# 25.4.3 Forces in G, E & B Fields

### Forces in G, E & B Fields

#### Gravitational Fields

• A gravitational force acts on anything with mass
• This is always an attractive force and acts on both stationary and moving masses
• Inside a uniform gravitational field, the gravitational force Fg is defined by:

Fg = mg

• Outside a uniform gravitational field (ie. outside the surface of a planet), the force is defined by Newton’s Law of Gravitation:

• Where:
• G = Newton’s gravitational constant
• M1 and M2 = two point masses (kg)
• r = separation between the centres of the masses (m)
• This equation tells us that the gravitational force will cause a force of attraction between any two masses

#### Electric Fields

• An electric force acts on anything with a charge
• This can be either attractive or repulsive and acts on both stationary and moving charges
• The electric force FE is defined by:

FE = QE

• Where:
• Q = size of the charge (C)
• E = electric field strength (N C-1)
• The electric force causes opposite charges to attract and like charges to repel
• When a charge is moving in an electric field, the electric force will cause the particle to move in a parabolic motion

#### Magnetic Fields

• A magnetic force acts on anything with a charge that is moving within a magnetic field
• The largest force occurs when the particle is moving perpendicular to the magnetic field lines
• The magnetic force FB for an isolated moving charge is defined by:

FB = BQv sin(θ)

• Where:
• B = magnetic flux density (T)
• v = velocity of the charge (m s-1)
• θ = angle between the charge’s velocity and the field lines (degrees)
• When a charge is moving normal to a magnetic field, the electric force will cause the particle to move in circular motion
• The force for a current-carrying wire inside an external B field is defined by:

FB = BIL sin(θ)

• Where:
• I = current (A)
• L = length of the conductor (m)
• θ = angle between the conductor and the field lines (degrees)

### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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