--- /dev/null
+# Fields
+
+Non-contact forces:
+- strong nuclear force
+- weak nuclear force
+- electromagnetic force (dipoles)
+- gravitational force (monopoles)
+
+---
+
+## Gravity
+
+### Newton's law of universal gravitation
+
+$$F_g=G{{m_1m_2}\over r^2}$$
+
+where
+$F_g$ is the gravitational force between $m_1$ and $m_2$
+$G$ is the gravitational constant, $6.67 \times 10^{-11} \operatorname{N m^2kg^{-2}}$
+$r$ is the distance between centre of $m_1$ and $m_2$
+
+
+- inverse square law
+- acceleration can be calculated from $F_g$, since $F=ma$
+- all objects with mass attract each other with $F_g$
+- $F_g$ acts equally on $m_1$ and $m_2$
+- acceleration of an object close to earth's surface can be approximated by ignoring its mass ($m_2 \approx 0$)
+- apparent weight may be different to gravitational (normal) weight
+
+### Gravitational fields
+
+$$g={F_g \over m}=G{M \over r^2}$$
+
+where
+$g$ is the gravitational field strength
+$F_g$ is the force due to gravity ($=G{{m_1m_2}\over r^2}$)
+$m$ is the mass of object in the field
+$M$ is the mass of the central body
+
+- arrows towards centre of object
+- closer arrows mean larger force
+- parallel field lines - uniform field strength (vector)
+
+Characteristics of gravitational fields:
+- monopoles
+- attractive force
+- extends to infinite distance, but diminishes with inverse square law
+
+### Work in a gravitational field
+
+Gravitational potential energy: $E_g = mg \Delta h$
+Work: $W = \Delta E_g = Fx$
+
+Area under force-distance graph = $\Delta E_g$
+Area under field-distance graph = $\Delta E_g / \operatorname{kg}$
+
+### Satellites
+
+## Electromagnetism
+
+### Electric fields
+
+- surrounds +ve and -ve charges
+- exerts force on other changes in its field
+- monopoles and dipoles
+- attractive/repulsive forces
+- can be constrained to a fixed distance (conductors / insulators)
+- current flows from +ve to -ve
+
+#### Field lines
+- +ve to -ve
+- start and end $\perp$ to surface
+- field lines never cross
+- point charges - radiate from centre
+
+#### Forces
+
+$$F=qE$$
+
+where
+$F$ is the force on charged particle
+$q$ is the charge of object experiencing force (Coulombs)
+$E$ is the strength of the electric field (Newtons / Coloumb or Volts / metre)
+
+#### Work in electric fields
+
+$$W=qV$$
+
+where
+$W$ is the work done on +ve point charge or in field
+$q$ is the charge of point charge being acted on
+$V$ is the potential (voltage) between points
+
+#### Coulomb's law
+
+
+$$F=k{{q_1q_2}\over r^2}$$
+
+where
+$k$ is Coulomb's constant $9.0 \times 10^9 \operatorname{N m^2 C^{-2}}$
+$q_1$ and $q_2$ are the charges on the interacting points
+
+
+#### Electric field at distance from a charge
+
+$$E=k{Q \over r^2}$$
+
+### Electromagnetism
+
+#### Lenz's law
+- Right hand grip rule (relationship between directions of $I, F$)
+
+#### Solenoids
+- Coil around core (like a transformer but field is transferred to kinetic energy)
+
+#### Magnetic force on charged particles
+
+$$F=qvB$$
+
+where
+$v$ is the component of velocity which is $\perp$ to magnetic field
+
+#### Right hand slap rule
+
+
+**Field, current and force are all 90 degree to each other**
+<pre>
+force
+| / field
+| /
+|/ 90 de=
+ \
+ \ +ve charge
+</pre>
+
+Force is given by $F=nBIl$
+
+### Motors
+
+#### DC
+
+- current-carrying wire experiences magnetic force $F$ equal to $nBIl$
+- torque: $\tau = r_{\perp} F$
+- split ring and brushes