P3 - Electric Circuits Flashcards
What is the charge on an electron?
Electrons are negatively charged
relative charge: -1
What happens when two objects are
rubbed together?
They become charged because negatively charged electrons are transferred from one object (which becomes positive) to the other (which becomes negative).
Where are electric fields found?
Around every electric charge.
What is an electric field?
A region of space in which the effects of
charge can be felt.
Objects with the same charge…
…repel.
Objects with the same charge repel
Objects with opposite charges…
…attract.
Objects with opposite charges….attract.
Define an insulator
An object which does not conduct
electricity. Electrons cannot flow through
the material.
How is static electricity produced?
When two insulators are rubbed
together, transferring electrons, to form a
positive and a negative charge.
How does sparking occur?
Sparking occurs when enough charge builds up between two statically charged objects. A spark is produced by the charge jumping through the air from the negative to the positive object to restore the charges (back to two neutral objects).
Define a conductor
A conductor is a material which can
conduct electricity; electrons are able to
flow through it.
Why can conductors not produce static
electricity?
Electrons flow through the materials
when they are rubbed together to cancel
out any charging effect.
Define an electric current
Current is the rate of flow of charge in an
electric circuit.
What is required in order for a
charge/current to flow?
● A potential difference
● A closed circuit
Describe the value of current across a
circuit
Current has the same value at any point
in a closed (series) circuit.
Give an equation linking charge and
current, giving SI units
charge (C) = current (A) x time (s)
Give an equation linking current and
voltage, giving all SI units
p.d. (V) = current (A) x resistance (Ω)
Define conductors in an electrical circuit
The components of the circuit (including
wires) which carry a charge and conduct
electricity.
Define resistors in an electrical circuit
Components such as resistors, lamps
and motors which resist the flow of
charge through them.
Why are wires not considered resistors?
Their resistance is so small it is
considered negligible.
How does resistance affect the current
flowing through a circuit?
The larger the total resistance in the
circuit, the smaller the current will be.
Describe an experiment to investigate
the resistance of a wire
● Use a length of wire connected to an ammeter (in series), a
voltmeter (in parallel) and a power supply.
● Connect two crocodile clips to the wire, one at each end, and
record the current and voltage.
● Vary the length of the wire (moving one of the clips),
recording V and I.
● Plot a graph of V against I; the gradient = resistance of wire.
Describe the key features of an ohmic
conductor
Resistance is constant, meaning the
conductor’s IV characteristic (graph of
current against voltage) has a linear
(straight line) gradient.
How does resistance change with
temperature?
As temperature increases, ions and electrons
have more kinetic energy. This results in more
collisions between the stationary metal ions,
making it more difficult for electrons to move
through the wire and carry a charge. Therefore
resistance increases with temperature.
What is a thermistor?
A resistor in which resistance decreases
as temperature increases.
What is an LDR?
A resistor in which resistance decreases
as light intensity increases…
e.g. used in automatic night lights.
Draw the circuit symbol for a switch
—-o—–o—- ——o__ o—–
closed open
Define potential difference
The work done per unit charge.
Give the equation linking potential
difference and work done, with all SI
units
p.d. (V) = work done (J) ÷ charge (C)
What is a series circuit?
A series circuit is one in which the same
charge passes through all components,
so the current through each component
is the same.
What is a parallel circuit?
A circuit in which the charge is split
between branches.
Describe the current across a series
circuit
Current is the same at all points in a
series circuit.
Describe potential difference in a series
circuit
Potential difference is split between
components, according to the ratio of
their resistances.
Describe current across a parallel circuit
Current is split between the branches of
the circuit.
Describe potential difference across a
parallel circuit
Each branch has a potential difference
equal to the potential difference of the
supply.
What is total resistance in a series
circuit?
The sum of the resistances of each
component.
What is the total resistance in a parallel
circuit?
The total resistance will be lower than that of the branch with the lowest resistance. This is because the charge is split; decreasing the charge to each component decreases its resistance.
What is a sensing circuit?
A circuit in which resistance is dependent
on an environmental factor such as light
or temperature, so therefore can be used
to monitor environmental changes.
How can sensing circuits be
constructed?
Using thermistors or LDRs.
What factors affect the energy
transferred when charge flows through a
component?
● Amount of charge.
● The potential difference across the
component.
What is the power rating of an
appliance?
It shows the power the appliance uses,
measured in Watts.
Define power
The rate of energy transfer, or the rate at
which work is done.
Give an equation linking power and
potential difference, giving all SI units
power (W) = current (A) x p.d. (V)
Give an equation linking power and
energy, giving all SI units
power (W) = energy (J) ÷ time (s)
Give an equation linking energy and
potential difference, giving all SI units
work done (J) = p.d. (V) x charge (C)
Give equations linking power with
resistance, including all SI units
power (W) = current^2 (A) x resistance (Ω)
P = I^2 x R
power (W) = p.d.^2 (V) ÷ resistance (Ω)
P = V^2 ÷ R
What is a step up transformer?
A transformer which increases voltage
decreasing the current
Why does current decrease when
voltage is increased?
Because of the principle of conservation
of energy; the output power cannot be
greater than the input power because the
energy cannot be increased.
Name the two coils in a transformer
Primary coil and secondary coil.
A step up transformer has more…
… turns on the secondary coil than on
the primary coil.
What is a step down transformer?
A transformer which decreases the
voltage (increasing the current).
A step down transformer has more…
…turns on the primary coil than on the
secondary coil.
Why is electricity transmitted through the
national grid with a high voltage?
This results in a low current, resulting in
less power dissipation (so less energy is
wasted in transport).
