Non-contact forces:
- strong nuclear force
- weak nuclear force
-- electromagnetic force (dipoles)
-- gravitational force (monopoles)
+- electromagnetic force
+- - electric fields (dipoles & monopoles)
+- - magnetic fields (dipoles only)
+- gravitational force (monopoles only)
+
+Gravitational & -ve electric monopoles - field lines radiate towards central object
+Magnetic & electric dipoles - field lines go from + to -, or N to S
---
- monopoles
- attractive force
- extends to infinite distance, but diminishes with inverse square law
+- charge produced by gravity = $GM$
### Work in a gravitational field
### Satellites
-## Electromagnetism
+## Magnetic fields
+
+### Characteristics
+- field lines always go from N -> S
+- dot means out of page, cross means into page
+- ${E_1 \over E_2}={r_1 \over r_2}^2$
+- flux: change in magnetic field
+
-### Electric fields
+## Electric fields
+
+### Characterisics
- surrounds +ve and -ve charges
- exerts force on other changes in its field
- can be constrained to a fixed distance (conductors / insulators)
- current flows from +ve to -ve
-#### Field lines
+### Field lines
- +ve to -ve
- start and end $\perp$ to surface
- field lines never cross
- point charges - radiate from centre
-#### Forces
+### Forces
$$F=qE$$
$q$ 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
+### Work in electric fields
$$W=qV$$
$q$ is the charge of point charge being acted on
$V$ is the potential (voltage) between points
-#### Coulomb's law
+### Coulomb's law
$$F=k{{q_1q_2}\over r^2}$$
$q_1$ and $q_2$ are the charges on the interacting points
-#### Electric field at distance from a charge
+### Electric field at distance from a charge
$$E=k{Q \over r^2}$$
-### Electromagnetism
-
-#### Lenz's law
+### Lenz's law
- Right hand grip rule (relationship between directions of $I, F$)
+- Eddy currents counter movement within a field
+- Represented by -ve sign in Faraday's law
-#### Solenoids
+### Solenoids
- Coil around core (like a transformer but field is transferred to kinetic energy)
-#### Magnetic force on charged particles
+### Magnetic force on charged particles
$$F=qvB$$
where
$v$ is the component of velocity which is $\perp$ to magnetic field
-#### Right hand slap rule
+### Right hand slap rule
**Field, current and force are all 90 degree to each other**
Force is given by $F=nBIl$
+
+### Faraday's law of induction
+
+$$\epsilon = -N{{\Delta \Phi_B}\over{\Delta t}}$$
+
+where
+$\epsilon$ is induced EMF (voltage)
+$N$ is the number of turns in the primary coil
+$\Phi_B$ is the magnetic flux (Wb or V / s)
+$\Delta t$ is the change in time for one cycle (can be derived from period or frequency)
+
+### Flux through coils
+$$\Phi_B = B_{\perp}A$$
+
+where
+$B_\perp$ is the field strength (Tesla)
+$A$ is the area of the field perpendicular to field lines
+
+if $B {\not \perp} A, \Phi_B \rightarrow 0$
+if $B \parallel A, \Phi_B = 0$
+
+- flux-time graphs ($t$ on $x$-axis): $\operatorname{gradient} \times n = \operatorname{emf}$
+
+
+**EMF is proportionate to change in flux**
+
+**Induced EMF opposes (counters) change in flux**
+
+### Transformer equation
+
+$${V_p \over V_s}={N_p \over N_s}$$
+$${I_p \over I_s}={N_s \over N_p}$$
+
+- core strengthens and "focuses" ac flux $\Phi$ through secondary coil
+
+
+### Root mean square
+
+$$V_{\operatorname{rms}} = {V_{\operatorname{p\rightarrow p}} \over \sqrt{2}}$$
+
+## Power transmission
+- 240 V / 50 Hz in Australia
+- higher voltages have lower $V_{\operatorname{loss}}$
+- ac is used because its voltage is easily changed with xfmrs
+
+### Safety
+- $\ge 30 \operatorname{mA}$ through heart is dangerous
+
+### Transmission $P_{\operatorname{loss}}$
+
+$$P_{\operatorname{loss}} = \Delta V I = I^2 R = {{\Delta V^2} \over R}$$
+
+where
+$R$ is the total resistance (derived from resistance per distance)
+
+To reduce power loss, use lower resistance (thicker) wires or increase voltage / reduce current with transformers
+
+
+
### Motors
#### DC