electricity Flashcards
what is resistance
how much a component resists the flow of charge carriers through it
p.d/current
What is an ohm
The resistance of a component when a p.d of 1v is produced per ampere of current
What is ohms law
For a metallic conductor kept at a constant temperature, the current in the wire is directly proportional to its p.d
Resistance of a wire is directly proportional to..
length
What is a thermistor
what happens to resistance when temperature increases
Semiconductor with a negative temperature coefficient (resistance drops as temperature increases) - non ohmic
rules about resistance in series and in parallel
series
Rt= R1 + R2 + R3
parallel
1/Rt = 1/R1 + 1/R2 + 1/R3
kirchoffs second law
In any circuit, the sum of the electromotive forces is equal to the sum of the p.ds around a closed loop.
in a series circuit the current is…
always the same
how is emf shared around a series circuit
shared across the components. if components have the same resistance then the p.ds will be shared equally. if different the component with greater resistance will have a greater proportion of the emf
in parallel circuit the e.m.f is
equal no matter how many branches there are
what is lost volts?
Lost volts is when the p.d measured at the power source is less than the actual e.m.f . This is due to not all the energy transferred to the charge is available for the circuit, as some is transferred into heat.
EMF=
terminal p.d + lost volts
how does increasing current affect lost volts
increasing current means more work is done by the charges, increasing the lot volts - lowering the terminal p.d.
what is a potential divider
varies the p.d to give any value up to the maximum e.m.f of a circuit
what is a potentiometer
variable resistor with 3 terminals and a sliding contact - adjusting this varies the Vout
diiference between p.d and e.m.f
Potential difference is the measure of energy (work done) by charge carriers
EMF is when work is done on the charge carriers
what is a volt
the p.d \acroos a component when 1j of energy is transferred per unit charge
1v= 1JC^-1
explain an electron gun
Electrons heated on a hot filament until they have enough energy to escape the surface (thermionic emission).
As there is accelerating p.d between the cathode and the anode the electrons speed towards it in a line- this creates a beam of electrons going through the hole all with similar kinetic energy
why does when a wire get hotter, its resistance increases
As temperature increases, positive ions inside the wire have more internal energy and vibrate more intensely , this means collisions increase and charge carriers do more work and transfer more energy
relationship between resistance and length
resistance is directly proportional to length
resistance and temperature in a thermistor
why
as temp increases, resistance decreases
This is because an increase in temperature increases the number density of electrons.
Kirchhoff second law
In any circuit, the sum of the electromotive forces is equal to the sum of all the p.ds
what is a potentiometer
variable resistor with 3 terminals and a sliding contact. Adjusting the contact varies the p.d giving a variable Vout
