Chapter 10: Current Electricity and DC Circuits

Question 1

What is electric current (I)?

Answer 1

The rate of flow of charges (I = Q / T)

Question 2

What instrument is used to measure current?

Answer 2

Ammeter

Question 3

How should the Ammeter be connected to a circuit, and why?

Answer 3

It should be connected in series. As the ammeter has an extremely low resistance, if it is connected in parallel, then all the current in the circuit will flow through the lower resistance path, and no current will flow through the component, creating a short circuit

Question 4

How does the conventional electric current flow?

Answer 4

From the positive terminal of the power source to the negative terminal

Question 5

What is electromotive force in terms of work done?

Answer 5

The amount of work done by a power source to drive a unit charge around an electrical circuit. (AKA battery voltage)

Question 6

What equation can be used to express electromotive force in terms of work done?

Answer 6

e.m.f. = Work Done (W) / Charge (Q)

Question 7

What is electromotive force in terms of energy?

Answer 7

The amount of non-electrical energy (chemical potential energy) converted into electrical energy when a unit charge passes through the power source

Question 8

What equation can be used to express electromotive force in terms of energy?

Answer 8

e.m.f. = Energy (E) / Charge (Q)
SI Unit: Volt (V)

Question 9

What is Potential Difference?

Answer 9

The amount of electrical energy converted into non-electrical energy when a unit charge passes between two points of an electrical component
OR
The difference in voltage between two points of an electrical component

Question 10

What equation can be used to express potential difference in terms of energy?

Answer 10

Potential Difference (V) = Energy (E) / Charge (Q)
SI Unit: Volt (V)

Question 11

What instrument is used to measure potential difference / voltage?

Answer 11

Voltmeter

Question 12

How should the Voltmeter be connected to a circuit, and why?

Answer 12

It should be conected in parallel. As the voltmeter has an extremely high resistance, if it is connected in series, the resistance of the circuit would be increased by an extremely large amount. Therefore, to measure the potential difference across two points in the circuit, the Voltmeter has to be connected in parallel

Question 13

What is Resistance?

Answer 13

The ratio of voltage across a conductor to the current flowing through it at that point

Question 14

What equation can be used to express Resistance in terms of Voltage and Current?

Answer 14

Resistance (R) = Voltage (V) / Current (I)
SI Unit: ohm

Question 15

What does Ohm’s law state?

Answer 15

Current flowing through a conductor is directly proportional to the potential difference across it provided all physical conditions stay constant

Question 16

What is an Ohmic conductor?

Answer 16

A conductor that follows Ohm’s law. That is to say that it’s resistance is constant

Question 17

What is a non-Ohmic conductor?

Answer 17

A conductor that does not follow Ohm’s law. The current flowing through it is not directly proportional to the voltage flowing through it, as the resistance in non-Ohmic conductors are not constant, and vary with different variables

Question 18

Why does the resistance of metals (good conductors) increase with higher current flow?

Answer 18

The metal heats up when a current flows through it, causing its temperature to rise. At higher temperatures, the metallic ions vibrate about their fixed positions more vigorously, which increases the number of collisions between the mobile electrons and fixed positive ions, which opposes and slows down the flow of electrons, causing resistance to increase

Question 19

What are some factors that affect the resistance of a conductor?

Answer 19

Temperature: As temperature increases, resistance increases
Length: As length increases, resistance increases
Area of cross-section: As area of cross-section increases, resistance decreases

Question 20

What is electrical resistivity?

Answer 20

A property of a material that quantifies how strongly it resist electrical current

Question 21

What is the SI unit for resistivity?

Answer 21

ohm-meter

Question 22

What equation describes the relationship between resistivity and resistance?

Answer 22

Resistivity (p) = Resistance (R) x Area (A) / Length (L)
OR
Resistance (R) = Resistivity (p) x Length (L) / Area (A)
SI unit: ohm-meter

Question 23

What is the difference between resistance and resistivity?

Answer 23

The resistance of a conductor depends on its length and area, while the resistivity of a conductor is constant, independent of its dimensions

Question 24

When are resistors in series?

Answer 24

When the same current physically flows through all the resistors

Question 25

How is the potential difference (voltage) across a resistor related to its resistance?

Answer 25

Directly proportional

Question 26

How can the potential difference across a power source in series be found?

Answer 26

Finding the sum of the potential differences across all the resistors

Question 27

When are resistors in parallel?

Answer 27

When there is more than one path the electrons can flow through

Question 28

How are the potential difference (voltage) across resistors in parallel related?

Answer 28

They are the same

Question 29

How is the current flow through each resistor related to the resistance of each resistor?

Answer 29

Inversely proportional

Question 30

How can total current be calculated in a parallel circuit?

Answer 30

The sum of all the currents at different branches

Question 31

How can the total resistance be calculated in a parallel circuit?

Answer 31

1 / Rt = 1 / R1 + 1 / R2 + 1 / R3 …

Question 32

How does a circuit short circuit?

Answer 32

When a path connected in parallel in the circuit has an extremely low resistance, such as when the path is only made of one wire, or consists of an ammeter. This path provides an easy path for the current, which draws an extremely large current to it

Question 33

What are some consequences of a short circuit?

Answer 33

Overheats wires and may cause fire, causes battery to go flat (run out of energy) very quickly

Question 34

What does a potential divider do?

Answer 34

It provides a voltage to an electrical component that is smaller than the supply voltage in a circuit. It can divide the supply voltage by any ratio

Question 35

Assuming the potential divider consists of two resistors, how can the voltage across either of the resistors be calculated?

Answer 35

Voltage across Resistor 1 = Supply Voltage x (Resistance 1 / (Resistance 1 + Resistance 2))
Works for both Resistor 1 and 2

Question 36

What are transducers?

Answer 36

Electrical or electronic devices that convert energy from one form to another

Question 37

What is the difference between an input and output transducer?

Answer 37

Input: Converts non-electrical energy into electrical energy (Microphones, Photocells, LDR)
Output: Converts electrical energy into non-electrical energy (LED, Speaker)

Question 38

How does a thermistor work?

Answer 38

It has a resistance that decreases when the temperature increases

Question 39

Why and where are thermistors used

Answer 39

They are used to measure and regulate temperature, like in refridgerators and ovens

Question 40

How does an LDR work?

Answer 40

It has a resistance that decreases when the light intensity increases

Question 41

Why and where are LDRs used?

Answer 41

They are used to measure and detect changes in light intensity, like in smoke detectors

Question 42

What steps should be taken to find the total resistance of a series-parallel circuit?

Answer 42

1. Label all the resistance and current values
2. Combine all the series branches into one equivalent resistance value (Rt = R1 + R2 + R3 …)
3. Convert the parallel resistances into one equivalent resistance value in series (1 / Rt = 1 / R1 + 1 / R2 + 1 / R3 …)
4. Repeat steps 2 and 3 until the circuit is simplified