When are step up transformers used?
Increasing voltage for transport across
the National Grid.
When are step down transformers used?
Decreasing voltage of the National Grid
supply to 230V for domestic use.
What is a magnetic field?
A region in which a magnetic object
experiences a force.
Where is a magnetic field strongest?
At the poles of a magnet.
Magnetic field strength decreases as…
Distance from the magnet increases.
Where does attraction occur?
Between opposite (unlike) poles of two magnets.
Where does repulsion occur?
Between like poles of two magnets.
Describe the key features of field lines in
a magnetic diagram
● They have arrows pointing from north
to south.
● The lines never touch, cross or
overlap.
How is the strength of a magnet
displayed in a diagram?
By the spacing of the field lines (the
closer together they are, the stronger the
magnet.)
What do the arrows on field lines
represent?
The direction of the force that would be
experienced by the north pole of a
magnet placed in the field.
What is a permanent magnet?
An object which always has poles, and is
therefore always magnetic.
What is an induced magnet?
A magnetic material which does not have fixed
poles. They can be induced to become magnets
(giving them poles) by placing them in a
magnetic field, but they lose their magnetisation
when the field is removed.
Explain how compasses work
When placed in the Earth’s magnetic
field, the north pole of a magnetic
compass points towards the Earth’s
north pole.
What do compasses provide evidence
for?
That the core of the Earth is magnetic.
What is the difference between Earth’s
magnetic and geographic poles?
Geographic north is at the top of a globe.
Magnetic north is at the bottom of a globe,
meaning magnetic south is at the top. This is why
the north poles of compasses point towards the
north pole (it is the magnetic south).
What is produced around a current
carrying wire?
A magnetic field.
How can you determine the direction of a
magnetic field around a wire?
Using the Right Hand Grip Rule. Produce a thumbs-up shape with your right hand and point your thumb in the direction of the flow of current. The field lines wrap around in the direction of your fingers.
What is a solenoid?
A coil of wire with a magnetic field, which
can be used as an electromagnet.
How does coiling a wire affect the
magnetic field?
It increases the field strength, as the
magnetic fields of each turn of wire are
added together.
How can you increase the strength of a
solenoid magnet?
● Using an iron core to carry field lines (as they
travel more easily through metal than air).
● Increase the number of turns in the coil.
● Increase the current.
Describe how a loudspeaker works
● Current flows into a coil, producing a magnetic field.
● The field interacts with a field from a permanent
magnet in the loudspeaker.
● The coil experiences a force, causing it to move.
● The movement of the coil causes the loudspeaker
cone to move, producing pressure variations which
produces sound.
What is the motor effect?
The force experienced by a
current-carrying wire placed in a
magnetic field, causing the wire to be
forced out of the field.
Describe the mechanism of the motor
effect
● When a current-carrying wire produces a
magnetic field within the field of a permanent
magnet, the two fields interact.
● The wire experiences a force pushing it away
from the magnetic field, at right angles to the
direction of the permanent field and the
current.
How can you predict the direction of the
motor effect?
How can you predict the direction of the
motor effect?
What is conventional current?
A model for current which flows in the
opposite direction to electrons.
Conventional current flows from positive
to negative.
What kind of current is used in Fleming’s
rule?
Conventional current.
Which factors affect the strength of the
motor force?
● The length of wire placed in the field
● The current in the wire
● The strength of the permanent field
Give an equation linking force with
current, including all units
Force (N) = magnetic flux density (T) x
current (A) x length of wire (m)
F = BIL
Describe how an electric motor works
● A rectangular wire, or coil, lies between two
permanent magnetic poles, so current flows up
one side and down the other.
● The motor effect produces a couple on the coil,
causing one side to move upwards and the other
to move downwards, causing the wire to rotate.
How is mains electricity produced?
Using electromagnetic induction.
What is electromagnetic induction?
The production of a potential difference
when there is relative movement
between a conductor and a magnetic
field.
In what direction is the potential
difference induced?
In the opposite direction to the
movement which produced it.
When is a current produced?
When the ends of the coil are connected
to a complete circuit.
How are electrical generators set up?
In the same way as a motor, with a rectangular loop of wire between permanent magnets. The main difference is the presence of a turbine to spin the coil.
Describe how an electrical generator
works
● A turbine spins the coil of wire between the
magnets.
● The wire cuts through the magnetic field,
experiencing a changing magnetic field.
● A potential difference is induced.
● A current is produced.
What kind of current is produced by an
ordinary generator?
An alternating current.
How can a direct current be produced?
Using a split ring commutator; this
disconnects and reconnects the wires
every half rotation, switching the current
so the motor spins continuously.
What is a dynamo?
A system which produces direct current
by continuously spinning a coil in a
permanent magnetic field.
How do transformers work?
● An alternating current flows through the primary
coil, producing an alternating magnetic field.
● This causes the secondary coil to experience a
changing magnetic field, inducing a potential
difference, which produces an alternating
current in the secondary coil.
Why do step up transformers increase
voltage?
There are more coils experiencing the
change, so a larger p.d. is induced.
State one assumption used in
transformer calculations
The transformer is 100% efficient (the
power is assumed to be the same in both
coils).
Give the transformer equations linking
number of coils, p.d. and current
n_1/n_2 = v_1/v_2= 1_2/I_1
How do microphones work?
● A current is produced which is proportional to the signal
strength of the sound.
● There is a fixed magnet in the centre, with the coil around it
free to move.
● Pressure variations from the sound waves cause the coil to
move, cutting the magnetic field lines and inducing a p.d.
and therefore a current.
● The current is transmitted to a loudspeaker.
