Flashcards in Electricity Y10 Deck (24)
What is voltage?
Energy transferred per unit charge passed
Volts = energy transferred ÷ charge
What is current?
Rate of flow of charge
Amps = charge ÷ time
What is resistance?
Ohms = voltage ÷ current
More resistance = less current.
What is power?
Watts = energy ÷ time
Watts = voltage x current
What does the current voltage graph of a resistor look like?
• straight line through origin ∴ V directly proportional to I
• constant gradient ∴ constant resistance
• constant resistance because constant temperature
What does the current voltage graph of a bulb look like?
• s shape, with rotational symmetry
• at bottom gradient steep ∴ low resistance. This is due to low temperature, so the bulb will also be dimmer
• at top gradient shallow ∴ high resistance. This is due to high temperature, so bulb will be bright
What does the current voltage graph of a diode look like?
• high resistance in one direction so current can only go in other direction
• only turns on at 0.6V
What is a thermistor?
A resistors which varies with temperature. Cold = high res. Warm = low res.
What is an LDR?
A resistor whose value depends on light intensity. Dark = high res. Light = low res.
What order should you do circuit calculations in?
What is the electron flow?
Negative to positive
What is the current direction?
Positive to negative
What is the difference between dc and ac?
Dc, e.g. Cell, only goes one way can't change direction
Ac, e.g. Mains, goes both ways ∴ can change direction, above and below axis
How can resistors be used in daily life?
• they slow down current ∴ the KE of the electrons turned into thermal energy
• can be used in appliance, e.g. Hair dryers
How can you investigate how current varies w voltage in wires, resistors, lamps and diodes?
• use an ammeter and measure current w and wo these elements.
• measure different voltage levels using a voltmeter w these elements
What is current like in series and parallel circuits?
Series: current is the same
Parallel: current divided by num of components
How do you read an oscilloscope?
• x axis is time base- time per cm. Can find freq of wave: 1/time
• y axis is y-gain. This is volts per cm. We can work out the amplitude of the wave, e.g. If y-gain is 2 V/cm, and it is 3 cm high, then V = 6
What does the earth wire do?
Longest pin ∴ goes in first and open other holes
If live wire becomes loose and touches the case, case becomes live. If you touch case, instead of current going through you, it takes path of least resistance through case, earth wire and to the ground ∴ creates a large current, which blows the fuse
What is double insulation?
No external bits of metal in appliance ∴ case can't become live ∴ don't need earth wire
What do the flex and cable grip do?
Flex is plastic coating on wire going in, as its an insulator
Cable grip: holds in flex and keeps it down
What does the fuse do?
• if current going through it is too large it melts causing gap in circuit, thus preventing fire
Comes in 3A, 5A, and 13A. Use smallest possible, but only acc to 10%
What does a circuit breaker do? What are its advantages over fuses?
• spring loaded switch held closed by soft iron bolt. An electromagnet is turned on if I too large and pulls bolt away ∴ switch opens and circuit breaks.
• more reliable (fuse only acc to 20%)
• more sensitive
• switch can be reset unlike fuse which breaks and so must be replaced
• respond mor rapidly to current surges than fuses
What are the pros and cons of parallel circuits?
Pros: • can disconnect or connect new elements without disrupting other elements, e.g. Can turn off a light without all the lights in your house going off
• if one part breaks, it wont all. Current can flow through other parts
Cons: • can't increase or decrease the voltage
• requires lots of wires