Y1: Electricity Flashcards
What is the function of an Ammeter
Measures the current, which is the stream of charged particles travelling through the wire
How do you use an Ammeter
- Must be connected in series (acts as a wire, with little resistance and little pd)
- Negative end of ammeter connects to power source side of the circuit
What is the function of a Voltmeter
It measures the difference in charge between the two points in a circuit.
How do you use a voltmeter
- To test an AC circuit, set the knob to V~, ACV or VAC
- To test a DC circuit, set the knob to V—, DCV or VDC
- Choose a range above the expected max pd. (raise if reading is overloaded)
- Must be connected in parallel across the component being measured.
What is the function of a Cell
- cells are composed of two electrodes that react with an electrolyte, creating a difference between the electrical potential of the electrodes, generating power.
- Cells are non-ohmic conductors
How do you use a cell
Connect it in series with the other electrical components, and it generates power.
What is the function of a Bulb
- Bulbs light up when an electric circuit passes through them.
- These are non-ohmic conductors, so don’t obey the relationship of ohms law. This means the current through the bulb is not directly proportional to the pd across it.
How do you use a bulb
Bulbs must be connected in series, with a blown bulb breaking the circuit.
What is the function of a Diode
- Diodes only allow current to pass in one direction, caused by the high resistance in one direction, and the low resistance in the other.
- Current can pass from the anode (A) to the cathode (K)
- Diodes are non-ohmic conductors
How do you use a diodes
Connected in series to the whole circuit.
What are the applications of a diode
- Recertification: Can convert AC mains power to DC for use in electrical equipment
(half wave recertification = 1 diode but loses half the signal)
(Full wave recertification = Multiple diodes in series and parallel)
What is the function of a Light emitting diode (LED)
- LEDs only allow current to pass in one direction, but as the current passes through, they release energy in the form of photons. This releases light in a process call electroluminescence.
- Not ohmic
How do you use an LED
Similar to a diode, they must be in series with the rest of the circuit.
What are the applications of an LED
- General lighting
- Display lighting (current creates light to show the circuit is on - eg. warning lights)
What is the function of a Resistor
- Resistors are passive electrical components (use energy) that create a resistance in the circuit.
- Ohmic resistors have a directly proportional relationship between current and potential difference, whereas non-ohmic resistors can change based on external factors such as temperature.
How do you use a resistor
Resistors can be used in series or parallel, but this changes the total resistance in the circuit.
- Series: R(T) = R(1) + R(2) +R(3)
- Parallel: 1/R(T) = 1/R(1) + 1/R(2) + 1/R(3)
What is the function of a Variable resistor
The wiper can adjust the length of resisting material that the current passes through, increasing or decreasing the total resistance.
- these can be ohmic and non-ohmic
How do you use a variable resistor
Similar to a normal resistor, they can be in series or parallel with each giving a different relationship between them.
- Series: R(T) = R(1) + R(2) +R(3)
- Parallel: 1/R(T) = 1/R(1) + 1/R(2) + 1/R(3)
- They also have a terminal known as a wiper to adjust their resistance in the circuit.
What is the function of a Thermistor (Thermally sensitive resistor)
It is a type of resistor whose resistance depends on temperature.
How do you use a thermistor
Used in parallel in the circuit
What is an NTC Thermistor
Negative temperature coefficient thermistor:
increase in temp = decrease in resistance (large amount of resistance created, when small temp change is detected)
What is a PTC Thermistor
Positive temperature coefficient thermistor:
Increase in temp = increase in resistance.
What is the function of a Light dependent resistor (LDR)
- LDRs are resistors that depend on the light levels they are exposed to
- There is an inversely proportional relationship between light levels and resistance (more light = less resistance)
How do you use an LDR
Should be used in parallel to the circuit.
What are the applications of an LDR
Switches for circuits (eg, outside security lights / street lamps)
What is the function of a Heater
The current passing through heats the coil, causing the heater to release thermal energy
H = I^2Rt
How do you use a heater
Heater is used in parallel with the circuit, producing heat in the surrounding wire.
What are the applications of a heater
Induction heater can be used for heating homes
What is the function of an Electric motor
- they are electro-mechanical machines that convert electrical energy into movement.
- AC and DC motors convert AC and DC power into movement
- They use the interactions between currents and magnetic fields to create a force to drive a motor
How do you use an electric motor
Used in parallel with the circuit, and the relationship between the current, force and magnetic field is shown with Flemings
left hand rule:
Thumb = force
First = field
Middle = Current
What are the applications of an electric motor
Fans, air conditioning, etc.
