P1 4/5 Electricity Flashcards
state the equation that links charge, current, and time
charge = current x time
Q = I x t
(C) (A) (s)
define current
the rate of flow of charge
measured in Amps
define potential difference
-> the force pushing charge around a circuit
-> energy used to move charge around a circuit (work done)
-> energy transferred per unit charge to move charge between two circuit points
* the voltage/potential diff across two circuit points
* also known as voltage
* measured in volts using a voltmeter
state the equation linking potential diff, energy transferred, and charge
energy transferred = potential diff x charge
E = V x Q
(J) (V) (C)
describe the relationship between resistance and current
when resistance increases, current decreases
-> providing the potential diff is fixed
describe current, voltage and resistance in a series circuit
- current through each component is the same - all ammeters will have the same reading
- voltage across circuit is sum of voltages in all components
- total resistance of the circuit is the sum of the resistances of all components
-> resistance increases as no. components increases
describe current in a parallel circuit
- total current is the sum of the current through each branch
- current splits between branches in proportion to resistance
-> more current flows through the path of least resistance
describe voltage in a parallel circuit
- voltage along each branch is the same as the supply voltage
-> if pd. of battery is 5V, each branch will share 5V between its components
describe resistance in a parallel circuit
- resistance of the entire circuit is less than the branch with the lowest resistance
-> if one branch is 5Ω and one is 10Ω, the resistance of the circuit will be lower than 5
-> resistance decreases as more resistors are added
state the equation linking resistance, current, and potential diff
potential diff = current x resistance
V = I R
(V) (A) (Ω)
state Ohm’s law
- in ohmic conductors, voltage is directly proportional to current
-> provided all conditions (eg. temp) are fixed - shown on a graph by a straight line of pos gradient through the origin
describe a fixed resistor
- ohmic conductor as it follows Ohm’s law - the voltage is directly proportional to the current (provided all conditions eg. temp remain constant)
*maintains constant resistance regardless of potential diff - particles in a resistor give a constant obstruction to current
- current increases as potential diff increases - direct proportion
describe a fixed resistor graph
straight line of pos gradient through the origin
describe a filament lamp
- resistance increases as potential diff increases
- filament wire heats up so particles move faster and more collisions happen between electrons in moving current+atoms - increased resistance
- current increases less for higher potential differences - gradient becomes shallower
describe a filament lamp graph
initially straight line through origin then gradient becomes shallower (s shape curve)
describe a diode
- high resistance to negative potential diff so doesn’t allow current to flow
- resistance above 0.7V is constant and current can flow
- particles in a diode act as valves+ only allow charge to travel in one direction
- current is 0 until pd=0.7 then current is directly proportional to pd
describe a diode graph
flat line at 0 and negative pd. values, then gradient becomes very steep at 0.7, current is directly proportional to pd once pd. is higher than 0.7V
describe the general method for RP: I-V characteristics in fixed resistor/ filament lamp/ diode
(resistor + lamp / diode)
* set up apparatus as shown
* use variable resistor to ensure voltmeter reads (3.5V / 0.7V)
* measure current with ammeter
* repeat steps for voltages of (3 to 0.5V / 0.6 to 0.1V )
* switch terminals of the battery and repeat steps with negative pd. values
describe the circuit+apparatus used for RP: I-V characteristics in fixed resistor/ filament lamp/ diode
- FIXED RESISTOR: fixed resistor+ battery/ ammeter/ variable resistor connected in series, with voltmeter connected at points before+after the fixed resistor
- FILAMENT LAMP: lamp+ battery/ ammeter/ variable resistor connected in series, with voltmeter connected at points before+after the lamp
- DIODE: diode+ battery/ ammeter/ variable resistor +FIXED RESISTOR connected in series, with voltmeter connected at points before+after the diode
describe a thermistor +graph +uses
- resistance decreases as temp increases
- thermal energy helps particles line up/form regular arrangement so charge can move through more easily
- current increases as temp increases
- shown on graph as decreasing curve
- used in fire alarms, air-conditioning, (fridge) thermostats
describe a light dependent resistor +graph +uses
- resistance decreases as light intensity increases
- light energy helps particles line up/form regular arrangement so charge can move through more easily
- current increases as light intensity increases
- shown on graph as decreasing curve
- used in night lights, outdoor lighting eg. street lamps
describe the method for RP: how the length of wire affects resistance
- set up circuit: place a cell, ammeter+test wire in series then connect a voltmeter to places in the circuit before+after the test wire
- place jockey on test wire at length of 100cm
- measure the potential diff using voltmeter and current using ammeter
- calculate resistance = potential diff / current
- repeat steps for lengths 90cm-30cm in intervals of 10cm
-> wire resistance is directly proportional to wire length
describe sources of error, prevention and improvements for RP: how length of wire affects resistance
- wire gets hot which changes the temp of the wire:
start taking results at the longest length as wire would heat up less
connect resistance wire to the circuit by the jockey for the shortest possible time to minimise current flow
pause between readings to allow wire to cool down
alternative method - have resistance wire submerged in water to help regulate temp - results precision - improved by repeating experiment+ calculating mean results
describe the conclusion for RP: how length of wire affects resistance
- as the length of wire increases the resistance increases
- shown on graph as straight line passing through the origin
- the current flowing through the wire contains charge that carries electrons, which collide with atoms in the wire - the longer the wire, the more collisions between electrons+ atoms so increased resistance
describe how the national grid works
- voltage from a generator in a power station is transported to a step-up transformer
- the transformer increases voltage (from around 25,000V-700,000V) so the current decreases - less current means less energy disspiated through heating of cables to thermal energy store of surroundings
- transmission cables carry high voltage electrical power - above ground to keep people safe from electric shocks
- a step-down transformer reduce voltage to 230V for safe use in consumer homes/industry
function of a fuse in a circuit
- thin wire that melts if too large a current flows through it - causes circuit to break
- this protects electrical circuits
function of a resistor and variable resistor in a circuit
resistor: controls amount of current in a circuit
variable: adjusted to control the amount of current
function of a diode and a LED in a circuit
diode: allows current to flow in one direction - prevents damage to other components
LED: emits light when current flows correct way through it
describe direct current
- power supply provides a direct pd.
- so current (electrons) flow in one direction from + to - of battery
describe alternating current
- pd. constantly changes direction
- so current (electrons) constantly change direction
describe the mains electricity
- alternating current/ supply
- UK domestic supply has frequency 50Hz and 230V
(current alternates 50 times per second)
describe the live wire
- bRown insulation - right side
- carries alternating pd. from plug socket to appliance
- can be dangerous even when circuit is switched off
- connected to a fuse which melts/blows to disconnect the circuit if too large a current passes through it
describe the neutral wire
- bLue insulation - left side
- completes the circuit
- very close to earth pd. = 0V
describe the earth wire
- green and yellow striped insulation - top
- safety wire - brings current to the earth if circuit develops a fault
describe the purpose of double insulated appliances
- outer case made of insulating plastic
- eg. hair dryers, mobile phone chargers, hand whisks
- do not need an earth wire
- two layers of insulating material between the electrical parts+ outside parts that can be touched - insulating plastic case and the insulated cable carrying the live+neutral wires
- no direct contact with internal electrical parts
- shown by symbol containing an inner+outer square
name the three equations for power
work done (energy transferred) = power x time
W = P x t
(J) (W) (s)
electrical power = current x pd.
P = I x V
(W) (A) (V)
power dissipated = current² x resistance
P = I² x R
(W) (A) (Ω)
