8. Charge and current

Question 1

Define electric curent

Answer 1

• Rate of flow of charge

Question 2

State the SI base unit for current

Answer 2

• Amperes (A)

Question 3

State the symbol used for electric current

Answer 3

• I

Question 4

State the equation to find current

Answer 4

• 𝐼 = 𝑑Q / 𝑑t.

Question 5

How can current in an electrical circuit be measured?

Answer 5

• Using an ammeter, which is placed in series

Answer 6

• amount of charge passing a given point in a circuit per unit time
• One ampere is the same as one coulomb of charge passing a given point per second

Question 7

What is electric charge

Answer 7

• Charge, Q, is a physical quantity, which can be either positive or negative.
• Some particles are charged, like protons and electrons
• There are two types of charge, positive and negative
• Any object that is not charged is called natural
• Objects with charge interact and exert forces on each other
• Like charges repel each other, opposite charges attract

Answer 8

• It is measured in coulombs (C),
• where 1 coulomb is defined as the electric charge flowing past a point in one second when there is an electric current of one ampere
• from delta Q = I delta t, 1C is equal to 1A s
• Any particle that has an electric charge is a charge carrier
• relative charges are measure against the constant e
  constant e is the elementary charge equal to 1.6 x10^-19
• It is the same charge on one proton
• A proton has a relative charge of 1+e and an electron -1e
• If we know the value of the current in a metal wire, we can also calculate the charge passing though it in a given time and even no of electrons

Answer 9

When we refer to the charge of ions and the components of atoms, we look at it as a quantised number – a proton has a charge of +1, and an electron has a charge of -1. However, these numbers represent multiples of the elementary charge, e
*, 1.6x10-19 C
* no electrons = charge / elementary charge

Answer 10

• The net charge of a particle is due to the gain or loss of electrons.
• In an atom, the number of protons equals the number of electrons, so the charges cancel each other out and the overall charge is neutral.
• Increasing the number of electrons will produce a negative ion with negative charge
• Removing electrons will produce a positive ion, as there are now more protons than electrons. with positive charge
• the size of the charge on a particular object can be expressed as a multiple of e
• the next charge on an object Q is given by
  Q = ± ne, where n is the number of electrons added or remove, and also the quantised charge value for the particle.
  Q is net charge
• we can describe the charge on an object as being quantised, this is because charge can only have certain values
• values must be integer multiple of e

Answer 12

Electric current is the rate of flow of charge, but charge can be carried in several ways, depending on the material the current is passing through. The current in metals is carried by electrons. In a metal, there is a lattice of positive ions, surrounded by free electrons. The positive metal ions are fixed in place, but the electrons can move around, and so when one side of the metal is made positive, and the other side is made negative, the electrons will be attracted to the positive side, and move through the metal as electric current.

Answer 13

Some liquids can conduct a charge. These conducting liquids are called electrolytes, and are commonly ionic solutions. This means they contain positive and negative ions. An example of this is water with salt, NaCl, dissolved in it. The salt splits in to Na+ cations and Cl- anions. When a pair of electrodes (the anode is the positive electrode and the cathode is the negative electrode) are placed in the solution, the cations will be attracted to the cathode, and the anions will be attracted to the anode. This produces an electrical current.

Answer 14

Conventional current was discovered and defined well before the discovery of the electron. It is the rate of flow of charge from the positive to the negative terminal, and this is how all electric currents are treated, regardless of the direction the charge carriers are moving in. In metals, the electrons flow from negative to positive, so the electron flow is in the opposite direction to the conventional current.

Question 15

What does Kirchhoff’s first law state?

Answer 15

• For any point in an electrical circuit, the sum of the currents in to that point is equal to the sum of the currents coming out of that point.
• This law is a consequence of the conservation of charge. Charge is a fundamental physical property, which cannot be created or destroyed, so it must be conserved.

Question 16

mean drift velocity

Answer 16

When electrons move through a metal, they frequently collide with the positive metal ions, resulting in random movement. When a power supply is connected, the free electrons are attracted towards the positive terminal, but they still collide with the positive metal ions. The mean drift velocity, v, is defined as the average velocity of the electrons as they travel down the wire, colliding with positive metal ions.

Answer 17

The number density, n, of a material represents the number of free electrons per unit volume. Conductors, such as metals, have very high number densities, around 1028 per m3. Insulators, such as plastics, have much smaller number densities, and semi-conductors like silicon have in- between values. When the value of n is lower, the electrons must travel faster to carry the same current.

Question 18

current

Answer 18

I = dQ / dt

Question 19

The total charge, Q in the wire is the product of the number of free electrons per unit volume, n, the elementary charge, e, and the volume, V:𝐼𝐼

Answer 19

I = neV / dt

Answer 20

The volume of the wire is equal to its cross sectional area, A, multiplied by its length. The length of the wire divided by the time taken for the electrons to cross this distance is equal to the mean drift velocity, v, so we can rewrite V/dt as Av

Question 21

I = Anev

Answer 22

𝐼𝐼 = 𝐴𝐴𝑛𝑛𝑛𝑛𝐴𝐴
We can rearrange this equation to find the mean drift velocity for electrons in a wire when the current, the cross sectional area of the wire, and the number density of the metal are known.