ⓉⓄⓅⒾⒸ 4 Flashcards
Magnetic forces
are due to interactions between magnetic fields. In a magnet, like poles repel
and opposite poles attract.
● Magnetic materials are materials that are attracted to magnets
and can be magnetised
● Non-magnetic materials are materials that are not attracted to
magnets and cannot be magnetised
Induced magnetism
○ They can be magnetised by stroking them with a
magnet, hammering them in a magnetic field, or putting
them inside a coil with a direct current through it.
○ They can be demagnetised by hammering them,
heating them or putting them inside a coil with an
alternating current through it.
magnetically hard magnets
Magnetic materials that can be permanently magnetised
magnetically soft magnets
Magnetic materials that are only temporarily
magnetised
permanent magnets vs electromagnets
● Permanent magnets are a hard-magnetic material that has been permanently magnetised
whereas electromagnets consist of a coil of wire wrapped around a magnetically soft core
and can be turned on and off.
● Permanent magnets are more useful when they do not need to be turned off such as a
fridge magnet, whereas electromagnets have the ability to be turned on and off so they can
be used for situations such as moving scrap metal.
Magnetic fields:
● Field lines around a bar magnet point from north to south
● The direction of a magnetic field line shows the
direction of the force on a north pole at that point.
● Field strength decreases with distance from the magnet
● Plotting compasses are small compasses which show the
direction and shape of a magnetic field.
Charge
is measured in coulombs. There are positive and negative charges; unlike charges
attract and like charges repel.
● Charging a body involves the addition or removal of electrons.
● Conductors allow electrons to flow through them whereas insulators impede the flow of
electrons.
How can charge be detected
using a gold leaf electroscope.
● If a positively charged rod is brought close to the
disc on top of the electroscope, electrons are
attracted to the top of the disc, away from the
bottom of the metal stem and the gold leaf. The
gold leaf will then be repelled from the metal stem
because they both become positively charged.
● If someone then touches the disc, electrons
flow from the ground into the disc as they are
attracted to the rod, and the electroscope now
contains a net negative charge. This is called
charging by induction.
Charges create electric fields
(when you place a charged object in an electric field it will experiences a force)
the direction of an electric field at a point is the direction of the force on a positive charge at
that point.
● Electric field lines point away from positive charges and
towards negative charges.
○ The field lines around a charged conducting
sphere are as if the charge was concentrated at
the centre of the sphere.
○ The field lines between two charged plates go in
straight lines from the positive plate to the
negative plate and are equally spaced apart.
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current
I is measured in amps and is the rate of flow of charge at a point in the circuit.
● The current is given by I=Q/t.
● It is measured with an ammeter placed in series.
● In metals, current is due to a flow of electrons. Because electrons are negatively
charged, conventional current (which is the rate of flow of positive charge) is in the
opposite direction to the flow of electrons.
Electromotive force (E.M.F)
of an electrical source of energy is measured in volts and is the
energy supplied by the source per unit charge in driving the charge round a complete
circuit.
Potential difference
V is measured in volts (1 V = 1 JC-1
) and is the work done per unit charge in
moving between two points in a circuit.
● It is measured with a voltmeter placed in parallel across the component.
● The higher the potential difference, the greater the current.
Resistance
The resistance of a component is given by the potential difference across it divided by the current
through it. The greater the resistance, the harder it is for current to flow through the component.
● As the length of a resistor increases, the resistance increases.
○ The resistance is directly proportional to the length.
● As the diameter of a resistor increases, the resistance decreases.
○ The resistance is inversely proportional to the cross-sectional area
Electrical working
● Energy is transferred from chemical energy in the battery to electrical energy used by circuit
components and then to the surroundings.
● The power of a component is given by P=IV.
● By using V=IR, this can be shown to be equivalent to P=I2R and P=V2
/R.
Electric circuits Series:
● Components are connected end to end in one loop
● The same current flows through every component
● The potential difference is shared across each component (i.e. the sum of the p.d.s across
the components is equal to the total p.d. across the supply).
● The total resistance is the sum of the resistances of each component RT = R1 + R2 + …
● The combined e.m.f. of several sources in series is the sum of the individual e.m.f.s
Electric circuits Parallel:
● Components are connected to the power supply in separate branches
● The current is shared between each branch (i.e. the sum of the currents in the separate
branches is equal to the current through the source)
● The potential difference is the same across every branch
● The total resistance of two resistors in parallel is less than the resistance of either resistor
by itself, and is given by 𝟏
𝑹𝑻
=
𝟏/𝑹𝟏
+
𝟏/𝑹𝟐
● Connecting lamps in parallel is advantageous because if one breaks, current can still pass
through the rest.
potential divider:
a circuit divides the source voltage
into smaller parts.
● The voltage across a certain component is
given by𝑉𝑜𝑢𝑡 = 𝑉𝑖𝑛 ×
𝑅/𝑅𝑇
where Vin is the
source voltage, R is the resistance of the
component and RT is the total resistance.
A thermistor
is a resistor whose resistance decreases
as the temperature increases.
light dependent resistor
is a resistor whose
resistance decreases as light intensity increases.
A relay
is an electromagnetically operated switch.
When a small current passes through the
electromagnet, it switches on and attracts an iron
arm. This arm rotates about a pivot and pushes the
contacts in another circuit together.
● They are used to switch on a circuit with a
high current using a circuit with a small
current
relay, thermistor and light dependent resistor can be used in conjunction to
operate light-sensitive switches
and temperature-operated alarms.
Diodes
only allow current to flow in one
direction, because they have a very high
resistance in the other direction. They can be
used as a rectifier (i.e. convert AC into DC).
Digital electronics Analogue:
signals vary continuously in amplitude, frequency or both
Digital electronics Digital:
signals are a series of pulses with two states, a high state and a low state.
Digital signals carry more information per second and maintain their quality better
over longer distances compared to analogue signals.