electricity Flashcards
draw the circuit symbol for a cell
draw the circuit symbol for a battery
draw the circuit symbol for a lamp
draw the circuit symbol for a fuse
draw the circuit symbol for a voltmeter
draw the circuit symbol for a ammeter
draw the circuit symbol for a diode
draw the circuit symbol for a resistor
draw the circuit symbol for a thermistor
draw the circuit symbol for a variable resistor
draw the circuit symbol for a LDR
draw the circuit symbol for a LED
what is electric current?
the flow of electrical charge
state the equation linking charge, current and time. give units for the quantities involved
Q = I t
charge (coulombs), current (amperes), time (seconds)
what can be said about the value of current at any point in a single closed loop?
current is the same at all points in a closed loop
what two factors does the current in a circuit depend on?
-potential difference (V)
-resistance (R)
what equation should be used to calculate potential difference if current and resistance are known? state the units for all three quantities
V = I R
potential difference (V), current (A), resistance (Ω)
what is an ‘ohmic conductor’? state the condition required
-a conductor for which current and potential difference are directly proportional
-resistance remains constant as current changes
-temperature must be constant
list four components for which resistance is not constant as current changes
-lamps
-diodes
-thermistors
-light dependent resistors (LDRs)
what happens to the resistance of a filament lamp as the temperature increases? why?
-resistance increases
-ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow
what is different about current flow through a diode?
-the current only flows in one direction
-resistance is very high in the other direction, preventing current flow
state what happens to the resistance of a thermistor as temperature increases
the thermistor’s resistance decreases
give two examples of when a thermistor may be used
-in a thermostat to turn a heater on below a certain temperature
-in a freezer to turn on a cooler when the temperature becomes too high
state what happens to the resistance of a LDR as light intensity decreases
the LDR’s resistance increases
give an application for a LDR
-street lights often use LDRs
-when light levels become too low, the light gains sufficient current to turn on
what are the two ways that a component can be connected in a circuit?
-series (same loop)
-parallel (adjacent loop)
how does the potential difference across two components vary when connected in series and parallel?
-series: total p.d is shared between each component
-parallel: p.d across each component is the same
if two resistors are connected in parallel, what can be said about their combined total resistance?
their total resistance is less than the smallest of the two individual resistances
if two resistors are connected in series, what can be said about their total resistance?
their total combined resistance is equal to the sum of the two individual resistances
describe the current in a series circuit
in a series circuit, the current is the same at all positions since the charge only has one path to flow through
describe the current in a parallel circuit
in a parallel circuit, the current is shared between the different branches. when the charge reaches a junction it splits
how should you connect an ammeter in a circuit to measure current?
ammeters should be connected in series with the component that they are measuring current through
how should you connect a voltmeter in a circuit to measure potential difference?
voltmeters should be connected in parallel to the component that they are measuring the potential difference of
why is it advantageous to connect lamps in parallel?
if one lamp blows, the rest will be unaffected and can still receive current (i.e. the circuit is still complete)
is mains electricity an a.c supply or a d.c supply? what do each of these stand for?
mains electricity is an a.c supply
a.c - alternating current
d.c - direct current
define alternating current and direct current
-alternating current: current that continuously changes direction at a specific frequency
-direct current: one directional current flow
what is the frequency and voltage of the UK mains electricity supply?
-frequency: 50 Hz
-voltage: 230V
how many wires are usually in the cables connecting electrical appliances to the mains? name these wires
-live wire
-neutral wire
-earth wire
state the insulation colour used on the earth wire
green and yellow stripes
state the insulation colour used on the live wire
brown
state the insulation colour used on the neutral wire
blue
explain when the earth wire does and doesn’t carry a current
-under normal circumstances, no current flows through the Earth wire
-if a fault occurs in the appliance (such as a surge or the casing becoming live), current will flow to the ground
what potential is the neutral wire at?
0 volts
state the potential difference between the live and earth wires
230 volts
what is the purpose of the neutral wire?
to complete the circuit by connecting the appliance back to the mains supply
for metal appliances, where is the earth wire connected to? why?
-earth wire is connected to the metal casing of the appliance
-if live wire becomes loose and touches the casing, the current will flow through the Earth wire, preventing electrocution
state two equations for the power of a circuit. give appropriate units
P = I V
P = I^2 R
power (watts), current (amperes), potential difference (volts), resistance (ohms)
state an equation linking energy transferred, power and time. give appropriate units
E = P t
energy (joules), power (watts), time (seconds)
state an equation linking energy transferred, charge flow and potential difference. give appropriate units
E = Q V
energy (joules), charge (coulombs), potential difference (volts)
what two main factors does the amount of energy transferred by an appliance depend on?
