P4 Flashcards
What equation links charge, current and time?
Q (charge, coulombs, C) = I (current, A) x t (time, s)
What equation links potential difference, current and resistance?
V (potential difference, V) = I (current, A) x R (resistance, ohms)
What is an ammeter?
Measures the current in amps flowing through the wire. It must always be placed in series with whatever you are measuring.
What is a voltmeter?
Measures the potential difference in volts across the wire. It must always be placed in parallel around whatever you’re investigating.
Describe the graph and I-V characteristics of ohmic conductors
Ohmic conductors = a wire or a resistor at a constant temperature
The current is directly proportional to the potential difference so you get a linear graph with a straight line. Because it’s at a constant temperature, there is no resistance.
Describe the graph and I-V characteristics of a filament lamp (or bulb)
Some energy is transferred to the thermal energy store of the filament which is designed to heat up.
As the current increases, the temperature of the filament increases so the resistance increases.
This means that less current can flow per unit pd so the graph gets shallower, hence the curve.
Describe the graph and I-V characteristics of a diode.
They will happily let current flow in one direction so the resistance is very high in the reverse direction.
What is an LDR?
It is a light dependent resistor.
It is dependent on the intensity of light so in bright light the resistance falls and in darkness the resistance is very very high.
They are used in automatic night lights, outdoor lighting and burglar detectors.
What is a thermistor?
It is a temperature dependent resistor.
When it’s hot, the resistance drops and in cooler conditions, there is the most resistance.
They are used for temperature detectors like a car engine temperature sensors and electronic thermostats.
What is a sensing circuit?
turn on or increase the power of components depending on the conditions they are in
What is a series circuit?
components are connected in a line between the +ve and -ve of the power supply (except for voltmeters which are always connected in parallel)
- total pd is shared between the various components
- same current flows everywhere
- resistance just adds up
- cell pd add up
What is a parallel circuit?
each component is separately connected to the power supply (except for ammeters which are always connected in series)
disconnecting one won’t really affect the others
it is how most things are connected in everyday life
- pd is the same across all components
- current is shared between branches
- adding a resistor in parallel reduces the total resistance
What are the results of the experiment which involves adding more and more resistors to series and parallel circuits?
the graph of a series circuit shows a positive straight line - more resistors means more overall resistance of the circuit
the graph of a parallel circuit shows a negative curve - more resistors and more loops increases the total current of the circuit which results in a decrease in total resistance of the circuit
What is ac and dc?
alternating current current is constantly changing direction because the voltage is constantly changing direction \+ve and -ve ends keep switching UK mains supply = an ac supply at 230V frequency of UK mains supply = 50 Hz cells and batteries supply dc
What is the composition of plugs?
three wires inside, each with a core of copper and a coloured plastic coating: the live, neutral and earth wire.
What is the live wire?
bottom right
brown
alternating potential difference
230V
What is the neutral wire?
bottom left blue 0V completes the circuit current flows when working normally
What is the earth wire?
top
green and yellow
safety
stops the casing of the appliance becoming live
only carries a current when there is a fault
0V
What causes an electric shock?
body=0V (like the earth)
the live wire has a current flowing so when there is contact, there is a large pd which causes a shock or a spark
if any link creates a low resistance path from the live wire to the earth, a huge current would flow
What are step-up and step-down transformers?
step up= power stations to the national grid - make the size of the ac voltage much bigger
step down= national grid to consumers 230v mains electricity in homes
What equation links power, energy and time?
P= E/t
What equation links power, pd and current?
P=VI
What equation links power, current and resistance?
P=IIR (current is squared)
What is a fuse?
between the live pin and the live wire
if too much current passes through, it melts and the live wire is cut off
they come as 3A, 5A or 13A
What is static electricity and how does it work?
insulating materials rubbed together
electrons scraped off one and dumped on the other
materials electrically charged (one +ve and one -ve but they are equal)
polythene and acetate rods being rubbed with a cloth duster
How does static electricity cause sparks?
electric charge builds up, pd increases between earth and object
when large enough pd, electrons jump the gap (attraction of opposite charge)