Electricity Flashcards
Current
The current is the charge passing a point in a circuit every second
(It is helpful to think of current as the charge per second)
Charge = current x time
Q = I x T (QIT)
Current is measured using an ammeter connected by series
In metals, the current is caused by a flow of free (delocalised) electrons
Conventional current
Electrons are negatively charged
This means that the electrons flow from negative to positive
Conventional current, however, is still defined as going from positive to negative
By definition, conventional current always goes from positive to negative (even though electrons go the other way)
Highest pressure being near the batteries
EMF
Potential Difference (Voltage) of the power source in a circuit which is measured in Volts
The electrical work done by a source in moving a unit charge around a complete circuit
(Think of the EMF as being the energy per coulomb.)
Measuring EMF by a voltmeter
Voltmeter must be parallel
The positive of the voltmeter must be connected to the positive of the battery vice versa
E = Work done/ charge
If EMF is work done and is eg: (1.5v) this means that the cell converts 1.5v into energy to DRIVE charge around the current
Series, you add the volts
Parallel, its the same
Potential difference
The work done by a unit charge passing through a component
Potential difference can be measured using a voltmeter just like EMF
V = Work done/ charge
Resistance
The property of matter that resists the flow of charges
Think of resistance as mud, when current tries to go through a light build with resistance it gets slower this is because it is losing energy
Therefore Current is directly proportional to potential difference as long as the temperature remains constant
However filaments lamps is different
The current increases at a proportionally slower rate than the potential difference
The unit is the Greek symbol ohms
R = V/I
R = resistance (ohms, Ω)
V = potential difference (volts, V)
I = current (amperes, A)
Resistance can also be found by adding the resistance in series but putting it in fraction when in parallel.
The resistance of a wire
As electrons pass through a wire, they collide with the metal ions in the wire
The ions get in the way of the electrons, resisting their flow
If the wire is thicker (greater diameter, cross sectional area) there is more space for the electrons and so more electrons can flow:
The thicker a wire, the smaller its resistance
If the wire is longer it means there is more resistance therefore resistance is directly proportional to length
The formula to work out resistance using a cross sectional area
The relationship between resistance, length and cross-sectional area can be represented mathematically
Resistance is inversely proportional to cross-sectional area (width, or thickness)
R = 1/A
Power equation for electrical devices and equation for electrical energy
P = IV
P = power
I = current
V =voltage/potential difference
E = VIT
E = electrical energy (joules)
V = voltage
I = current
T = time
P = I^2 x R
P = V^2/R
Series
Components are connected ends to end in one loop
Same current flow through each component
Potential difference is shared across each component due to amount of resistance
If you have two resistors of the same resistance it will be split equally, however if you have one resistor to be 15 ohms and another 5 ohms it will be split in that ratio this is because the current is the same (V = I x R ) so the bigger the resistance the bigger the voltage
The total resistance is the sum of the resistances of each component
If there is more voltage in series it means that there is more current
If there is more components it means that there is less current
Parallel
Components are connected to the power supply in a separate branch
Current is shared between each branch (current spilts, doesn’t always have to be equal)
The potential difference is the same across every branch
The total resistance = 1/R(t) = 1/R(1) + 1/R(2) etc
Connecting lamps in pallerel is good as if one breaks current still passes it can also be individually controlled by a switch
All components get he same voltage and you can control them independently
KWh (the kilowatt per hour)
Energy usage in homes and businesses is calculated and compared using the kilowatt hour
The kilowatt hour is defined as:
A unit of energy equivalent to one kilowatt of power expended for one hour
To convert joules to KWh
KWh x (3.6 x 10^6) = J
J / (3.6 x 10^6) = J
Potential energy divider
When two resistors are connected in series, the potential difference across the power source is shared between them
A potential divider splits the potential difference of a power source between two components
- The resistor with the largest resistance will have a greater potential difference than the other one
- If the resistance of one of the resistors is increased, it will get a greater share of the potential difference, whilst the other resistor will get a smaller share
Therefore when two resistors are connected in series the potential difference across the power source is divided between them
Formula
V(out) = (R(1)/R(1) +R(2)) x V(input)
Or
V(in) = V(out) / (R(1)/R(1) +R(2))
remember that the higher the resistance the more energy it will take to ‘push the current through’ and therefore the higher the potential difference.
Electrical safety
Damaged Insulation - if someone touches an exposed wire there is a risk of shock
Overheating of cables - fires or melt the insulations
Damp conditions - if moisture comes into contact with lives wires causes a short circuit (which could cause a fire).
Diode
A diode only allows one way flow of current through it (denoted by the arrow or direction of the triangle in the circuit diagram).
It’s good for changing alternating current into direct current
Since alternating current is when it’s always changing diode stops that change
Thermistors
When a thermistor temperature rises the resistance of it decreases this could be due to the current or being directly heated
When the resistance falls so does the potential difference so the other resistor will get a larger share of the potential difference
Ohmic conductors and filaments lamps graphs
When current increases so does voltage as seen in a normal fixed resistor (ohmic conductor)
For filament lamp when the voltage increases so does the brightness this means that temperature increases therefore increasing resistance so thats why current does increase anymore
