8 - Charge and Current

Question 1

Define current

Answer 1

The rate of flow of electric charge

Question 2

Current equation

Answer 2

I(current) =Q(charge) / t(time)

Question 3

What is charge?

Answer 3

A physical property that all bodies possess and is either positive or negative. Through electrostatic forces, these charges can interact.

Question 4

How do charges interact?

Answer 4

Like charges repel and unlike charges attract.

Question 5

What is the SI unit for charge?

Answer 5

C or Coulomb

Question 6

What is one coulomb of charge defined as?

Answer 6

The amount of charge that passes a point per second when the current is 1A.

Question 7

What is the fundamental charge?

Answer 7

1.6x10^-19 - All charges are multiple of the fundamental charge (Charge is quantised) and 1e (electron) has a charge of -1.6x10^-19

Question 8

What is net charge?

Answer 8

The net charge of an object is expressed as a multiple of e: Q=±ne and depends on the loss or gain of electrons.

Question 9

How is current produced in a wire?

Answer 9

Metals don’t have a full outer shell of electrons so when they bond, they deposit the outer shell electrons into a sea of delocalised electrons, resulting in positive ions. The negatively charged electrons will be attracted to the positive terminal and repel the negative terminal. The electron move freely from the negative terminal to the positive terminal, resulting in a flow of electrons, resulting in a current.

Question 10

What is the difference between electron flow and conventional current?

Answer 10

Electron flow is the direction of electron flow from the negative terminal to the positive terminal due to electrons being negatively charged. The opposite is conventional current. Conventional current moves from the positive terminal to the negative as it assumes positive charged carriers (protons) cause current flow. Conventional current was before the discovery of the electron.

Question 11

What is an electrolyte?

Answer 11

A solution that has ions within it, which can carry charge through the liquid.

Question 12

What causes current to be produced in the wire during electrolysis?

Answer 12

The flow of electrons. The electrons move to the positive terminal from the negative (opposite of conventional current). The cathode is the side the electrons move towards and the opposite is the anode.

Question 13

How do ions in the solution move towards the cathode?

Answer 13

The cathode is negatively charged so positively charged ions in the solution (cations) will move towards the cathode.

Question 14

How do ions in the solution move towards the anode?

Answer 14

The anode is positively charged so negatively charged ions in the solution (anions) will move towards the anode.

Question 15

How is current in an electrolyte created?

Answer 15

The movement of ions towards the cathode and anode is a flow of charge and so creates an electric current within the solution.

Question 16

What is the conservation of charge?

Answer 16

In any closed system, the total charge is the system is conserved. Electric charge can neither be created or destroyed. The total amount of charge in the universe is constant.

Question 17

State Kirchhoff’s first law

Answer 17

At any junction in a circuit, the total current entering the junction is equal to the total current leaving the junction (ΣI(in)=ΣI(out)).

Question 18

What is number density?

Answer 18

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 19

n of conductors

Answer 19

Conductors have large number densities of conduction electrons typically of order 10^28

Question 20

n of semiconductors

Answer 20

Semiconductors 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 as silicon. Semiconductors have number densities typically of order 10^17.

Question 21

semiconductors and heat

Answer 21

Semiconductors have lower number densities than metals so electrons must travel faster in order to carry same current, increasing the temperature of the semiconductor (silicon in computer needs cooling systems).

Question 22

n of insulators

Answer 22

Insulators have the lowest value of n. There are few or no conduction electrons and number density is almost zero.

Question 23

What is drift velocity?

Answer 23

Charged particles do not travel in a straight line through a conductor as they collide with other particles in the material therefore we use the average speed the particle travels at along the conductor. This is called the drift velocity.

Question 24

New Current equation

Answer 24

I = Anev
Current = cross-sectional area of wire x number density x electron charge x drift velocity

Question 25

Effects on drift velocity with changing cross sectional area

Answer 25

The narrower the wire, the greater the drift velocity must be in order for the current to be the same. The mean drift velocity is inversely proportional to the cross-sectional area of the wire.