# IB Physics/Fields and Forces

## Topic 6 Fields and Forces

### 6.1 Gravitational Force and Field

#### 6.1.1 State Newton's universal law of gravitation

• Every single point mass attracts every other point mass with a force that is proportional to the product of their masses and is inversely proportional to the square of their separation
1. $F=G{\frac {Mm}{r^{2}}}$ • G = universal gravitation constant (6.67 x 10-11)Nm2kg-2 determined by Henry Cavendish
• M = source mass (where the field is coming from)
• m = test mass (mass being affected by force, though it has a gravitational force on its own)
• r = the distance between the centres of each mass

#### 6.1.2 Define gravitational field strength

A space where a small test mass feels a force due to its mass.

1. $g={\frac {F}{m}}$ #### 6.1.3 Determine the gravitational field due to one or more point masses

• gravitational field can be shown using gravitational field lines
• gravitational field lines must be evenly dispersed around the point mass
• more lines indicates a greater field magnitude

#### 6.1.4 Derive an expression for gravitational field strength at the surface of a planet, assuming all its mass is concentrated at the centre

In other words, an equation that expresses gravitational field strength in terms of the distance away from the source must be found; a function g(r) that calculates the gravitational field strength when the distance away is known. This is because in the situation, we want to find the gravitational field strength when we know how far the surface is from the source.

1. $g={\frac {F}{m}}$ $F=G{\frac {Mm}{r^{2}}}$ $g={\frac {G{\frac {Mm}{r^{2}}}}{m}}$ • The m's cancel so:
1. $g=G{\frac {M}{r^{2}}}$ ### 6.2 Electric Force and Field

#### 6.2.1 State two types of charge

• positive (+) = a deficiency of electrons
• negative (-) = an excess of electrons

#### 6.2.2 State and apply The Law of Conservation of Charge

• The net charge of an isolated system is conserved. The charge stays constant.
• Charge can neither be created nor destroyed.
• Using this law, we can always predict how many positive and negative charges exist

#### 6.2.3 Describe and Explain the difference in electrical properties of conductors and insulators

• Conductors
• substances that allow electron to easily flow through them
• Examples: metal graphite
• Superconductor: a perfect conductor, all substances become superconductors at 0 Kelvin
• Insulator
• Substances that do not allow electrons to flow easily through them
• Examples: plastics, rubber
• There are no perfect insulators

#### 6.2.4 State Coulomb's Law

• The electric force between two charges are proportional to the product of their charges and inversely proportional to the square of the distance between them
• It acts along the line joining the two charges
1. $F=k{\frac {q_{1}q_{2}}{r^{2}}}$ • F = Force of electric charge attraction/repulsion
• q1 = source charge
• q2 = test charge
• r = distance between centre of charges
• k = coulomb constant (8.99 x 109)Nm2C-2 in a vacuum
• In other media use ${\frac {1}{4\pi E_{0}}}$ where E0 is the permitivity constant of free space

#### 6.2.5 Define electric field strength

• The force felt per unit charge by a small positive test charge at that point in the electronic field.
1. $E={\frac {F}{q}}$ • Electric fields exist around charges and combinations of charges

#### 6.2.6 Determine the electric field strength due to one or more point charges

1. $E=k{\frac {q}{r^{2}}}$ #### 6.2.7 Draw the electric field patterns for different charge configurations

• Need pictures
• Electric field always flows from positive to negative

### 6.3 Magnetic Force and Fields

#### 6.3.1 State the moving charges give rise to magnetic fields

• Oersted's basic principle of electromagnetism: moving charges produce a magnetic field

#### 6.3.2 Draw magnetic field patterns due to currents

• charge moving through wire
• solenoid
• freely moving charge in a magnetic field

#### 6.3.3 Determine the direction of the force on a charge moving in a magnetic field

• Right Hand Rule #3
• fingers are magnetic field
• thumb is direction of velocity
• palm is force
• or Left Hand Rule (FBI rule)
• name fingers of your left hand with letters FBI, starting from thumb, and hold the three fingers so that there are right angles between each two.
• F (thumb) stands for force
• B (index finger) is the magnetic field direction
• I (middle finger) is the direction of current (same as that of velocity of positive charge and opposite to velocity of negative charge)

#### 6.3.4. Define the magnitude and direction of a magnetic field

• Magnetic field moves from north to south and is flipped within a magnet