4.1 ✅ Current and Charge

Question 1

What is electric current?

Answer 1

Electric current can be defined as the rate of flow of charge.

• In simple terms , electric current is the amount of charge passing through a given point in a circuit per unit time.

Question 2

What is the formula for Electric Current?

Answer 2

I = ΔQ / Δt
Current = Charge Transferred / Time taken

• Current (I) is measured in Amps (A)
• Charge (Q) is measured in Coulombs (C)
• Time is measured in Seconds (s)

Question 3

Explain Electric current means in simple terms

Hence, explain in simple terms what 1 ampere of current means.

Answer 3

In simple terms, electric current is the amount of charge passing through a given point in a circuit per unit of time

1 ampere is the same as one coulomb of charge passing through a given point per second 1(Cs ⁻¹)

Question 4

What is electric charge?

Answer 4

Electric Charge is a physical property, like mass, volume or temperature.

Extra Info: It is a measure of “chargedness”

Question 5

What are the two types of Charge, and give examples for each?

Answer 5

Positive charge (e.g. Protons) and negative charge (e.g. Electrons)

Question 6

What do you call an object that is not charged at all?

Give one example

Answer 6

An object that is not charged at all is called neutral

For example, neutrons

Question 7

What do objects that are charged do?

Answer 7

Objects that are charged interact and exert forces on each other - like charges repel each other and opposite charges attract.

Question 8

What is Electric Charge measured in?

Answer 8

Electric Charge is measured in coulombs (C)

Question 9

Define coulomb

Answer 9

A coulomb is defined as the __electric charge flowing past a point in one second__ , where there is an electric current of 1 ampere.

Question 10

What is One Coulomb (1C) in base units?

Answer 10

One Coulomb (1A) is equivalent to One Ampere Second (1 A s) in base units

Question 11

What is a charge carrier?

Answer 11

A charge carrier is any particle that has an electric charge (be it negative or positive)

Question 12

Any particle that has an electric charge (be it negative or positive) is known as a ____________

Answer 12

Any particle that has an electric charge (be it negative or positive) is known as a charge carrier

Question 13

Give examples of charge carriers

Answer 13

Question 14

INFO on elementary charge

Answer 14

• In the past, we refer to ions as having a charge of +1 or -2, such as in Chemistry.
• But these are relative charges that are measured against the constant e.
• This constant e is known as the elementary charge and is equal to 1.60 x 10⁻¹⁹ C
• This means that a proton has a charge of +1e and an electron has a charge of -1e and not just 1

Question 15

What is the elementary charge e equal to?

Answer 15

1.60 x 10⁻¹⁹ C

Question 16

What is the charge on one proton equal to?

Answer 16

The charge on one proton is equal to the elementary charge: 1.60 x 10⁻¹⁹ C

Question 17

Explain what the net charge of an object is?

Answer 17

The Net charge is the overall charge of an object. It results from either a gain or loss of electrons by the object.

If the object gains electrons then it becomes positively charged, but if the object loses electrons then it becomes negatively charged

Question 18

Explain how we can express the net charge on a particle or an object?

Answer 18

We can express the net charge on a particle or an object as quantised and a multiple of e

Question 19

Give a formula for the net charge on an object?

Answer 19

The net charge on an object can be given by the formula:

Q = ±ne

• where Q is the net charge on the object in coloumbs
• n is the number of electrons
• e is the elementary charge

Question 20

What do we describe charge as being & why?

Answer 20

Charge is described as being quantised as charge can only have certain values that are integer multiples of e.

For example, an object with 1.92 x 10⁻¹⁸ C has a charge of +12e

Question 21

Electric current is defined as the rate of flow of charge.

What do we have to be aware of in this statement?

Answer 21

The flow of charge does not necessarily mean the flow of electrons, but any charge carrier.

In metals, the charge carriers are electrons, but in liquids or electrolytes, the charge carriers tend to be ions

Question 22

Define what current is for:
- In metals
- In electrolytes

Answer 22

Current is defined as the movement of electrons in metals and movement of ions in electrolytes

Question 23

Explain the “Modelling of electric current in metals”

Answer 23

1. A metal is as a regular, crystal, lattice structure of positive metal ions - surrounded by a number of free electrons.

Free electrons are the small number of electrons from each metal ion that are free to move around (although most of the electrons are fixed to their metal atom)

2. Electric current is the rate of flow of charge, or movement of charge carriers. In order to get the electrons to flow, we need to make one end of the metal positive, and the other negative, so that the electrons will move slowly towards the positive end - and essentially create a __flow of charge__.

Question 24

What are free electrons?
In reference to “Modelling Electric Current in metals”

Answer 24

Free electrons are the small number of delocalised electrons from each metal ion that are free to move around.

Question 25

The greater the rate of charge flow, the greater the ____________ in the wire.

Answer 25

The greater the rate of charge flow, the greater the **electric current** in the wire

Question 26

What could cause an increase of electric current in a wire?

Answer 26

An increase of electric current is due to the increase in the rate of charge flow. This is because of:

Question 27

What is Conventional Current?

Answer 27

**Conventional current** was defined long before the discovery of electrons as a current flowing from a positive terminal towards a negative one

Question 28

What is the direction of electric current treated as?

Answer 28

The direction of electric current is treated as positive to negative, regardless of the direction the charge carriers are moving in

Question 29

Describe the relation between the flow of conventional current and the actual flow of electrons?

