5.2 Electric Circuits Flashcards
Law of Conservation representted by Kirchoff’s Laws
- Conservation of CHARGE: total current into a junction = total current out of a junction
- Conservation of ENERGY: loss in electric potential = gain in electric potential in a closed loop
Diode
allows current to flow in one direction only; used to convert AC into DC
Fuse
melts and breaks the flow of current;
used to prevent overheating/fire
When electrons flow through a component in the circuit (e.g. a resistor)…
- electrons transfer electric potential energy to the component
- component transforms electric potential energy into other forms of energy (eg. thermal, light)
measuing the potential difference across a component =
measuring the difference in the electric potential energy of electrons before they enter the component and after they leave it
Ohm’s Law
V = IR
For a conductor at a constant temperature, the current through it is proportional to the potential difference across it
the gradient of an ohmic resistor’s I-V graph =
gradient = I/V = 1/R
What happens to the I/V graph of an ohmic resistor?
straight line through origin (since I is directly proportional to V)
What happens to the I/V graph of an filament bulb?
- Middle section is straight and passes through origin (ohmic)
- Gradient decreases as V increases → more I flows → temp increase → R increase → I increases at a slower rate (non-ohmic)
What happens to the I/V graph of an diode?
- increasing gradient on RHS because diode is forward biased
- zero current on LHS because diode doen’t conduct
resistivity
ρ = RA/L
resistance per unit length of a material with unit cross-sectional area
Wires are made from what material? Why?
copper: good conductor, low resistivity at room temperature
In a series circuit:
- same current throughout
- split p.d. depending on each component’s resistance
In a parallel circuit:
- same p.d. across all loops
- total current is sum of the currents in each parallel branch
advantage of parallel circuit
- single power source supplies all with same p.d.
- if one breaks, current can still flow through the rest
explain the heating effect of current
electrons flow through the conductor
–> collide with ions within the metal conductor, making them vibrate more –> heat up
for a given resistor, if the current or voltage doubles, the power dissiplated will
x 4
(because P = IV = I^2R=V^2/R)
The main purposes of a potential divider are:
- To provide a variable potential difference
- To provide a specific potential difference
- To split the potential difference of a power source between two or more components
potential divider equation:
V out = (R2/R1+R2) (V in)
If the resistance of one of the resistors is increased
it will get a greater share of the potential difference, whilst the other resistor will get a smaller share (V=IR)
primary vs secondary cell
primary: non-rechargable, chemical used up, disposed after use; electrons flow from - to +
secondary: rechargable (chemical reaction reversible); during recharge, electrons are forced from + to - by an external current
capacity of a cell
the amount of charge that it contains AND is able to discharge
Lamp A is connected to a 240 V supply and lamp B is connected to a 12 V battery. Both lamps have the same current, yet 240 V lamp glows more brightly. Explain in terms of energy transfer
Voltage is energy transferred per coulomb charge.
Since the lamps have the same rate of flow of charge (current), the energy transferred to each coulomb of charge in the 240 V lamp is 20 times greater than for the 12 V lamp.
explain the difference between p.d. and e.m.f
both are energy transfered per unit charge, but the type of energy transfer is different
- p.d. is the energy transfered from electrical to other forms per unit charge
- (eg. when charges pass through a resistor, their electrical energy is converted to heat, creating a potential different across the resistor)
- e.m.f is the energy transfered from other forms to electrical per unit charge
- (eg. when charges pass a power supply, the chemical energy in power supply is converted to electric energy and transfered to the charges)
electromotive force (emf)
power supplied per unit current / energy supplied per unit charge; in moving charge completely around the circuit
Explain why the headlights of a car may go slightly dimmer when the engine is started.
the starter motor needs more I → increased I increases lost volts (Ir, where r is internal resistance)
Explain why a high-voltage supply for use in school demonstrations has a very large internal resistance included inside it.
if someone touches and current flows, the p.d. will immediately drop to a very low value as Ir is so large
electronvolt (eV)
work done to move 1 electron through a p.d. of 1V
electric potential difference
energy per unit charge; when a positive test charge is moved between two points
electric current
the rate of flow of charge past a given cross-section
how do you determine the energy stored/transferred by a constant current from a discharging graph (V-t)
area under graph = Vt
energy = VIt = current x area
The current needed to recharge a secondary cell must
flow in the opposite direction to the natural current flow