What is current (I)
Rate of the flow of charge (amperes)
I = Q/t
What is charge (Q)
The flow of electrons through the wire (Coulombs)
Q = It
What is potential difference
Work done (energy transferred) to move one coulomb of charge between 2 points (Volts)
V = W/Q = E/Q
What is resistance
A measure of how difficult it is for the current to flow (P.d per amp)
R=V/I
What is ohm’s Law
Through an ohmic conductor under constant conditions, the current is proportional to the potential difference, with the resistance as the constant of proportionality
V=IR
What is an ideal volt/ammeter
- Voltmeter = infinite resistance, so no current
- Ammeter = no resistance, and therefore no potential difference
What are the IV characteristics of an ohmic conductor
Current is directly proportional to potential difference under constant conditions
What are the IV characteristics of a filament lamp
Proportional, until the current becomes too high, increasing the temp and therefore increasing the resistance, so the current can’t continue to increase as the P.d does.
What are the IV characteristics of a diode
Components have a forward bias, so there is very little negative current.
Threshold voltage is also required to be reached for current to start flowing in positive direction, due to resistance within the component.
What is Resistivity
Measure of how difficult it is for current to flow through a material (Ωm)
ⲣ = RA/L
How do you determine the resistivity of a material
- Measure the diameter of the wire and calculate the cross-sectional area
- Record I and V of the wire at different lengths
- Calculate the resistance for each length (V=IR)
- Plot a graph of R vs. Length (m)
- Gradient = R/L
∴ ⲣ = Gradient x A
What is a semi conductor
Material where the conductivity increases (resistance decreases) as energy is applied
eg. NTC Thermistors - Increasing temp releases charge carriers to reduce resistance/resistivity of the material
What is a superconductor
A material with no resistance, achieved by cooling it below it’s critical temperature (~10K)
No resistance means no heating, and therefore means no wasted energy
What are some uses for superconductors
- Power cables
- Strong electromagnets
- Efficient circuits
However, it is difficult and expensive to maintain the low temperatures
What is Power
Rate of energy transferred (W)
P = E/t
What 3 equations link power, current, potential difference and resistance
P = IV
P = (V^2)/R
P = (I^2)R
What 3 equations link electrical energy, current, potential difference and resistance
E = IVt
E = (V^2)t/R
E = (I^2)Rt
What is internal resistance (r)
The resistance inside a battery, due to electrons colliding when the chemical energy makes them move
What is Load/external resistance (R)
The total resistance of all the components in the circuit
What is the Electromotive force - Emf (ɛ)
The amount of electrical energy the battery transfers per coulomb of charge (P.d of the battery - V)
ɛ = E/Q
What is the terminal potential difference (V)
The potential difference across the load resistance (energy transferred per coulomb)
If no internal resistance, ɛ=V
What is the lost potential difference (v)
Energy transferred per coulomb to overcome the internal resistance
What is the sum of the Terminal and lost potential difference
Energy transferred per coulomb in load resistance (V)
+
Energy per coulomb wasted overcoming the internal resistance (v)
=
Energy per coulomb supplied by the source (ɛ)
∴ ɛ = V+v = I(R+r)
∴ V = ɛ -Ir
How do you measure the Emf of a battery
- Record and plot V vs. I
- As V = ɛ -Ir:
Gradient = -r
Y-intercept = ɛ
What is Kirchhoff’s first Law
Conservation of charge:
Total current entering a junction = Total current leaving a junction
What is Kirchhoff’s second Law
Conservation of energy:
Total Emf around a series circuit = Sum of the Pds across each component
ɛ = 𝛴IR
What is the relationship of current in a series circuit
I1 = I2 = I3
Due to Kirchhoff’s 1st Law
What is the relationship of Potential difference in a series circuit
ɛ = V1+ V2 + V3
Due to Kirchhoff’s 2nd Law
What is the relationship of resistance in a series circuit
Rt = R1 + R2 + R3
As V=IR, and I is constant
What is the relationship of Emf for cells in series
ɛt = ɛ1 + ɛ2 + ɛ3
Due to Kirchhoff’s 2nd Law
What is the relationship of current in a parallel circuit
It = I1 + I2 + I3
Due to Kirchhoff’s 1st Law
What is the relationship of Potential difference in a parallel circuit
ɛ = V1= V2 = V3
Due to Kirchhoff’s 2nd Law
What is the relationship of resistance in a parallel circuit
1/Rt = 1/R1 + 1/R2 + 1/R3
As I=V/R, and V is constant
What is the relationship of Emf for cells in parallel
ɛ1 = ɛ2 = ɛ3
Due to Kirchhoff’s 2nd Law
What is a potential divider
A circuit where the potential difference of the source (Vs) is split across the resistors in the ratio of their resistances
V out = R2/(R1+R2) x Vs
How can variable resistors be used in potential dividers
Used as R1, allowing V out to be altered
Decreased resistance in R1 = Increased V out
How can LDRs be used in potential dividers
Used as R1, allowing V out to be altered
Increase in light = Decrease in resistance in R1 = Increased V out
How can thermistors be used in potential dividers
Used as R1, allowing V out to be altered
Increase temp = Decrease resistance in R1 = Increased V out
What is a Potentiometer
A variable resistor used in a potential divider to replace R1 and R2, with a slider to alter the resistance of each, to alter V out
