A battery is two or more cells put together.
The longer end represents the positive
terminal, the short represents negative
P.d and E.M.F
One Volt is the p.d across a component when 1J of
energy is transferred per unit charge
1V=1JC^-1
P.d is work done by the charge carriers per unit of charge. The difference in
energy before and after a point. Electrical energy transferred into other forms.
E.m.f is work done on the charge carriers per unit charge, by
a cell or battery. The charge carriers gain electrical energy
The formula is
the same as the
one above
The electron gun
A small metal filament is heated until some elctrons get
enough kinetic energy to escape the surface of the metal,
in a process called thermionic emission.
This is placed in a vacuum, with a high, accelerating
p.d between the cathode and anode
The freed electrons are accelerated towards the anode, gaining
If there is a small hole in the anode, a fine beam of electrons is created.
As electrons accelerate, they gain kinetic energy.
P.d=eV, V being the accelerating p.d.
Work done on electron = gain in kinetic energy
eV=1/2mv^2
Resistance
Resistance is the ratio of p.d
Across a component/current
in a component
The unit is the ohm, were 1Ω=1VA⁻¹
Ohms law states: For a conductor kept at a
constant temperature, current in the wire is
proportional to the p.d across it
When a wire gets hotter, resistance increases and therefore current
decreases. This is because positive ions vibrate with a greater
amplitude, increasing the frequency of the collisions. The electrons
transfer more energy, which is given off as heat.
I-V characteristics
The circuit shown left is used to
collect data for an I/V graph, but the
fixed resistor can be any component
A filament lamp is non ohmic, since p/d
isnt proportional to Current. The
resistance of the lamp is also not constant
Resistance increases as the p.d across it increases, since
the wire increases with temperature until it glows.
Fixed resistors have constant resistance, regardless
of temperature. Voltage is proportional to
temperature, therefore they follow ohms law
Shallower line = higher resistance, since
R=Inverse of gradient (R=V/I, not I/V)
Diodes
A diode only allows current to flow in one particular
direction (shown by the way the 'arrow' faces)
Some diodes are light emitting diodes, shown
by two arrows. They function the same way as
normal diodes, but light up when current flows
A diode is non ohmic, and it's behaviour depends on the polarity
The I/V graph for a diode is shown left. When p.d
is negative, resistance is almost infinite. No
current can flow the wrong way through a diode
When positive, current doesn't flow initially. A
threshold p.d has to be reached (this is the point
and L.E.D starts emitting light, the threshold p.d
determines the colour/frequency of light)
From this point, for every small increase in p.d,
resistance drops sharply.