Topic 6 - Electricity Flashcards
Power equations
Power (W) = current (A) × voltage (v)
P = IV
Power (W) = Work done (J) / time (s)
P = W/t
Power (W) = Energy (J) / time (s)
P = E/t
Voltage equations
Voltage (v) = current (A) × resistance (Ω)
V = IR
Voltage (v) = energy (J) / charge (C)
V = E/Q
Charge equation
Charge (C) = current (A) × time (s)
Q = It
Energy equations
Energy (J) = charge (C) × voltage (v)
E = QV
Energy (J) = current (A) x voltage (v) x time (s)
E = IVt
Current
The rate of flow of charge. Current flows from the positive terminal to the negative terminal of a cell.
Voltage
The “electrical pressure” that drives the current around the circuit. It is the energy transferred per unit charge passed.
Resistance
Anything in the circuit that slows the flow of current down. The higher the resistance of a circuit, the lower the current.
Ohm’s Law
Current is directly proportional to voltage across a fixed resistor and constant temperature
Diode
An electronic component that conducts current in primarily in one direction. It has low resistance in one direction and high resistance in the other.
Thermistor
A temperature-dependent resistor.
↑ temperature, ↓ resistance, ↑ current
The graph is NON-LINEAR
LDR
A light-dependent resistor - changes resistance depending on light intensity.
↑ light intensity, ↓ resistance, ↑ current
Can be used as a sensor in cameras or automatic lights that come on when it gets dark.
Ammeter
A device that measures the strength of current flowing through a component. It must be placed in series (anywhere in the main circuit) but never in parallel.
Voltmeter
A device that measures the voltage across the component. It must be placed in parallel around the component under test
Variable resistor
A type of resistor used to control the value of current and voltage by changing the resistance
A.C. supply
A current that continuously changes its direction, going back and forth around a circuit. It is produced by electrical generators (e.g. mains electricity)
D.C. supply
A current that is steady, constantly flowing in the same direction in a circuit, either positive or negative. It is produced by cells and batteries
LEDs
Light-emitting diodes emit light when a current flows through them in the forward direction. They can be used to indicate the presence of current in a circuit.
Series Circuits
- Components are connected in a line (except voltmeter)
- If one component disconnects the whole circuit is broken
- Vtotal = V1 + V2 + V3… = voltage of battery
- Rtotal = R1 + R2 + R3…
- Itotal = I1 = I2 = I3…
- The more components, the lower the current
Parallel Circuits
- If one component disconnects the circuit still works
- Vtotal = V1 = V2 = V3
- Itotal = I1 + I2 + I3…
- Rtotal = 1/R1 + 1/R2 + 1/R3…
- The way the current splits depends on the resistance of each individual component
- More components in parallel = lower total resistance
Fuse
A safety device designed to cut off the flow of electricity to an appliance if the current becomes too large. The wire inside the glass cylinder heats up and melts.
Earthing
Used to protect you from an electric shock. It does this by providing a low resistance path for a fault current to flow to earth
Circuit breaker
An electrical safety device used in some circuits to protect it from damage if too much current flows. It breaks the circuit by opening a switch.
Electrical power
the rate at which an appliance transfers energy
Static electricity
The build up of charge on insulating materials, which can cause sparks when discharged. A common cause of static electricity is friction
Spark
Electrons jumping across the gap between the charged object and the earthed object due to large voltage difference.
Van de Graaff generator
A machine that causes friction between a rubber belt and plastic rollers in order to build up positive electrostatic charge on a metal dome.
- When the belt rubs against the lower roller, friction is created, causing electrons to move from the belt to the roller. The belt is left with a positive charge.
- Electrons from the roller repel electrons away from the tip of the comb, leaving the comb with a positive charge
- The comb is attracted to the negative roller, but the positive ions cannot reach the roller due to the belt being in the way
- Therefore, the positive ions get transported up the belt to the dome
Insulation
When an insulating material (e.g. rubber) is used to cover metallic wires to cancel the risk of electrocution.
Double insulation - insulation around the wires + a non-metallic case for a second layer
Movement of electrons in electrostatic experiments
- All objects are initially electrically neutral
- When electrons are transferred through friction, one object becomes negative and the other positive
- These objects are attracted to other neutral objects by repelling like charges so the opposite charges are in front
Use of electrostatic charge in photocopiers
- An image of the document is projected onto a positively charged copying plate
- Where light falls onto the plate, the electrical charge leaks away
- Negatively charged black toner particles are attracted to the remaining positive areas
- Toner is transferred to a piece of paper and heated to make it stick
- Paper is taken off the plate and turned over to reveal text
Use of electrostatic charge in inkjet printers
- A nozzle makes a fine spray of tiny ink droplets, which are then given an electrostatic charge
- The charged droplets can be directed, using pairs of charged plates, with great precision to form letters and images on paper
Refuelling an aircraft
- There is buildup of charge when the fuel + pipe (insulators) rub against each other
- This can cause an explosion because fuel can get ignited by the spark
- You can join a wire between the metal plane and ground, which allows electricity to flow along, preventing the buildup of static charge on the plane
- Make nozzles out of metal so charge is conducted away
Saw Mill
- Static electricity from electrical equipment can create sparks
- Sawing wood creates fine wood dust
- If this dust gets dispersed, the spark can land on it and create an explosion in the enclosed space
Volt
Joule per coulomb
