Electrical Circuits Flashcards
Switch
Opens / closes a circuit
Cell
Provides power to a circuit
Lamp / Bulb
Emits light by heating a filament
Resistor
Slows down the flow of electricity (current), providing resistance to a circuit
Variable resistor
Same as resistor, but the level of resistance can be changed
LDR
Changes resistance depending on the intensity of light. As the intensity increases, the resistance decreases
Thermistor
Changes resistance depending on temperature. As the temperature increases, resistance decreases.
Diode
Allows current to flow in one direction
LED
Light emitting diode
Converts electrical energy to light
Ammeter
Measures current
Voltmeter
Measures P.D
Fuse
Breaks a circuit when at a certain current
Energy
Capacity for doing work
Symbol: E
Unit: J, Joule
Equation: E = QV, E= Pt, E = FS
Base units: kgm^2 s-2
Frequency
Number of oscillations in a given time period (1s)
Symbol: f
Unit: Hertz, Hz
Equation: f = 1/T, f = v/λ
Base units: s^-1
Current
Rate of flow of electrical charge
Symbol: I
Unit: Amperes, A
Equation: I = Q/t, I = V/R, I = P/V
Base units: A
Potential difference
The energy supplied per coulomb of charge
Symbol: V
Unit: Volts, V
Equation: V = E/Q, V = IR
Base units: kgm^2 s^-3 A^-1
Power
Rate of energy transfer / work done
Symbol: P
Unit: Watts, W
Equation: P = E/t, P = IV, P = I^2R
Base units: kgm^2 s^-3
Charge
The physical property of matter that causes it to experience a force when placed in an electromagnetic field
Symbol: Q
Units: Coulombs, C
Equation: Q = It, Q = E/V
Base units: As
Resistance
The ratio of P.D across a component to current though it
Symbol: R
Unit: Ohms, Ω
Equation: R = V/I, R = P/I^2
Base units: kg ^2 s^-3 A^-2
Resistivity
Specific property of a material that measures its electrical resistance
Symbol: ρ
Units: Ohm meter, Ωm
Equations: ρ = RA/L (A = cross sectional area, L = length of wire)
Base units: kgm^4 s^-3 A^-2
Time period
Time take for one complete oscillation of a wave
Symbol: T
Unit: second, s
Equation: T = 1/f
Base units: s
Time
Measurable duration of an action, process, or condition
Symbol: t
Unit: second, s
Equation: t = E/P, t = Q/I
Base units: s
Ohm’s law
Voltage is directly proportional to current at a constant resistance.
V = R x I
Kirchhoff’s first law
The total current into a junction equals the total current out a junction
I 1 = I 2 + I 3 + ……
Kirchhoff’s second law
In any loop around a circuit, the sum of the EMFs equal the sum of the P.Ds
Current in a series circuit
Current is constant in all parts
I 1 = I 2 = I 3
Current in a parallel circuit
Current is split between branches based on resistance
I total = I 1 + I 2 + I 3
Voltage in a series circuit
Shared amongst components
V total = V 1 + V 2 + V 3
Voltage in a parallel circuit
Each loop gets the full voltage. The P.D is split between components in the loops
V total = V 1 = V 2 = V 3
Resistance in a series circuit
Total resistance in the sum of the individual resistance of each component
R total = R 1 + R 2 + R 3
Resistance in a parallel circuit
Reciprocal of total resistance in the sum of the reciprocals of the individual resistances
1/R T = 1/R 1 + 1/R 2 + …. 1/R n
Typical trigger voltage for a diode
0.6V
Forward bias
When a diode is setup in a way that allowed current to flow through
Backward bias
When a diode is set up in a way that does not allow current to flow through
Potential divider
A circuit with two or more resistors in series with each other. There is a source of fixed P.D between them, which is divided between the components in the circuit.
Relationship between resistance of a component to the P.D over it in a potential divider
The proportion of resistance to the total resistance is equal to the proportion of the P.D to the total P.D over the component
Potential divider equation
V1 = (V0 x R1) / (R1 + R2 + … Rn)
What is the gradient of an IV graph?
1/R
Therefore, a steeper gradient is a lower resistance
Three factors that influence resistance in a wire
Resistivity (directly proportional)
Length of the wire (directly proportional)
Cross sectional area (inversely proportional)
Difference between Resistivity and resistance
Resistivity is a constant material property whereas resistance is sample specific, depending on length, cross sectional area, and Resistivity
What is the magnitude of the charge per carrier unit?
1.6x10^-19
What is the actual value of charge per carrier unit?
-1.6x10^-19
Which direction do electrons move?
Negative to positive.
What is drift velocity?
The average velocity that a particle attains due to an electric field.
Describe the motion of electrons in an ionic lattice with no P.D
Moving about in random directions. No net velocity.
Describe the motion of electrons with a P.D
Moving about in random directions, however with a mean velocity. Net velocity isn’t 0.
Describe the atomic structure of a wire.
Ions surrounded by de localised electrons
Drift velocity equation
I = nAve where;
- I = current, A
- n = number density if metal, m^-3
- A = cross sectional area, m^2
- v = drift velocity, ms^-1
- e = electron charge, coulombs
What is internal resistance?
The resistance within the cell
When is the terminal P.D equal to its emf?
Only when no current is drawn, since Vlost = I x r internal
Equation for total current with emf, R and r
I total = emf / (R+ r)
Equation for emf with vlost and vload
Emf = Vload + vlost
Or
Emf = I total R + I total r
4 physical properties that influence current in a wire
Cross sectional area
Drift velocity
Number density of metal
Charge carried per electron
Energy transfers in a wire with resistance
Electrical energy -> work done -> thermal energy
How do thermistors work?
With more temperature, electrons are pushed into the conduction band form the valence band due to having more energy.
The electrons are excited and are no longer part of the atom.
Hence, more electrons are available to carry charge, so resistance decrease. (Number density increases)
How do LDRs work?
Whenever light falls on the photoelectric material, it absorbs energy causing electrons in the valence band to be excited into the conduction band. More electrons available to carry charge = lower resistance.
