Charge and Current

Question 1

How is electric current produced?

Answer 1

• The flow of charge particles

Question 2

Electric current

Answer 2

• The rate of flow of charge
• I = Q/t
• Measured in Amps
• Current measured by an ammeter connected in series

Question 3

Definition of 1 Amp

Answer 3

• 1 Amp is the same of as 1 coulomb of charge passing a given point per second
• (Cs^-1)

Question 4

What is Charge?

Answer 4

• Is a physical quantity that can be positive or negative
• Measured in coulombs

Question 5

Definition of 1 coulomb

Answer 5

• 1 coulomb is the same as the flow of charge in a time of 1 second when the current is 1 amp
• (Is)

Question 6

Conventional Current

Answer 6

• The rate of flow of charge from the positive to negative terminal
• I metals the electrons flow from negative to positive, so the electron flow is in the opposite direction to conventional current

Question 7

Kirchhoff’s 1st Law

Answer 7

• At any point in an electrical circuit the sum of current entering a point is equal to the sum of current leaving that point
• Conservation of charge

Question 8

What is the Conservation of Charge?

Answer 8

• Charge cannot be created or destroyed, only conserved

Question 9

What happens when electrons move through a metal?

Answer 9

• They frequently collide with positive metal ions, resulting in random movement
• When power supply is connected, the free electrons are attracted towards positive terminal by they still collide with positive metal ions

Question 10

Mean Drift Velocity

Answer 10

• The average velocity of the electrons as they travel down the wire colliding with positive metal ions

Question 11

Number density of a material

Answer 11

• n
• Is the number of free electrons per unit volume
• Conductors have very high number densities
• Insulators have smaller number densities
• When the value of n is lower, electrons must travel faster to carry the same current

Question 12

How to derive: I = Anev

Answer 12

• I = Q/t
• Total charge in the Q in the wire is the product of the number of free electrons per unit volume, n, e, V:
  I = neV/t
• Volume = A x l
• However length/t = v, therefore:
  I = Anev

Question 13

Relationship between resistivity and n

Answer 13

• Resistivity is directionally proportional to n

Question 14

Relationship between current and n

Answer 14

• Ability to carry current increases with increase in n