Answer 29

The flow of electrons travels in the opposite direction to the flow of conventional current

Question 30

What are electrolytes?

Answer 30

Electrolytes are liquids that can carry an electric current (and essentially conduct electricity)

Question 31

In electrolytes, what is electric current described as? | And what is it not described as?

Answer 31

In electrolytes, electric current is **not** described as the flow of electrons, but a flow of ions.

Question 32

What are electrolytes made up of?

Answer 32

Electrolytes are either made up of **molten ionic compounds**, or more commonly **ionic solutions** (which means that they contain positive (Cations) and negative ions (anions))

Question 33

Explain if pure water is an ionic solution

Answer 33

Pure water is an excellent **insulator**, but water from taps and and rivers is an ionic solution - it contains dissolved ions - so it can carry an electric current.

Question 34

Give an example of "Modelling Electric current in Electrolytes"

Answer 34

* A common example of an ionic solution is the salt Sodium Chloride (NaCl) dissolved in water: * The salt separates into Na⁺ cations and Cl⁻ anions * If a positive and negative electrode, known as a cathode and anode respectively, are placed into the solution, the Na⁺ cations will move towards the cathode to gain an electron, and the Cl⁻ anions will move towards the anode to lose an electron * This movement of ions creates a flow of charge, and essentially an electric current ## Footnote Note about a full electode with metals (like the image above): When the Na⁺ ions reach the cathode, they accept an electron, and when the Cl⁻ ions reach the anode they donate an electron, so electrons can flow through the metal part of the circuit.

Question 35

Explain what we use to measure electric current, how we use it, and why

Answer 35

An **ammeter** is used to measure the electric current at any point in a circuit. It is always placed directly in series in the circuit at the point where you want to measure the current. As ammeters are placed in series they should have the lowest possible **resistance** in order to reduce the effect they have on the current – an ammeter with high resistance would decrease the current it should be measuring. The ideal (perfect) ammeter has zero resistance, and so has no effect on the current it measures.

Question 36

What does the **conservation of charge** state

Answer 36

The **conservation of charge** states that electric charge can neither be created nor destroyed. The total amount of electric charge in the universe is constant. ## Footnote Remember that charge is a fundamental physical property (Refer to flashcard 4)

Question 37

What does Kirchhoff's First Law state? | Also include the equation

Answer 37

Kirchoff's First Law states that for any point in an electrical circuit, the sum of currents into that point is equal to the sum of the currents out of that point ## Footnote ΣIᵢₙ = ΣIₒᵤₜ

Question 38

How can we write Kirchhoff's First Law as an equation?

Answer 38

Question 39

What is Kirchhoff's First Law based on?

Answer 39

Kirchhoff's First Law is based off the **conservation of charge**, where the charge (measured in coulombs) is the product of the current (in amperes) and the time (in seconds). Charge cannot be destroyed, so the charge carriers entering a point in a given time must equal the total number of charge carriers leaving that same point during that time

Question 40

Explain the link between the **conservation of charge** and **Kirchhoff's First Law**?

Answer 40

**Kirchhoff's First Law** is a consequence of the **conservation of charge**, as charge cannot be destroyed, so the charge carriers entering a point must equal the total number of charge carriers leaving that point during the same time

Question 41

Explain what **Number density** is

Answer 41

Number density is the number of free electrons **per** cubic metre of material. This is due to the fact that some metallic materials have more free electrons than others

Question 42

Explain how a higher number density affects the material

Answer 42

The higher the number density, the greater number of free electrons per m³, and so the better the electrical conductor

Question 43

What are the 3 groups we can classify materials into based on their number density?

Answer 43

* Conductors - which have a very high number density - of the order **10²⁸ m³** * Insulators that have a much lower number density value * Semiconductors have number densities in between the two - with number densities of around **10¹⁷ m³**

Question 44

Explain the difference between semiconductors and metals?

Answer 44

Semiconductors have a much lower number density than metals

Question 45

Explain what semiconductor materials must do in order to carry the same current as a conductor?

Answer 45

Semiconductors have a much lower number density than metals, so in order to carry the same current, the electrons in the semiconductors need to move much faster. ## Footnote Must remember: This also increases the temperature of a semiconductor, as movement => kinetic energy => temperature

Question 46

Good to remember about movement of electric current in wires and switches

Answer 46

Good to remember about movement of electric current in wires and switches

Question 47

What is mean drift velocity?

Answer 47

The mean drift velocity is the average velocity of the charge carriers travelling through the conductor

Question 48

Explain why the mean drift velocity of electrons are relatively slow

Answer 48

The mean drift velocity of electrons are relatively slow because the electrons only travel a small distance before colliding with a metal ion

Question 49

What is the formula for electric current involving mean drift velocity?

Answer 49

*`I = A n e v`* Where: - *I* is current in Amps - *A* is the **cross-sectional area** of the conductor in m² - *n* is the **number density** - *e* is the **elementary charge** (1.60 x 10⁻¹⁹) - *v* is the **mean drift velocity** of the charge carriers in **ms⁻¹**

Question 50

How do we maintain the current if the cross sectional area of a wire becomes smaller? ## Footnote Think about mean drift velocity

Answer 50

If the **cross-sectional area** of a wire becomes smaller, in order to maintain the same current, the mean drift velocity must **increase** so that the rate of flow of charge is **maintained**.