Electricity Flashcards
How do you know which way current flows from a cell/battery diagram?
flows in the direction which the longer line (+ve terminal) is e.g https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQIejczZ-QXezZDKqaLgBNUddh2pQyMgHEx0A&usqp=CAU
What is current? How can we work this out?
-the rate of flow of charge
I= Q/t
where:
I= current (A)
Q= charge (C)
t= time (s)
What do we mean by convectional current?
when current is +ve coming from the +ve terminal to the -ve, moving in the opposite direction to e- flow (as e- always move away from -ve terminal to positive)
What is potential difference? How can we work this out?
-the work done per unit charge
V=W/Q
where:
V= p.d. (V)
W= work done (J)
Q= charge (C)
What is resistance? What equation can use to work this out?
-the opposition to the flow of charge
R=V/I
where:
R= resistance (Ω)
V= potential difference (v)
I= current (A)
What are the symbols for a diode and a light emitting diode (LED)? What do they do?
diode: allows current to only flow in one direction
https://hightechlabtz.files.wordpress.com/2015/08/screen_shot_2013-05-16_at_115737-13ead003ff403ffe69d.png?w=333&h=188
LED: emits light when current flows through it
https://keystagewiki.com/images/thumb/4/4f/LightEmittingDiodeSymbol.png/300px-LightEmittingDiodeSymbol.png
What is a thermistor? What is its symbol?
-a resistor that changes value with the temperature
-increase temp, resistance decreases
https://upload.wikimedia.org/wikipedia/commons/thumb/1/10/Thermistor.svg/120px-Thermistor.svg.png
What is a light dependent resistor (LDR)? What is its symbol?
a resistor that changes with the light intensity that falls upon it
-increase LI, resistance decreases
What is a capacitor? What is its symbol?
a capacitor is a circuit component that temporarily stores electrical energy through distributing charged particles on plates to create a potential difference
https://d1hj4to4g9ba46.cloudfront.net/questions/917774_523371_ans_2f2fe057d5cd41959777ce06a5b2a8e1.png
What is Ohm’s law? What is the equation representing it?
for a metallic conductor at a constant temperature, the current through it is proportional to the potential difference across it
V= IR
where:
V= potential difference (V)
I= current (A)
R= resistance (Ω)
Draw the graph for Ohm’s law (for a resistor)
https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2020/09/9.2.5-Ohms-law-graph.png
Draw an I-V graph showing the characteristic for: a diode (including LED), an ohmic resistor and a filament bulb
https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2020/09/9.2.3-IV-conductor-and-diode-graphs.png
How would the I-V graphs show resistance for a thermistor and a LDR?
thermistor:
-the line would be the temperature instead of resistance
-if the thermistor was a NTC (-ve temp coefficient) then if you increase T, R decreases
-if it was a PTC (+ve temp coefficient) then if you increase T, R increases)
LDR:
-the line would be light intensity instead of resistance
-if light intensity increases then the resistance decreases
Describe the shape for the I-V graph of a filament bulb when the p.d is small. Why does increasing the current increase the resistance?
-when p.d. is small, the wire obeys Ohm’s law (I-V is directly proportional)
-as current flows through the lamp, electrical energy is converted into heat energy so the metal ions vibrate with increased amplitude
-this leads to an increase rate of electrons backscattering which therefore increases resistance
What do we mean by resisitivity?
a measure of how strongly a material opposes the flow of electrical current
-material dependant
How can we work out resisitivity?
How can we work out cross-sectional area of a wire?
(πD^2)/4
(pi x diameter^2)/4
Draw a R-T graph for a thermistor and label it
What are some uses of thermistors?
used in circuits in ovens, fire alarms and digital thermometers
What is an insulator and how does it work?
-a material in which electric current does not flow freely
-e- are bound to atoms which are localised and aren’t free to move
What is a metallic conductor and how does it work?
metals that conduct electricity
-some e- are delocalised, these are conduction electrons which flow when a voltage is applied
What is a semiconductor and how does it work?
materials which have a conductivity between conductors and non-conductors or insulators
-the number of delocalised charge carriers increases as you increase temp or light intensity making them better conductors (as if they increase, R decreases)
What is a superconductor?
a material with no resistance below a critical temperature
What do we mean by the critical temperature of a superconductor?
the temperature at which a material becomes superconducting
Draw and label the graph of resistivity and temp for a superconductor and a normal metal
https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/04/5.2.3-Superconductivity-Graph.png
What are some uses of superconductors?
-the production of strong magnetic fields (e.g. maglev trains, MRI scans, confinement in particle accelerators)
-the reduction of energy loss / dissipation in the transmission of electric power
Resistivity
RP5: describe this practical
determine the resistivity of a 2 metre constantan wire
Resistivity
RP5: describe the graph and give the y=mx equation
R= ρ/A x L
graph: resistance and length are directly proportional
How do we find the total resistance from resistors in series?
Rt= R1 + R2 + R3+…
How do we find the total resistance from resistors in parallel?
1/Rt= 1/R1 + 1/R2 +1/R3+…
then once you find out 1/Rt, make it the reciprocal to find Rt
What is the current for all components in a series circuit?
the same for all components
It= I1 = I2 = I3 =…
What is the current for all components in a parallel circuit? What does the amount given out to components depend on?
