Section 2: Electricity Flashcards
Current
Rate of flow of charge (usually electrons) around a complete circuit
Current is measured in amps (A) with an ammeter in series
Voltage
potential difference
Energy carried around a circuit by electrons
v = amount of energy transferred per coulomb of charge
1 v = 1 joule per coulomb
Measured in volts (v) with a voltmeter in parallel
Current and electrons
Current is always from positive to negative
Electrons in a circuit always flows away from the negative terminal to the positive
Battery voltage
Sum of voltage of all other components
Draw symbols for:
- cell/battery
- switch
- ammeter
- voltmeter
- bulb
- LDR
- thermistor
- resistor
- variable resistor
check it
Equation for current
current = charge / time I = Q/t
Equation for voltage
voltage = energy / charge V = E/Q
Resistance
Resists the flow of the current
Measured in ohms Ω
Light dependent resistor
Resistance decreases as light increases, depends on light levels
- In bright light, resistance is low, more current can flow through
- In darkness, resistance is the highest
Useful for various electronic circuits e.g automatic night light and burglar detectors
Equation for resistance
resistance = voltage / current
Ω V A
Equation for power
power = voltage x current
(watts)
or joules per second
Thermistor
Resistance decreases as temp increases, depends on temp
- low temp, resistance high
- high temp, resistance low
Useful for temp detectors e.g fire alarms, thermostats
2 things for a circuit to work
- Complete circuit
wires runs from battery all the way - No short circuits
no way past a lamp going through other components
(otherwise lamp will not light)
Series Circuit
-current and voltage
- All components are connected in a line, end to end
- If one component is removed the circuit is broken and all stops working
-Current is the same everywhere A1 = A2 = A3
-Potential difference is shared between all components
V = V1 + V2
voltage round a series circuit adds up to voltage source
-Total resistance is the sum of all individual resistors
R = R1 + R2 + R3
- Cell voltages add up
e. g two 1.3 V cells in series would supply 3V in total
Parallel Circuit
-current and voltage
- Each component separately connected to the battery
- If one component is removed it doesn’t affect all others
-Current is shared between branches
A = A1 + A2
-There are junctions where current split or rejoins, total current into junction = leaving junction
-Total current = total current of separate branches
-Potential difference the same across all components
V1 = V2 = V3
-All components gets the full p.d, so identical bulbs are the same brightness
- Resistance is always less than that of the branch with the smallest resistance
- Resistance lower because current has more than one branch to take
- A circuit with 2 resistors in parallel will have a lower resistance than a circuit with either of the resistors by themselves
Current - voltage graph for resistors
V-I graphs
-Current through a resistor (at constant temp) is directly proportional to the voltage (Ohms law)
- Steeper the slope, the lower the resistance
R = V / I
-x-axis : potential difference
-y-axis : current
- straight line through, x = y
(see graph)
V - I graph for bulbs
- Bulbs do not follow ohms law
- The resistance increases as the temp increases, so it’s not directly proportional
- x-axis : potential difference
- y-axis : current
- ’s’ shape
(see graph)
V - I graph for diodes
- Used to regulate voltage in circuits and make logic gates
- Has very high resistance in one direction, so current can only flow in other direction
- x-axis : potential difference
- y-axis : current
- flat and then a steep rise
(see graph)
What can be used to indicate the presence of a current in a circuit?
lamps and LEDs
What happens to the current if I increase the resistance ?
What if i decrease it?
increasing resistance will decrease the current, this is achieved by adding more components or components with higher resistance
-increase the current, achieved by removing or replacing with components of a lower resistance
Draw a series circuit
draw a parallel circuit
check it
What is voltage in terms of energy transfer?
amount of work done per unit of charge
What is a series circuit useful for?
everything is connected in a line so the voltage is shared out between components.
- useful for low powered things like fairy lights
- however if one component breaks then whole circuit stops working
What is a parallel circuit useful for?
different components connected separately to supply
- practical because if one component breaks the circuit still works
- better for higher powered thing due to p.d being same for all components
- however power supply used quickly
What is the risk of water in domestic electricity?
water conducts electricity and energy from circuit can flow through it, creating a fire and electrocution risk
What is the risk of frayed cables/damaged plugs in domestic electricity?
the insulation has wore down, exposing live wires, electricity can be conducted
What is the risk of over loading in domestic electricity?
can cause fire
What are the features of a plug?
Label it
- plastic casing which is an insulator
- 3 pins made from brass which is a good conductor
- fuse between live terminal and pin. This breaks if the current is too high
- cable secured in the plug by cabe grip, which should grip the cable itself, and not individual wires
neutral wire : blue
earth wire : green/yellow stripes
live wire : brown
what equation connects energy, current, time and voltage
energy(J) = current(A) x voltage(V) x time(s)
What is the use of the earth wire ?
- many electrical appliances such as cookers, fridges have metal casing.
- earth wires creates the path of low resistance for current to flow if live wire touches the casing so an electric shock is avoided
What is the use of the fuse?
- made from a metal that has a low melting point so if a large current flows through the circuit and the fuse wire gets very hot the metal melts which breaks the circuit
- prevents electric shock and fires
- needs to be replaced once used
What are the use of circuit breakers?
if too large a current flows through a circuit then a switch opens making the circuit incomplete.
once fault in circuit corrected, the switch is reset by pressing a reset button
What is the use of live wires?
provides the path which electrical energy from power station travels
what equation connects power, current and voltage?
power(W) = current (A) x voltage(V) P = I x V
How to choose the correct fuse size to be fitted to a device?
- Fuses come in standard ratings of 3A, 5A and 13A
- chose one which allows the correct current to flow but blows if it gets a bit larger
- e.g if correct current in circuit is 4A then choose a 5A fuse
What is the use of insulation?
- covering live wire with a material that doesnt conduct
- Double insulation is when all electrical parts of an appliance is insulated so that the user cannot touch it. e.g when an appliance has a casing made of plastic
Prove E = IVt from V = E/Q and Q =It
E = Q x V E = (I x t) x V E = I x t x v
What equation connects charge, current and time?
charge (coulombs) = current(A) x time(s)
Q = I t
What is Ohm’s law?
V = I x R voltage(V) = current(A) x resistance (Ω)
How does changing resistance affect the current in a circuit?
Increasing the resistance will decrease the current
Decreasing the resistance will increase the current
What’s an experiment to investigate Current and Voltage characteristics?
Have a circuit with a voltmeter around a lamp and an ammeter.
- turn power supply up till p.d across lamp is 12
- record p.d + current
- calculate resistance r = V/I
- change voltage to see relationship
What is the difference between direct current (d.c) and alternating current (a.c)?
d. c:
- current flows in only one direction, such as when supplied by batteries and cells
- line on graph is flat
a. c:
- current constantly changes directions, such as when supplied by Mains electricity
- It has the frequency of 50 Hz, which means it changes direction, and back again 50 times a second
- line on graph like a transverse wave
- p.d of the live terminal varies between large + ve and -ve value
- p.d of neutral terminal is close to 0
- although the mean voltage is about 230 V, the peak voltage is higher
