Current & Charge

Question 1

What is current?

Answer 1

The rate of flow of electric charge

Question 2

How does charge in an LED light work?

Answer 2

Charge can only flow if the component is connected in a positive to negative direction.

Question 3

Is charge a vector or a scalar quantity?

Answer 3

(Charge is a fundamental in physics)
It is a scalar - even though it travels from positive to negative terminals charge is described by magnitude alone

Question 4

Equation for current

Answer 4

I = ▵Q / ▵t
Or
I = P/V
Or
V= IR —> I = V/R

Question 5

Equation for total charge

Answer 5

▵Q = ne

e = elementary charge
n = number density

Question 6

Base units for electric charge

Answer 6

A s

Question 7

What does quantised mean?

Answer 7

Charge is quantised
- It is a multiple of the elementary charge
- All electrons have the charge of 1.6x10-19
- Total/net charge = 1.6x10-19 x n

Question 8

What is (N)?

Answer 8

Number of unbalanced charges - (number of electrons or charged ions)

Question 9

Equation to find number density

Answer 9

n = ▵Q/ e

Question 10

What is Kirchhoff’s first law?

Answer 10

The sum of currents entering a junction is equal to the sum of current exiting a junction
- Conservation of charge

Question 11

What is number density?

Answer 11

The number of charge carriers per unit volume

n = N/V

N = (Number of unbalanced charges), number of electrons or charged ions

V = Volume (m³)

Question 12

Units for number density

Answer 12

1/m³ —> m⁻³

Question 13

What is the relationship between current and velocity?

Answer 13

If charge carriers are travelling at a faster speed, the current will increase

They are directly proportional

Question 14

What is drift velocity?

Answer 14

The mean velocity of charges along the length of a conductor

Question 15

Equation for drift velocity

Answer 15

I = nAev (rearrange)
V = I/nAe

v = velocity/speed that electrons or charge carries are moving along the wire
e = elementary charge or charge of the ions
A = cross sectional area
n = number of unbalanced charges/electrons or charged ions

Question 16

4 factors that affect current

Answer 16

Velocity - as the velocity of the charge carriers increases the current auto increases

Charge - if the charge carriers are ions, they will have a much higher charge than electrons, therefore there’ll be a higher current

Cross sectional area - If the area of the wire increases there will be more space for electrons/charge carries to travel through - therefore a higher current

Number density - more charge carries means a higher current