Electricity Flashcards
What’s electric charge
A physical property that all bodies possess. Two different types, positive and negative charges.
How do charges react
Via electrostatic forces
Either attract or repel depending on the charges
What’s definition of one coulomb
The amount of charge that passes a point per second, when there is a current of one amp present
What is the value of the fundamental charge (it’s quantised)
1e= 1.6 x 10 (-16) Coulombs
Electric current definition
The rate of flow of electric charge
Electric current equation
Current= charge/ time I= Q/t
Net charge
The charges of most objects results from the loss or gain of electrons. Net charge can be expressed of a multiple of e:Q= +-ne
Conventional Current
Defined as a current from a positive terminal towards a negative one
Electron Current
The flow of electrons (from negative to positive)
What are electrolytes
Liquids that can carry charge
Measuring electric current
Ammeters used to measure rate of flow of charge.
In order measure current, the charge must flow through the ammeter.
In order that the ammeter doesn’t effect the current, it must be placed in series and have a negligible resistance
Homogeneity of Equations
In any equation, every term has to have the same SI base units. If this is true then the equation is homogeneous.
Conservation of charge
In any closed system the total charge in the system is conserved .
It states that electricity charge can’t be destroyed or created only transferred. The total amount of charge in the universe is constant.
Kirchhoff’s first law
In any junction in a circuit, the total current leaving the junction is equal to the total current entering the junction.
Cell function
Pushes electrons around the circuit
Ameter function
Measure current
Voltmeter function
Measures the potential difference at one point
Filament lamp
Lights up when current passes through it
LED function
Emits light when current passes through it
Switch function
Breaks circuit
Fuse function
Melts and breaks the circuit if current is too high
Resistor function
Limits the current in the circuit
Thermistor function
Increases resistance, when temperature decreases
Variable resistor function
Allows resistance to be varied
What’s Voltage
Voltage is a measure of the electrical energy power transferred per unit charge (energy provided by the charged carriers).
Defined: V=W/Q (work done/ charge)
Where are voltmeters placed?
Always placed in parallel to the circuit (used to find the potential difference between 2 points)
One volt is defined as…
The energy transferred per coulomb of charge as the charge moves between 2 points in the circuit.
1V= 1J C (-1)
What’s Potential Difference (P.D)?
P.D is the energy transferred BY the charge carriers
V=W/Q
What’s Electromotive Forces?
EMF is the energy transferred TO the charge carriers (by the cell/ battery)
E=W/Q
What term is used to describe when work is done on the charge carriers
EMF
E= delta W/ delta Q
Kirchhoff’s Second Law
In any circuit, the sum of EMF is equal to the sum of the potential difference (P.D) around a closed loop.
Resistance
The resistance of a component is defined as the ratio between the potential difference across a component and the current flowing through it.
Equation for resistance:
R=V/I
Measured: Ohms
Base unit: kg m2 s-3 A-2
What’s Ohms Law:
For a metallic conductor at Constant Temperature the potential difference across the conductor is directly proportional to the current flowing through the conductor. V=IR
When resistors are connected in a series circuit you can calculate the total resistance using:
Rt=R1+R2+R3
When resistors are connected in a parallel circuit you can calculate the total resistance using:
1/Rt= 1/R1 + 1/R2 + 1/R3
Physical quantity?
A measurement of something
Internal Resistance?
The amount of energy available to the rest of the circuit is the total emf supplied to the cell, minus the energy lost in the cell (lost volts) as heat.
What is the loss energy caused by (in a circuit)?
The internal resistance
What are the formulas that can be formed from kirchhoff’s 2nd law?
E=Ir+IR
E=Ir+V
V=E-Ir
Does current effect internal resistance?
Current does not effect resistance, as I and V are directly proportional therefore as current increases/ decreases so does voltage. So as a result resistance doesn’t change
Resistance:
Measure of the opposition to current flow in an electrical circuit.
Do rechargeable batteries and car batteries have a low or high internal resistance and why?
Small internal resistance, this allows them to be recharged using higher currents without overheating or wasting a lot of energy (so recharging is faster).
Do high-voltage power supplies used in classrooms have a low of high internal resistance and why?
They have a very high internal resistance/ often millions of ohms.
Acting as a safety feature, preventing the power supply from delivering a fatal electric current.
What factors effect resistance?
Length
Cross section area
Material
Temperature
What’s is resistance directly proportional to?
The length of the component
Resistances is inversely proportional to
Cross sectional area
Equation for resistance
Resistance= constant (resistivity) x length / cross sectional area
R= pl / A
Gradient= p / A p= gradient x A
Resistivity of a material (definition):
The measure of how much a particular material opposes electron flow
Equation for resistivity:
Resistance x Cross sectional area/ length
Units for resistivity
Ohms x m
What happens to the components of a circuit, as temperature increases and why?
The resistance increases, as the particles have more kinetic energy, resulting with more paths to be cut off for the electrons, as collisions increase.
What’s a potential divider
Is a simple circuit that uses resistors (or thermistors/LDR’s) to supply a variable output potential difference
What’s the equation for Vout in a potential divider
Vout= (R2/R1+R2) x Vin
Equation for current (potential divider)
Current= Vin/ R1+R2
What does loading refer to
Connecting a component or circuit to Vout, placing a component in parallel with R2. This lowers the resistance of this part of the circuit, and so lowers Vout
What would happen if you add a large load (large resistance)?
Has very little effect on Vout
What would happen if you add a small load (small resistance)?
Significantly reduce Vout
When does Vout drop?
When a component is connected across it, cause in a parallel the more resistance you add, the lower the total resistance would be.
