4 Current electricity Flashcards
What happens to certain insulating materials when they are rubbed together?
they become charged
What happens to charged objects when electrons are transferred (2)?
- insulating materials that become positively charged when rubbed lose electrons
- insulating materials that become negatively charged when rubbed gain electrons
What is the rule of attraction?
like charges repel
unlike charges attract
What is static electricity?
charge ‘held’ by an insulator or an insulated conductor
How many electrons and protons are in an uncharged atom?
Equal number which neutralises the charge
What is a charged atom called?
an ion
What is the one sub-atomic particle which can be transferred to or from an atom?
electrons
What happens when a polythene rod is rubbed with a dry cloth? (charging by friction)
- transfers electrons to the surface atoms of the rod from the cloth
- the polythene rod becomes negatively charged and the cloth is positively charged
What happens when a perspex rod is rubbed with a dry cloth? (charging by friction)
- transfers electrons from the surface atoms of the rod on to the cloth
- the perspex rod becomes positively charged and the cloth becomes negatively charged
What does a circuit diagram show?
how components in a circuit are connected together
How do we identify components?
each has its own agreed symbol
What does a cell do?
necessary to push electrons around a circuit
a battery consists of 2 or more cells
What does a switch do?
enables you to turn the current in a circuit on or off
What does an indicator do (bulb)?
designed to emit light as a signal when current passes through it or as a light source
What does a diode do?
allows current to pass in one direction only
What does a light emitting diode (LED) do?
emits light when a current passes through it
What does an ammeter do?
measures the electric current which passes through it
What does a fixed resistor do?
limits the current in a circuit
What does a variable resistor do?
allows the current to be varied
What does a fuse do?
designed to melt and break the circuit if the current passing through it is greater than a certain amount
What does a heater do?
designed to transfer electrical energy to heat the surroundings
What does a voltmeter do?
used to measure potential difference i.e.voltage
What is an electric current?
a flow of charge/electrons
What does the size of an electric current show?
the rate of flow of electric charge
What is electric charge measured in?
coulombs (C)
What is electric current measured in?
amperes (A)
What is the equation for finding the current?
I = charge flow (C) ------------------------- time taken (s)
I = Q
t
What is the equation for potential difference?
V = work done (J)
———————
charge (C)
V = W
Q
What happens to electrons trying to pass through a torch bulb?
electrons have to pass through lots of vibrating ions in the metal filament
the ions resist the passage of electrons through the torch bulb
What do we measure resistance in?
ohms (omega sign)
What is the equation for resistance?
R = potential difference (v)
———————————-
current (A)
R = V
I
What is Ohm’s law?
the current through a resistor is directly proportional to the potential difference across the resistor
Why is a wire known as an ohmic conductor?
its resistance is constant
reversing the pd makes no difference to the shape of the line as the resistance is the same no matter what direction the current is in
What happens to the current-potential difference graph of a filament bulb and why?
- line curves away from the current axis, current is not directly proportional to the pd, the filament bulb is a non-ohmic conductor
- resistance increases as the current increases and therefore increases with temperature
- reversing the pd makes no difference to the shape of the curve, resistance same as current
Why does the resistance of the metal filament in a bulb increase as its temperature increases?
ions in the metal filament vibrate more as the temperature increases so they resist the passage of the electrons through the filament more
What happens to the current-potential difference graph of a diode and why?
- in the forward direction, the line curves towards the current axis so the current is not directly proportional to the pd, a diode is not an ohmic conductor
- in the reverse direction the current is negligible so its resistance in the reverse direction is much higher than that in the forward direction
- LED emits light when a current passes through it in the forward direction
What happens to the current-potential difference graph of a thermistor and light-dependent resistor (LDR) and why?
thermistor:
- at a constant temperature the line is straight and therefore the resistance is constant
- if the temperature is decreased, its resistance decreases
LDR:
- same idea but in terms of light emitted
What are the rules for components in a series circuit?
- the current is the same in each component
- adding the potential differences gives the total potential difference
What happens when you add the resistances of components in a series circuit?
gives the total resistance of resistors in series
For cells placed in series, what is the total potential difference?
(acting in the same direction)
the sum of their total individual potential differences
What are the rules for components in a parallel circuit?
- the total current is the sum of the currents passing through the separate components
- the bigger the resistance of a component, the smaller its current
What is the potential difference in a parallel circuit?
the same across each component
How do you calculate the current through a resistor in a parallel circuit?
current (A) = potential difference (V)
———————————–
resistance (Omega)
