Chapter 2: Electricity Flashcards
What is voltage/potential difference (V)?
The amount of energy (joules) per coulomb of charge. (or work done by a cell per coulomb of charge transported)
What is current (A)?
Amount of charge flowing past any point on a circuit in a given time
What is a battery?
2 or more chemical cells (—-+| i—-) in series (—-+| i–| i—-)
What is the symbol for a lamp?
Circle with a cross inside it
What is a fixed resistor?
A resistor whose resistance does not change (symbol = rectangle)
What are the symbols for ammeters and voltmeters?
Circles with A and V inside
What is a variable resistor?
A resistor that allows the current to be varied (symbol = rectangle with diagonal arrow)
What is a fuse?
A device that melts and breaks the circuit when the current is higher than its rating (symbol = transparent rectangle)
What is a diode?
A device that only allows current to flow in one direction (symbol = circle with “play” sign)
What is a light-emitting diode (LED)?
A diode that emits light when a current flows through it (symbol = diode symbol with arrows coming off it)
What is a light-dependent resistor?
A device with low resistance in bright light and high resistance in dark conditions (symbol = rectangle in circle with arrows going into it)
What is a thermistor?
A device with a low resistance at high temperatures and vice versa (symbol = fixed resistor symbol with diagonal line flat at the beginning)
What equation links charge flow to current and time?
Q = It Q = charge (C) I = current (A) t = time (s)
What happens when voltage is increased but the resistance remains the same?
The current increases (V = IR can be used to answer these types of questions)
What is meant by the “voltage” of a battery?
How much electrical potential energy it gives to electrons entering it. The potential difference across a 3V battery is always 3V.
What equation links voltage, current and resistance?
V = IR V = voltage (volts) I = current (amps) R = resistance (ohms)
What happens to the voltage and the current at different parts of a circuit?
Voltage changes, current stays the same
How are ammeters and voltmeters placed?
Ammeters are placed in series, voltmeters are placed in parallel
What is an ohmic resistor?
A resistor where, at a constant temperature, the current through the resistor is proportional to the PD across it
What does “series” mean?
The current does not split
What does “parallel” mean?
The current splits into different paths
What are the rules for series circuits?
Same current through each component, voltage is shared between components, the total resistance of two components is the sum of their individual resistances
What are the rules for parallel circuits?
Voltage and current are divided evenly among the components, the total resistance of two resistors in parallel is less than the resistance of the smaller individual resistor
What is direct current?
Current where the electrons always flow in the same direction
What is alternating current?
Current where the electrons change direction multiple times per second
How is the alternating current measured?
The frequency is measured in hertz
Why is the peak AC potential difference higher than the voltage rating?
To make up for the time where the voltage is close to 0
How is mains electricity supplied?
230V alternating current at 50Hz
What is the live wire?
The brown wire that carries the AC from the power station (230V)
What is the neutral wire?
The blue wire that completes the circuit (<1V)
What is the earth wire?
The green and yellow wire whose purpose is to stop an appliance becoming live (0V, only carries a current if there’s a fault)
What is earthing?
A method in which an electrical appliance’s metal case is connected to the earth wire, preventing it from becoming live in the case of an accident
Why do resistors heat up when charge passes through them?
The electrons collide with atoms in the resistor, converting the electrons’ electrical potential energy to heat energy
What is the equation that links energy transferred, voltage and charge?
E = VQ E = Energy (J) V = Voltage (V) Q = Charge (coulombs)
What equation links energy, power and time?
E = Pt E = Energy (J) P = Power (W) t = time (s)
What equation links power, current and voltage?
P = IV P = Power (W) V = Voltage (V) I = Current (A)
What equation links power, current and resistance?
P = RI^2 P = Power (W) R = Resistance (ohms) I = Current (A)
Why should the current in the overhead wires be reduced as much as possible?
To reduce the power output into the cables (meaning they heat up)
What happens to the resistance of a wire as its length increases?
Its resistance increases
How is current in transmission cables reduced?
A transformer takes the current from the generator, increases the voltage and decreases the current before it flows through the overhead cables. Near our homes, another transformer step the voltage down to 230V to make it safe.
What is static electricity?
Static electrical charge on an insulator
How is static electricity created?
When two insulators are rubbed together, electrons are rubbed off one material and onto the other. One material is now positive charged and the other negatively charged.
What causes arcs and sparks?
Electrons jumping across the air from one material to another to correct the imbalance in charge
What is an electric field?
A field that exerts a force on a charged object
What are the properties of electric fields?
A charged object creates an electric field around itself
The electric field is strongest closest to the object
The direction of the field is the direction a positively-charged particle would move
The strength of the field can be increased by adding more charge to the object
Why would a human get an electric shock when he touches a live wire?
There would be a potential difference between the wire (230V) and the earthed human (0V), causing a current of charged electrons to flow from the wire to the ground, through the human. This earths the charge.
