4 Electricity and magnetism Flashcards
equation for resistance
R = V/I
What is the relationship of the resistance of a metallic wire to its length?
Restistance is directly proportional to its length.
- as length increases the resistance increases.
What is the relationship of the resistance of a metallic wire to its cross-sectional area
resistance is inversely proportional to its cross-sectional area.
- As area increases the resistance decreases
equation for electrical power
P = Iv
equation for electrical energy
E = Ivt
potential divider what
splits p.d between two resistors in proportion to the resistances
current in series circuit
current at every point in a series circuit is the same
resistors in parallel circuits
the combined resistance of two resistors in parallel is less than that of either resistor by itself
1/Rt = 1/R1 + 1/R2 + 1/R3…
advantages of connecting lamps in parallel in a lighting circuit
the bulbs in the parallel circuit will be brighter than those in the series circuit
total resistance in series
Rt = R1 + R2 + R3…
the sum of currents entering a junction in a parallel circuit is…
equal to the sum of currents that leave the junction
Voltage in a parallel circuit
Voltage is the same in every branch
describe the action of a variable potential divider.
the resistor with the greatest resistance will have a larger potential difference than the others.
equation for two resistors used as a potential divider
R1/R2 = V1/V2
hazards of damaged insulation
if someone touches the exposed piece of wire, they could be lethally shocked
hazards of overheated cables
Passing too much current through too small of a wire can cause it to overheat. Leading to electrical fires.
hazards of damp conditions
if moisture comes into contact with live wires, the moisture could conduct electricity either causing a short circuit within a device (which could cause a fire) or posing an electrocution risk
what does a mains circuit consist of
live wire, a neutral wire and an earth wire
why must a switch be connected to the live wire for the circuit to be switched off safetly
so that the appliance is always at zero potential difference
live wire
copper wire coated with brown plastic along which the current enters the device. It’s carry the power from the source to a switch or appliance.
way to remember color
(live = earth = dirt = brown)
fuse
a glass or ceramic canister containing a thin wire that melts if the current gets too high
neutral wire
copper wire coated with blue plastic that also connects to the cable in the wall and completes the circuit. It carries the circuit back to the original power source
(neutral - feeling slightly blue)
earth wire (not needed)
copper wire coated in striped plastic that provides a path for current to flow from the case of the device to the ground if there is a fault
(earth - sun and earth - yellow and green)
use and operation of trip switches
Basically fuses: when too much current passes through a given circuit the switch is ‘tripped’ and the circuit is broken
How to choose appropriate fuse ratings and trip switch settings
Find the amps of the circuit then multiply that by 125%
why must the outer casing of an electrical appliance must be either non-conducting (double insulated) or earthed?
Double insulation protects the user of the appliance from an electrical shock by preventing any possibility of the external casing becoming live
what protects the circuit and cabling for a double-insulated appliance?
a fuse without an earth wire protects the circuit and the cabling for a double insulated appliance