-the current is split across the different branches (or junction)
-the total current into a junction must equal the total current out of a junction
-the amount of current in each branch depends on the total resistance of the components within that branch
It= I1 + I2 + I3 +…
What is the p.d. for all components in a series circuit?
-the emf of the power supply is shared amongst all the components in different amounts, depending on their resistance
Vt= V1+ V2 +V3 +…
What is the p.d. for all components in a parallel circuit?
-the voltage of all the components in each branch is equal to the e.m.f of the power supply
Vt= V1 = V2 = V3 =…
What is the p.d. of cells in series?
All p.d. of cells in series are combined
What is the p.d. of cells in parallel?
All p.d of cells in parallel are the same, they do not combine
What must always be conserved in a circuit? What are these rules called?
Charge, energy (therefore current and emf must be conserved)
-kirchoff’s first law (charge/current)
-kirchoff;s second law (energy/emf)
What is Kirchoff’s first law? Give an example
at any junction in a circuit, the total current leaving the junction, is equal to the total current entering a junction
-https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2020/09/10.1.2.1-Kirchoffs-first-law.png
What is kirchoff’s second law? Give an example
around a given loop, the sum of the emf is equal to the sum of the p.d. dropped
-https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2020/10/10.1.2.2-Kirchoffs-second-law-in-a-series-circuit.png
-https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2020/09/10.1.2.2-Loops-in-parallel-circuit.png
What is electrical power? How can we work this out?
rate of change of work done
P= W/t
where:
P= power (W)
W= work done (J)
t= time taken (s)
How can we find the power dissipated by an electrical device?
P= IV
where:
p= power (W)
I= current (A)
V= p.d. (V)
What are the other two equations to work out power using resistance?
P= I^2 x R P= V^2 / R
where: where:
P= power (W) P= power (W)
I= current (A) V= p.d. (V)
R= resistance (Ω) R= resistance (Ω)
What is a potential divider?
circuits which produce an output voltage as a fraction of its input voltage
What are potential dividers used for?
-to provide a variable potential difference
-to enable a specific potential difference to be chosen
-to split the potential difference of a power source between two or more components
Give some examples of where potential divider’s are used in
used in volume controls and sensory circuits using LDRs and thermistors
What is the equation finding the voltage given out from a potential divider?
https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2020/09/10.2.1-Potential-divider-diagram-and-equation.png
What affects the p.d. given out in a potential divider circuit?
-the resistor with the largest resistance will have a greater potential difference than the other one from V = IR
(whilst the other resistor will get a smaller share)
-in potential divider circuits, the p.d across a component is proportional to its resistance from V = IR
-POTENTIAL DIVIDERS ALWAYS SHARE THE P.D. FAIRLY
What type of resistors are used in potential divider circuits? What must happen to other resistors in the circuit if these specific resistors have a change in resistance?
-variable and sensory e.g. thermistors and LDRs
https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2020/09/10.2.4-Sensory-potential-divider-circuits.png
REMEMBER: p.d. is proportional to R (R changes depending on light intensity for an LDR and temp for a thermistor)
-Since the total p.d of the components must be equal to p.d. given in, if the p.d of the sensory resistor decreases then the p.d of the other resistor in the circuit must increase and vice versa
What do we mean by emf?
the total/maximum voltage available to the circuit
How can we work out the emf of a power supply?
ε= E/Q
where
ε= emf (V)
E= energy transferred (J)
Q= charge (C)
How can emf be measured from a circuit?
by connecting a high-resistance voltmeter around the terminals of the cell in an open circuit
What do we mean by the terminal p.d.?
the potential difference across the terminals of a cell
When does the terminal p.d. equal the emf?
when there is no current flowing and no internal resistance
Why is the emf higher than the terminal p.d. when current flows?
-a potential difference develops across the internal resistance
-this p.d. dropped across the internal resistance is called the ‘lost volts’
-this as a result lowers the terminal p.d.
What do we mean by lost volts?
the p.d. from the emf needed to overcome the internal resistance inside the battery (when current flows)
How can we work out the terminal p.d?
V= ε - v
where:
V= terminal p.d. (V)
ε= emf (V)
v= lost volts (V)
What equations can we use to work out the lost volts from a cell/battery?
v= Ir
v= ε - V
where:
v= lost p.d. (V)
I= current (A)
ε= emf (V)
V= terminal p.d. (V)
How else can we work out emf from the internal resistance?
https://cdn.savemyexams.co.uk/cdn-cgi/image/w=1920,f=auto/uploads/2021/04/5.4.1-EMF-equation.png
What do we mean by internal resistance?
the resistance of the materials within the battery
emf and internal resistance
RP6: describe this practical
investigate the relationship between e.m.f and internal resistance by measuring the variation of current and voltage using a variable resistor
emf and internal resistance
RP6: describe the graph. What is the y=mx equation?
-negative correlation graph where the p.d. and current are inverse to each other ( p.d. increases, current decreases)
V = –rI + E
where:
V= p.d. (V)
I = current (A)
-r = –internal resistance (Ω)
E = emf (V)
