4.2
electrical quantities
4.2.1 electric charge
there are two types of electric charge: positive and negative.
- like charges repel each other
- unlike charges attract each other
electrostatic charge can be produced by friction.
- electrons are transferred between objects
- only negative charge moves
charge is measured in coulombs (c).
electric fields
an electric field is a region where a charge experiences a force.
- field direction is the direction of force on a positive charge
- fields exist around charged objects
4.2.2 electric current
electric current is the flow of electric charge.
definition:
i = q / t
- current is measured in amperes (a)
- charge is measured in coulombs (c)
- time is measured in seconds (s)
conduction in metals
electric current in metals is due to the movement of free electrons.
current types
- direct current (d.c.) flows in one direction
- alternating current (a.c.) changes direction
- conventional current flows from positive to negative
4.2.3 electromotive force and potential difference
electromotive force (e.m.f.)
e.m.f. is the work done by a source per unit charge.
e = w / q
potential difference (p.d.)
p.d. is the work done per unit charge between two points.
v = w / q
- both are measured in volts (v)
- voltmeters measure potential difference
4.2.4 resistance
resistance is the opposition to the flow of current.
r = v / i
- measured in ohms (Ω)
- higher resistance means lower current
for a metallic conductor:
- resistance increases with length
- resistance decreases with cross-sectional area
4.2.5 electrical energy and electrical power
electrical energy is transferred from sources to components.
electrical power
p = iv
electrical energy
e = ivt
- energy is measured in joules (j)
- power is measured in watts (w)
the kilowatt-hour (kwh) is a unit of electrical energy.
used to calculate the cost of electricity.