P3 - Electric Circuits Flashcards
What is the charge on an electron?
What is the charge on an electron?
Electrons are negatively charged
(relative charge: -1).
What happens when two objects are
rubbed together?
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?
Where are electric fields found?
Around every electric charge.
What is an electric field?
What is an electric field?
A region of space in which the effects of charge can be felt. When another charge enters the field, both charges interact and experience a force.
Objects with the same charge…
Objects with the same charge…
…repel.
Objects with opposite charges…
Objects with opposite charges…
…attract.
Define an insulator
Define an insulator
An object which does not conduct
electricity. Electrons cannot flow through
the material.
How is static electricity produced?
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?
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
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?
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
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?
What is required in order for charge to flow?
● A potential difference
● A closed circuit
Describe the value of current across a
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
Give an equation linking charge and current, giving
SI units
Charge (C) = Current (A) x Time (s)
Q
I x TGive an equation linking current and
voltage, giving all SI units
Give an equation linking current and potential
difference, giving all SI units
P.D. (V) = Current (A) x Resistance (𝝮)
V
I x RDefine conductors in an electrical circuit
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
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?
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?
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
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
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.
Draw the IV characteristic of an ohmic
conductor (at a constant temperature)
Check
Explain the IV characteristic of
a filament lamp
Explain the IV characteristic of a filament
lamp
When current flows, the filament produces heat. This increases the resistance of the filament (R=V/I)
Explain the IV characteristic of
a diode
Explain the IV characteristic of a diode
Current through a diode flows in one direction only, as it has a very high resistance in the opposite direction.
explain the IV characteristic of a diode Current through a diode flows in one direction only, as it has a very high resistance in the opposite direction.
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?
What is a thermistor?
A resistor in which resistance decreases
as temperature increases.
What is an LDR?
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
check
Check cgp for the all symbols for everything
ok
Define potential difference
Define potential difference
The work done per unit charge.
Give the equation linking potential
difference and work done, with all SI
units
Give the equation linking potential difference and
work done
P.D. (V) = work done (J) ÷ charge (C)
W
V x QWhat is a series circuit?
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?
What is a parallel circuit?
A circuit in which the charge is split
between branches.
Describe the current across a series
circuit
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
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
Describe current across a parallel circuit
Current is split between the branches of the circuit.
Describe potential difference across a
parallel circuit
Describe the 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?
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?
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?
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?
How can sensing circuits be constructed?
Using thermistors or LDRs.
What factors affect the energy
transferred when charge flows through a
component?
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?
What is the power rating of an appliance?
It shows the power the appliance uses,
measured in Watts.
Define power
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
Give an equation linking power and potential
difference, giving all SI units
Power (W) = Current (A) x P.D. (V)
P
I x VGive an equation linking power and
energy, giving all SI units
Give an equation linking power and energy, giving all
SI units
Power (W) = Energy (J) ÷ Time (s)
E
P x TGive an equation linking energy and
potential difference, giving all SI units
Give an equation linking energy and potential
difference, giving all SI units
Work done (J) = P.D. (V) x Charge (C)
W
V x QGive equations linking power with
resistance, including all SI units
Give equations linking power and resistance,
including all SI units
power (W) = current² (A) x resistance (Ω)
P = I² x R
power (W) = p.d.² (V) ÷ resistance (Ω)
P = V² ÷ R
What is a step up transformer?
What is a step up transformer?
A transformer which increases voltage
(decreasing the current).
Why does current decrease when
voltage is increased?
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
Name the two coils in a transformer
Primary coil and secondary coil.
A step up transformer has more…
A step up transformer has more…
… turns on the secondary coil than on
the primary coil.
What is a step down transformer?
What is a step down transformer?
A transformer which decreases the voltage (increasing the current).
A step down transformer has more…
A step down transformer has more…
…turns on the primary coil than on the
secondary coil.
What is the transformer equation?
What is the transformer equation? primary voltage x primary current = secondary voltage x secondary current IpVp = IsVs so Pp = Ps This illustrates how power, and therefore energy, is conserved.
Why is electricity transmitted through the
national grid with a high voltage?
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?
When are step up transformers used?
Increasing voltage for transport across the National Grid.
When are step down transformers used?
When are step down transformers used?
Decreasing voltage of the National Grid
supply to 230V for domestic use.
What is a magnetic field?
What is a magnetic field?
A region in which a magnetic object
experiences a force.
Where is a magnetic field strongest?
Where is a magnetic field strongest?
At the poles of a magnet.
Magnetic field strength decreases as…
Magnetic field strength decreases as…
Distance from the magnet increases.
Where does attraction occur?
Where does attraction occur?
Between opposite (unlike) poles of two magnets.
Where does repulsion occur?
Where does repulsion occur?
Between like poles of two magnets.
Describe the key features of field lines in
a magnetic diagram
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?
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?
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?
What is a permanent magnet?
An object which always has poles, and is
therefore always magnetic.
What is an induced magnet?
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
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?
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?
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?
What is produced around a current carrying wire?
A magnetic field.
How can you determine the direction of a
magnetic field around a wire?
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?
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?
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?
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
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?
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
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? Using Fleming’s left hand rule. Thumb = Movement First finger = Field Second finger = Current
What is conventional current?
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?
What kind of current is used in Fleming’s rule?
Conventional current.
Which factors affect the strength of the
motor force?
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
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
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?
How is mains electricity produced?
Using electromagnetic induction.
What is 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 what direction is the potential difference induced?
In the opposite direction to the
movement which produced it.
When is a current produced?
When is a current produced?
When the ends of the coil are connected
to a complete circuit.
How are electrical generators set up?
How are electric 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
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?
What kind of current is produced by an ordinary
generator?
An alternating current.
How can a direct current be produced?
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?
What is a dynamo?
A system which produces direct current by continuously spinning a coil in a permanent magnetic field.
How do transformers work?
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?
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
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
Give the transformer equations linking the number of
coils, P.D. and current
N₁/N₂ = V₁/V₂ = I₂/I₁
How do microphones work?
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.
