Module 4: C8 - Charge And Current

Question 1

What is Electric Charge

Answer 1

Electric charge is a physical property that all bodies possess and there are two different types of charge:

• Positive (+) Charge
• Negative (-) Charge

Question 2

How do charges interact

Answer 2

Charges interact via the electrostatic force and depending on the types of charges, there are two possibilities:

• Two of the same charges will repel
• Two opposite charges will attract

Question 3

What is the base unit of Electric Charge

Answer 3

The coulomb (C) is the base unit of electric charge.

Question 4

What is a Coulomb

Answer 4

One coulomb (1C) of charge is defined as the amount of charge that passes a point per second when there is a current of one amp (1A)

Question 5

What is the charge of a proton and an electron

Answer 5

Charge of a proton is 1.6x10^-19C

Charge of an electron is -1.6x10^19C

Question 6

What is Elementary Charge (what is 1e equal to in C)

Answer 6

All charges are found to be a multiple of the fundamental charge e. (Charge is quantised)

1e = 1.6x10^-19

Question 7

Equation for the flow of Electric Charge

Answer 7

I = Q/t

• Q = Charge is measured in coulombs (C)
• I = Current is measured in amps (A)
• t = Time measured in seconds (s)

Question 8

Example Question:

How much charge has flowed when 2.5A flows for 20 seconds?

Answer 8

I = 2.5A
t = 20s
Q = It

2.5 x 20 = 50C

Question 9

Example Question:

The current in a lamp is 6.2A. Calculate the number of electrons passing through one point in the lamp in 2.0 minutes

Answer 9

I = 6.2A
t = 120s
Q = It

6.2 x 120 = 744C

744/1.6x10^-19 = 4.65x10^21

4.65x10^21 electrons pass through one point of the lamp in 2.0 minutes

Question 10

Net Charge Definition

Answer 10

The charge on most objects results from either a gain or loss of electrons by the object.

Question 11

How can the net charge be expressed

Answer 11

The net charge can be expressed as a multiple of e: Q = ±be

Question 12

Example Question:

A mobile phone charger draws 500mA and takes 4.0 hours to charge a phone. Calculate the charge transferred in that time

Answer 12

I = 0.5A
t = 14400s
Q=It

0.5 x 14400 = 7200C

Question 13

What is Electric Current

Answer 13

Electric current is the rate of flow of electric charge

Question 14

Explain how electric current will be produced if one end of a wire is positive (+) and the other end is negative (-)

Answer 14

The structure of a metal is made up of a rigid structure of positive metal ions, surrounding in a ‘sea’ of free electrons. When one end of the wire is positive and the other end is negative, electrons will be repelled from the negative side of the wire (as 2 negative charges repel) and then will be attracted to the positive end of the wire (as positive and negative charges attract)

Question 15

What is Conventional Current?

Answer 15

Conventional current was defined long before the discovery of electron as a current from a positive terminal towards a negative one. Conventional current or simply current, behave as if positive charge carriers cause current flow. Conventional current flows from the positive terminal to the negative.

Question 16

What is Electron Flow

Answer 16

The flow of electrons is termed electron current. Electrons flow from the negative terminal to the position.

Question 17

Describe the difference between conventional current and electron flow in a metal wire.

Answer 17

Conventional current in a metal wire would be when charge moves from a positive terminal to a negative terminal, meaning the charged particles that would be moving throughout the wire would be protons. This idea of conventional current was wrong. The correct version that is actually electron current. Electron current is when charged particles (electrons) move from the positive terminal to the negative terminal in a circuit.

Question 18

What is an Electrolyte

Answer 18

An electrolyte is a solution in which ions are charged and able to conduct while being free to move around the substance and

Question 19

How is current caused in an Electrolyte

Answer 19

The fact that charged ions are free to move about the structure (anions are attracted to the anode, while cations are attracted to the cathode), means the solution is an electrolyte. Solutions that can be electrolytes are aqueous solutions and ionic solutions (as charged ions are free to move in these)

Question 20

What can make a larger current in a wire

Answer 20

• A greater number of electrons moving last a given point each second (for example, a wire with a greater cross-sectional area)
• The same number of electrons moving faster through the metal

Question 21

How does Increasing Temperature affect Current/Resistance

Answer 21

If the temperature increased, then the positive ions would vibrate more, creating more resistance for the moving electrons, leading to a lower current.

Question 22

What is an Ammeter and how is it used

Answer 22

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.

(The ideal ammeter has zero resistance)

Question 23

What is the Conservation of Charge

Answer 23

In any closed system, the total charge in the system is conserved.

Conservation of charge states that electric charge can neither be created nor destroyed. The total amount of charge in the universe is constant.

Question 24

What is Kirchhoff’s First Law

Answer 24

At any junction in a circuit, the total current leaving the junction is equal to the total current entering the junction.

Question 25

How can you show Kirchhoff’s Law in General Terms

Answer 25

Σ I in = Σ I out

Question 26

How can you Show Kirchhoff’s law in a circuit

Answer 26

I1 + I2 = I3

           |
           /\   I1
           | —>—— |   I2         |
          \/   I3

Question 27

A single wire is connected to two other wires, A and B. If the current is the single wire is 15A, and 1.9x10^21 electrons pass along wire A in 1.0 minutes, calculate the current in wire B to the nearest ampere.

Answer 27

(1.9x10^21) x (1.6x10^-19) = 304C

304/60 = 5.067 A

15A - 5.067A = 9.933A
=10A

Question 28

What is Number Density

Answer 28

The number density (n) is the number of free electrons per unit volume.

The higher the number density, the greater the number of free electrons per m^3 and so the better the electrical conductor.

Question 29

Definition of a Conductor, Semiconductor, and Insulator

Answer 29

Conductor - Large number density of conduction electrons

Semiconductor - They have number densities approximately 1 million times smaller than metals such as copper. The conduction electrons travel faster than in conductors and can be changed by adding an impurity to a material such a silicon.

Insulator - Few or no conduction electrons. Number density is almost zero.

Question 30

What is Drift Velocity

Answer 30

Charged particles do not travel in a straight line through a conductor, because they collide with other particles in the material.

We therefore use the average speed the particle travels at along the conductor. This is called the drift velocity.

Question 31

Equation for Electric Current (involving mean drift velocity)

Answer 31

I = Anev

Current = Cross Sectional Area x Number Density x Elementary Charge x Mean Drift Velocity

Question 32

Show I = Anev is homogeneous with respect to its base units

Answer 32

I = Anev

A = m^2Cms^-1m^-3
A = cm^3s^-1m^-3
A = cs^-1
A = Ass^-1
A = A

Question 33

Describe the affect on the Mean Drift Velocity if all other factors are constant for

I = Anev

a) Current Increases
b) Cross-Sectional Area Decreases
c) Number Density Doubles

Answer 33

I / Ane = v

a) Mean Drift Velocity Increases

b) Mean Drift Velocity Increases

c) Mean Drift Velocity Decreases (by half)

Question 34

How does changing the cross-sectional area affect the mean drift velocity

Answer 34

The narrower the wire, the greater the drift velocity must be in order for the current to be the same.

Question 35

Worked Example:

A piece of wire carrying a current narrows such that it radius halves. What effect does this have on the mean drift velocity of the electrons in the wire?

Answer 35

A = πr^2
π1/2^2 = π/4

Therefore, Mean Drift Velocity will increase by a factor of 4.