

What is Kirchhoff’s 2nd Law?

Answer 10

The algebraic sum of all voltages around any closed loop is zero

Question 11

What does Kirchhoff’s Voltage Law imply about voltage in a circuit?

Answer 11

Voltage is dropped through components with none left over in a closed loop

Question 12

True or False: Voltage exists at a single point in a circuit.

Answer 12

False

Question 13

What year were Kirchhoff’s circuit laws first described?

Answer 13

1845

Question 14

Who first described Kirchhoff’s circuit laws?

Answer 14

Gustav Kirchhoff

Question 15

What is the significance of Kirchhoff’s laws?

Answer 15

They are essential tools for the analysis of electric circuits

Question 16

What is Ohm’s Law used for?

Answer 16

To calculate an unknown value for either voltage, current, or resistance

Ohm’s Law is expressed as V = IR, where V is voltage, I is current, and R is resistance.

Question 17

What does Kirchhoff’s 1st Law state?

Answer 17

The total current flowing towards a node is equal to the total current flowing away from the node

This is also known as Kirchhoff’s Current Law.

Question 18

What is the principle behind Kirchhoff’s 2nd Law?

Answer 18

The total EMF must equal the total PD in a circuit

EMF stands for electromotive force and PD stands for potential difference.

Question 19

How can unknown PD be calculated using Kirchhoff’s 2nd Law?

Answer 19

By subtracting the known PDs from the total EMF

This allows for the determination of unknown voltages in a circuit.

Question 20

In a parallel circuit, what is true about the PD across resistors?

Answer 20

The same PD is across all resistors connected in parallel

This is important for accurate calculations of current through each resistor.

Question 21

What is the voltage across the 6 Ω resistor in the example?

Answer 21

6 V

This voltage is the same as the voltage across the 12 Ω resistor due to their parallel connection.

Question 22

How do you calculate the current through a resistor using Ohm’s Law?

Answer 22

I = V/R

I is current, V is voltage, and R is resistance.

Question 23

What is the voltage across the 2 Ω resistor in the example?

Answer 23

3 V

This was calculated using Ohm’s Law after finding the current through the resistor.

Question 24

What is the total EMF calculated in the example?

Answer 24

9 V

This is derived from the sum of the PDs across the resistors connected in the circuit.

Question 25

Fill in the blank: According to Kirchhoff's 1st Law, the total current at a node is equal to the total current _______.

Answer 25

flowing away from the node

Question 26

True or False: Kirchhoff's 2nd Law can be used to find unknown voltages in a circuit.

Answer 26

True

Question 27

What does Internal Resistance (IR) refer to in a power supply?

Answer 27

A small resistor in series with the pure EMF of a power source ## Footnote This resistance influences the terminal voltage depending on the current supplied.

Question 28

What is the terminal voltage of a power supply when it is off-load?

Answer 28

The terminal voltage is equal to the EMF ## Footnote This occurs when no current flows through the internal resistance.

Question 29

What happens to the terminal voltage when the power supply is on-load?

Answer 29

The terminal voltage decreases compared to the EMF ## Footnote This is due to the potential difference (PD) across the internal resistance.

Question 30

How is terminal voltage calculated when a power supply is on-load?

Answer 30

Terminal voltage = EMF - PD across internal resistance ## Footnote The PD increases with higher current.

Question 31

What is the effect of a short-circuit on a power supply's current?

Answer 31

A high current flows, limited only by the IR of the battery ## Footnote This can lead to dangerously high short-circuit currents.

Question 32

What can excessive short-circuit current do to a power source?

Answer 32

It can quickly destroy the power source ## Footnote Many power supplies have low IR, leading to high short-circuit currents.

Question 33

What factors contribute to achieving a stable terminal voltage in a power supply?

Answer 33

Choice and combination of power sources, voltage regulators, and load resistors ## Footnote These components help maintain constant voltage and current.

Question 34

How is a battery's Internal Resistance (IR) calculated?

Answer 34

IR = (Open-Circuit Voltage - Closed-Circuit Voltage) / Load Current ## Footnote IR increases as the battery discharges.

Question 35

What is the typical IR of an AA battery cell?

Answer 35

About 0.1 Ω ## Footnote This value affects the voltage between the terminals depending on the current.

Question 36

What is the danger associated with larger power sources compared to smaller ones?

Answer 36

Larger power sources can be potentially fatal ## Footnote For example, a 20 V and 200 A source is much more dangerous than a small dry cell.

Question 37

What should users be cautious about regarding the ratings on batteries?

Answer 37

The terminal voltage and maximum current ratings ## Footnote These ratings help prevent mishandling and dangerous situations.

Question 38

True or False: The terminal voltage of a power supply is always the same as its EMF.

Answer 38

False ## Footnote The terminal voltage can be lower than the EMF when on-load.

Question 39

Fill in the blank: The actual voltage across the terminals of a power supply is called the _______.

Answer 39

terminal voltage ## Footnote It varies based on the current supplied to an external load.