Booklet 9: Energy, Power and Resistance Flashcards
Potential Difference
the energy transferred per unit charge from electrical to other forms
potential difference = energy lost by charge / charge
V (V) = W (J) / Q (C)
Electromotive Force
the energy gained per unit charge, from other forms to electrical
e.m.f = energy gained by charge / charge
E (V) = W (J) / Q (C)
Volt
the potential difference between two points when 1J of work is done to move a charge of 1C
2 Factors Affecting Size of Current
eg. lamp
the p.d. or voltage across the lamp- greater the voltage, greater the current
the resistance of the lamp- the greater the resistance, the smaller the current
Resistance
the ratio of the voltage to the current
R=V/I
Resistance (Ω) = potential difference (V) / Current (I)
also remember V=IR
Ohm
the resistance of a component when a potential difference of 1V drives a current of 1A through it
Ohm’s Law
for a metal/Ohmic conductor at constant temperature, the current on the conductor is directly proportional to the potential difference across its ends
straight line through origin of I-V graph
Resistor IV Characteristics
straight line through origin I ∝ V, Ohmic conductor so R is constant same characteristics if current is forwards or backwards steep gradient = lower resistance shallow gradient = higher resistance
Filament Lamp IV Characteristics
passes through origin
higher voltages = line starts to curve, non-Ohmic (no constant proportionality between I and PD)
higher resistance = higher current because greater number of electrons knock more into the metal lattice, more knocking means more resistance because more vibrating/kinetic energy/temperature
Diode or LED IV Characteristics
graph is at zero then goes up past the origin
one-way valve, allows conventional current through but not a current in any other direction
NTC IV Characteristics
temperature up, resistance down
similar to Filament Lamp IV graph, except higher
Resistivity
a property of a material which is a measurement of its resisting power
or
the product of resistance (of a component made of the material) and its CSA divided by its length
ρ=RL/A
resistivity (Ωm) = Resistance (Ω) * length (m) / CSA (m^2)
Resistance-Length Graph
gradient = ρ/A
ρ=gradient * A
Electrical Power Equations
(using P=VI and V=IR)
P=I^2 * R
P=V^2 / R
Energy (Electricity)
since power = voltage x current and energy = power x time
energy = voltage x current x time
