Download Ques Electricity - South Newcastle Trust

Static electricity
Some insulating materials become electrically charged when they are rubbed together. Charges that
are the same repel, while unlike charges attract.
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Negatively charged particles called electrons move from one material to the other
The material that loses electrons becomes positively charged
The material that gains electrons becomes negatively charged
Both materials gain an equal amount of charge, but the charges are opposite
Electricity in circuits
Electrical circuits can be represented by circuit diagrams. The various electrical components
are shown by using standard symbols in circuit diagrams. Components can be connected in
series, or in parallel. The characteristics of the current and potential difference (voltage)
are different in series and parallel circuits.
You need to be able to draw and interpret circuit diagrams and symbols:
Equations you need to know:
Open
Switch
Closed
Switch
Lamp
Cell
Battery
Voltmeter
Resistor
Fuse
Current is the flow of electrons,
measured with an ammeter.
The size of the electric current is the
rate of flow of electric charge. You
can calculate the size of a current
using this equation:
I = Q ÷ t

Ammeter
Light
emitting
diode (LED)
Variable
resistor
Diode
the charge in coulombs)
Thermistor

Light

dependent
resistor
(LDR)
I is the current in amps A
Q is the charge in coulombs, C
t is the time in seconds, s
Potential difference is (pretty much)
the same as VOLTAGE – the push of
the batteries, the energy per electron
in the circuit. To measure, use a
voltmeter. To calculate use: V= I x R
(Voltage = Current x resistance)
Or V = W ÷ Q (V is the potential
difference in volts, W is the work done
(energy transferred) in joules, Q is
When two or more components are connected in series, the same current flows through each
component. When two or more components are connected in series, the total potential difference
of the supply is shared between them. This means that if you add together the voltages across
each component connected in series, the total equals the voltage of the power supply.
Resistance
Resistance is measured in ohms. It can be calculated from the potential difference across a
component and the current flowing through it. The total resistance of a series circuit is the
sum of the resistances of the components in the circuit.
Resistors, filament lamps and diodes produce different current-potential difference graphs.
The resistance of thermistors depends on the temperature, while the resistance of lightdependent resistors (LDRs) depends on the light intensity.
Resistance – What you need to know for Higher tier
An electric current flows when electrons move through a conductor, such as a metal wire. The
moving electrons can collide with the ions in the metal. This makes it more difficult for the
current to flow, and causes resistance. As the temperature increases, the metal ions vibrate
more. There are more collisions with the electrons and so the resistance increases.
The resistance of a long wire is greater than the resistance of a short wire because electrons
collide with ions more often.
The resistance of a thin wire is greater than the resistance of a thick wire because a thin wire
has fewer electrons to carry the current.
A graph of current against potential difference shows you how the
current flowing through a component varies with the potential difference
across it.
The current flowing through a resistor at a constant temperature is
directly proportional to the potential difference across it (STRAIGHT
LINE). This is the graph of current against potential difference for a
resistor at constant temperature (ie constant resistance).
The resistance of a lamp increases as the temperature of its filament
increases. The current flowing through a filament lamp is not directly
proportional to the voltage across it (S-SHAPE). This is the graph of
current against potential difference for a filament lamp.
Diodes have a very high resistance in one direction. This means that
current can only flow in the other direction. This is the graph of current
against potential difference for a diode:
A light-emitting diode, LED, produces light when a current flows through
it in the forward direction. LEDs are often used for indicator lights in
electrical equipment such as computers and television sets. As LEDs use a
much smaller current than other types of lighting, their use is increasing. They also have less
wasted energy as heat than filament bulbs.
AC/DC Circuits: with direct current (eg batteries) the current flows in ONE DIRECTION only.
However, in alternating current (eg household mains) the current constantly CHANGES
DIRECTION. It changes direction 50 times a second (50Hz).
Fuses: Fuses protect electrical circuits and appliances. The fuse breaks the circuit if a fault in
an appliance causes too much current flow. This protects the wiring and the appliance if
something goes wrong. Always choose a fuse ABOVE the working current of the appliance
1. A set of Christmas tree lights is made from twenty identical
lamps connected in series.
(a)
Each lamp is designed to take a current of 0.25 A. The
set plugs directly into the 230 V mains electricity supply.
(i)
Write down the equation that links current, potential
difference and resistance.
.........................................................................................................................
......................................................................................................................
(ii)
(1)
Calculate the resistance of one of the lamps. Show clearly how you work out
your final answer and give the unit.
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
..........................................................................................................................
Resistance = ......................................................................
(iii)
(4)
What is the total resistance of the set of lights?
..........................................................................................................................
..........................................................................................................................
Total resistance = ........................................................
(b)
(1)
How does the resistance of a filament lamp change as the temperature of the filament
changes?
...............................................................................................................................
...............................................................................................................................
...............................................................................................................................
.............................................................................................................................
(1)
(Total 7 marks)
3. (a)
The diagram shows the voltage-current graphs for three different electrical
components.
Which one of the components A, B or C could be a 3 volt filament lamp?
Explain the reason for your choice.
(3)
2. The diagram shows a simple type of car rear window heater. The six heating elements are
exactly the same.
Each heating element has a resistance of 5 Ω. The current passing through each element
is 0.4 A.
(i)
Calculate the total resistance of the six heating elements.
Show clearly how you work out your answer.
.....................................................................................................................................
.....................................................................................................................................
Total resistance = .............................. ohms
(2)
(ii)
Why is the current passing through each element the same?
.....................................................................................................................................
.....................................................................................................................................
(1)
(iii)
What is the total current passing through the whole circuit?
.....................................................................................................................................
(1)
(iv)
How is the 12 volt potential difference of the car battery shared between the six
heating elements?
.....................................................................................................................................
.....................................................................................................................................
(1)
(Total 5 marks)