-how long the appliance is being used for
-the power of the appliance
describe the energy transfers in a battery powered torch
-battery converts chemical energy into electrical energy
-bulb converts electrical energy into light as well as waste energy in the form of heating
describe the energy transfers in a battery powered motor
-battery converts chemical energy into electrical energy
-motor converts electrical energy into kinetic energy as well as waste energy in the form of heating due to friction
what three things determine the power of a circuit device?
-the potential difference across the circuit
-the current through the circuit
-the amount of energy transferred in a given time
what is the purpose of the national grid?
to link power stations to consumers so that they have access to a source of electricity
what are the two types of transformers used in the national grid?
-step-up transformers
-step-down transformers
where are step-up transformers found in the national grid? what do they do?
-step-up transformers are used when connecting power stations to transmission cables
-they increase the potential difference
where are step-down transformers found in the national grid? what do they do?
-step-down transformers are used in connecting transmission cables to domestic buildings (like houses)
-they decrease the potential difference
why do transmission lines transfer electricity at high potentials?
-a high potential, results in a low current
-the lower the current, the less energy that is wasted as heat
-therefore it is more efficient
why does the potential need to be decreased between transmission lines and houses?
-lower potentials are safer for domestic use and reduces the likelihood of severe electrocution
-appliances are designed for 230V
outline the basic steps of investigating how a wire’s resistance depends on its length
-set-up circuit
-attach resistance wire to metre rule
-move the crocodile clip along the wire in increments, taking measurements for voltage and current each time
-calculate the resistance for each length
what two types of meters are needed in the circuit, and how should they be connected?
-voltmeter: connected in parallel to the resistance wire
-ammeter: connected in series with the resistance wire
what equation is used to obtain the resistance from the current and voltage readings?
R = V / I
what should the graph of resistance against length look like?
the two variables are directly proportional so should produce a straight line going through the origin
in reality, why might the line produced not pass through the origin?
there may be a zero error. this may be because it is hard the attach the stationary crocodile clip exactly at the zero end of the metre rule
why should the wire used be thin?
-for a given material, the thinner the wire, the higher the resistance
-this makes the recorded resistance values large enough to be measured and interpreted
why should a low potential difference be used when carrying out this experiment?
to prevent the current getting too high and the wire heating up
what is the issue if the wire heats up during the experiment?
resistance of a wire will increase if the temperature increases, creating an extra variable which will interfere with the results. temperature should remain a control variable
why does resistance increase with temperature?
the metal ions have more kinetic energy so vibrate more. this causes them to collide more frequently with current-carrying electrons, providing more resistance against the flow of electrons
what safety precautions should be taken when carrying out this experiment?
-avoid touching the wire since it may be very hot
-keep potential difference values low to avoid overheating
draw two resistors in series
draw two resistors in parallel
if both resistors are identical, which combination of resistors will have the least resistance?
the resistors in parallel will have a lower resistance than the resistors in series
relative to the resistance of the resistors, describe the total resistance of the parallel combination
the total resistance will be lower than the resistance of the smallest of the two individual resistances
outline the basic steps of the I-V characteristics practical
-set-up circuit with lamp/resistor/diode
-alter the potential difference in regular increments
-record the current for each p.d
-plot a graph of current against p.d
-repeat for the other two components
what two types of meters are needed in the circuit, and how should they be connected?
-voltmeter: connected in parallel to the component
-ammeter: connected in series with the component
what component is required to alter potential difference if you’re not using a variable power pack?
a rheostat or variable resistor
what is an ‘ohmic conductor’? state the condition required
-a conductor for which current and potential difference are directly proportional
-resistance remains constant as current changes
-temperature must be constant
what specific type of ammeter may be required in this experiment and why?
a milliammeter since the currents and current changes involved may be quite low
what must also be present in the circuit when the diode is being tested? where should it be connected?
a protective resistor should be connected to prevent the current levels getting too high. It should be connected in series with the diode
for which component does the polarity of the power supply matter and why?
the diode, since diodes only allow current to flow through in one direction
what must be kept constant to get reliable results? how can you achieve this?
temperature should remain constant so that the resistance of the components isn’t affected. you should disconnect the supply when not taking readings to avoid unnecessary heating
how do you determine the resistance of a component from an I-V graph?
the resistance at a given point, is the inverse of the gradient of the line drawn from that point to the origin. work out the gradient and use 1/gradient to obtain the resistance
what would you expect the I-V graph of a resistor to look like?
what would you expect the I-V graph of a diode to look like?
what would you expect the I-V graph of a filament lamp to look like?
what happens to the resistance of a filament lamp as the temperature increases? why?
-resistance increases
-ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow
