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CfE Higher Physics Unit 3
Electricity
Higher Physics
Electricity
Pupil Notes
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CfE Higher Physics Unit 3
Electricity
Alternating and Direct Current
Links to National 5 – AC and DC supplies; voltage; current
At National 5, the only knowledge needed was the difference between AC/DC
supplies, like a battery or power pack.
At Higher level, the relationship between direct and alternating current and
how it affects a circuit will be looked at.
When a circuit is connected to a battery, the current always flows round the
circuit in one direction.
This is called direct current (d.c.)
When a circuit is connected to the mains supply, the current flows round the
circuit in one direction and then in the opposite direction.
This is called alternating current (a.c.)
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CfE Higher Physics Unit 3
Electricity
D.c. supplies will not change the level of voltage being supplied to a circuit for
the entire time that it is turned on for. The graph below shows what a d.c.
supply voltage will look like.
Voltage (V)
Time (s)
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CfE Higher Physics Unit 3
Electricity
As an a.c. supply voltage will change the direction of current periodically whilst
the circuit is on, the graph also follows this pattern. The graph being below the
x – axis indicates an opposite direction of flow.
Voltage (V)
Time (s)
From a.c. signal graphs, the peak voltage and frequency of the signal can be
calculate. When these signals are created they are generated with an
associated y-gain and time-base function.
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CfE Higher Physics Unit 3
Electricity
Voltage (V) = Height (number of boxes) x voltage/y gain
1
1
Frequency (Hz) =

period length  time base
Example
A signal from an a.c. supply is displayed on an oscilloscope. The time base is set
at 0.01 s/div. The y-gain is set at 4 V/div. Calculate the peak voltage and
frequency of the signal.
Peak Voltage  Height x Voltage Gain
Peak Voltage  3 x 4  12 V
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1
Length x Time Base
1
Frequency 
 25Hz
4x0.01
Frequency 
CfE Higher Physics Unit 3
Electricity
Peak and R.M.S. Voltage
As the voltage in an a.c. supply is always changing, it is very difficult to use the
peak voltage to power anything as this only last for a short period of time.
In order for an a.c. supply to have any use, the R.M.S. (root mean square)
voltage is used instead – this is almost equivalent to having a d.c. supply.
Vpeak
Vrms
The relationship between the peak voltage and the R.M.S. voltage is as follows
Vpeak  2 Vrms
The same relationship can be used when linking peak and R.M.S current
Ipeak  2 Irms
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CfE Higher Physics Unit 3
Electricity
Circuits
Links to National 5 – Charge; Current; Voltage; Ohm’s Law; Circuit Rules; Power
Revision of Previous Work
An electrical current is a flow of (negative) charge around a circuit. The amount
of charge that flows around a circuit depends on the size of the current and
the time the charge flows.
Q  It
Q = Charge (C)
I = Current (A)
t = Time (s)
The current in a component in a circuit can be measured by placing an ammeter
in series with the component and the potential difference (p.d.) across a
component measured by placing a voltmeter in parallel with it.
A component is said to have a resistance if a potential difference (p.d.) is
needed to drive a current through it.
V  IR
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CfE Higher Physics Unit 3
Electricity
Power tells us how much energy is used each second. There are four power
formulae that can be used. Each of these was derived in National 5.
P
E
t
P  IV
P  I2R
V2
P
R
P = Power (W)
I = Current (A)
V = Voltage (V)
R = Resistance (Ω)
E = Energy (J)
t = Time (s)
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CfE Higher Physics Unit 3
Electricity
Series and Parallel Circuits
In a series circuit the current is the same at all points. In a series circuit the
sum of the voltages across all the components is equal to the supply voltage.
In a parallel circuit, the voltage across each component is the same as the
supply voltage. In a parallel circuit, the sum of the current flowing through each
branch is equal to the supply current.
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CfE Higher Physics Unit 3
Electricity
Electrical Sources and Internal Resistance
Links to National 5 – Circuit Rules; Ohm’s Law
In Higher, we do not just use the terms voltage, current and resistance. We
have to be much more specific about which feature in a circuit we are talking
about.
E.M.F
A battery provides an electromotive force (e.m.f.) to drive current around a
circuit. The e.m.f is defined as the electrical energy supplied to each coulomb
of charge that passes through the battery. The units of e.m.f are the same as
p.d. (potential difference). At Nat 5, we would have described this as the supply
voltage.
A voltmeter can be used to find the e.m.f. of a battery or the p.d. across a
component. Unlike in National 5, we do not expect to see the p.d. across a
component to be the same value as the e.mf. Consider the following circuit:
In this circuit, the only component affecting current or drawing a p.d. is the
variable resistor. Using Ohm’s Law (and at Nat 5), we would expect to see the
current falling as the resistnace increased. We would also expect the voltage to
remain the same as there is no other component.
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CfE Higher Physics Unit 3
Electricity
In reality, the graph plotted is more like the following:
The p.d. across the varibale resistor will slowly decrease as the current
decreases. The circuit appears to lose volatge.
Lost Volts
These “lost volts” are due to a small internal resistance within the battery. for
circuits containing internal resistance, the battery is in cased in a dotted box.
Internal resistance is denoted by a lower case ‘r’. The
circuit at the side is still a series circuit so
therefore the current flowing through the load (big)
resistor and internal resistance will be the same.
A larger form of Ohm’s Law can be used.
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CfE Higher Physics Unit 3
Electricity
Electromotive Force  Potential Difference  Lost Volts
E.M.F.  P.D.  L.V.
E  IR  Ir
Example
The e.m.f. of a battery is 12 V. When an 8 Ω resistor is connected to the
battery a current of 1 A flow in the circuit.
(a) What is meant by an e.m.f. of 12 V?
(b) Calculate the p.d. across the 8 Ω resistor.
(c) Calculate the ‘lost’ volts.
(d) Calculate the internal resistance.
(a) The maximum amount of energy provided to each coloumb of charge in the circuit is
12J OR when the current is zero, the p.d. across the battery is 12 V.
(b) V  IR
V 1x8
V  8V
(c) EMF  PD  LV
12  8  LV
LV  12 - 8  4V
(d) V  Ir
4 1xr
r4
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CfE Higher Physics Unit 3
Electricity
Calculating E.M.F and Internal Resistance from a graph
When a circuit is set up the p.d. across the battery will drop immediately. This
means that the p.d. across the component will not equal the E.M.F. Below is a
graph showing the previous statement.
In order to measure the E.M.F of the supply, the line of graph must be brought
back to meet the y-axis.
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CfE Higher Physics Unit 3
Electricity
Where the altered line meets the y-axis determines the E.M.F of the supply.
For the above graph the E.M.F. of the battery will be 6V.
To calculate the lost volts and internal resistance of the circuit, we must take
into consideration the current axis of the graph also.
As the internal resistance will always be in series with the rest of the circuit,
we take a point on the graph that is easy to read to record the current.
From the graph, when the P.D. is 5v, the current is 5A. If the P.D. is 5V and the
E.M.F. is 6V, the lost volts will be 1V.
Using Ohm’s Law (V = Ir), the internal resistance ‘r’ is calculated to be 0.2 Ω.
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CfE Higher Physics Unit 3
Electricity
Capacitors
A capacitor is a device that is used to store charge. Capacitors can be used to
keep a circuit running if there is a failure with the battery or power supply. A
capacitor consists of two pieces of metal in close proximity to each other – but
they are still separated by a dielectric material.
How it works
An electron arrives from the
negative terminal of the battery
and repels an electron from the
opposite side of the capacitor.
The electrons build up on one side
of the capacitor. This causes a
P.D. to develop across the
capacitor which opposes the P.D.
of the battery.
Eventually the P.D. across the
capacitor equals the P.D. of the
battery. This means no more
charge can flow and one side of
the capacitor is full of electrons.
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CfE Higher Physics Unit 3
Electricity
The amount of charged stored in the capacitor is directly proportional to the
P.D. across the capacitor.
The ratio between charge and potential difference is known as the capacitance
which is measured in Farads.
C
Q
V
C = Capacitance (F)
Q = Charge (C)
V = Potential Difference (V)
N.B. A capacitance of 1F is MASSIVE!! You will be expecting a small number in
calculations, like millifarads or microfarads.
Example
Calculate the charge stored in a 2000 μF capacitor when the p.d. across the
capacitor is 12 V.
Q
V
Q  CV
Q  2000x10 6  12
C
Q  0.024C
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CfE Higher Physics Unit 3
Electricity
Energy Stored in a Capacitor
Using our graph from before:
The energy stored in a
capacitor can be calculated
using the area under the
graph.
E
1
bh
2
1
E  QV
2
(1)
E = Energy (J)
Q = Charge (C)
V = Potential Difference (V)
Using Equation 1, as well as the equation for capacitance, we are able to derive
two other formulae used to calculate the energy stored in a capacitor.
Method 1
1
E  QV
2
Method 2
1
E  QV
2
Q  CV
1
E  CVV
2
E
1
E  CV 2 (2)
2
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V
1 QQ
2 C
1 Q2
E
2 C
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Q
C
(3)
CfE Higher Physics Unit 3
Electricity
Example
The p.d. across a 2000 μF capacitor is 12 V. Calculate the energy stored in the
capacitor.
*There is no mention of charge in the question so no need to consider
Equations 1 or 3*
1
E  CV 2
2
E
1
2
 2000x10 6  12 
2
E  0.144J
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CfE Higher Physics Unit 3
Electricity
Charging a Capacitor
When the capacitor is initially turned on (i.e. the switch is closed) the electrons
will be able to flow freely and there will be no P.D. across the capacitor. Over
time, the leectrons build up on the capacitor and it makes the flow of current
get less and the P.D. increases until the capacitor is fully charged.
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CfE Higher Physics Unit 3
Electricity
Discharging a Capacitor
When the battery is removed, the capacitor acts as the source of energy for
the current. The electrons will be able to flow freely with maximum energy.
Over time, the energy will begin to dissipate in the circuit and the flow of
electrons will begin to fade to zero.
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CfE Higher Physics Unit 3
Electricity
Electrons at Work – Semiconductors
You should know from previously carried out work that a conductor is a material
that allows electrons to flow freely through it. An insulator is a material that
does not allow electrons to flow freely through it.
A semiconductor is a material that is somewhere in between. The amount of
conductivity (the ability to allow electrons to flow) is dependent on the
temperature of the semiconductor, as well has the form of energy being
supplied to it (e.g. UV light).
Band Theory is used describe these conductivity properties of conductors,
insulators and semiconductors.
All materials have two layers – a valence band and a conductive band. The
conductive band will allow electrons to flow freely through it, whereas the
valence band holds the electrons tightly.
In a conductor:
In a conductor, the conduction band is not
completely full and will have space for
electrons to move about freely
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CfE Higher Physics Unit 3
Electricity
In a insulator:
The valence band is full and the conduction
band has no free electrons.
The band gap between them is two large for
electrons to pass between so there is no
conductivity.
In a semi-conductor:
There is a gap between the conduction and
valence band. The gap is small enough for
electrons to “jump” from the valence band to
the conduction band. At room temperature,
there electrons have just enough energy. As
the temperature rises, more electrons can
“jump, therefore increasing conductivity.
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CfE Higher Physics Unit 3
Electricity
Doping
Semiconductors are usually made of materials such as silicon or germanium.
Each of these elements contains a certain number of electrons. By doping a
semiconductor with another element, the conductivity of the semiconductor
changes – without the need to increase temperature.
Take silicon: it has four electrons on the outside shell. A semiconductor made
only from silicon is known as a pure semiconductor.
The semiconductor can be made into an N-type material (negatively charged) by
adding an element with one extra electron, such as arsenic. The semiconductor
becomes negatively charged due to there being an extra electron floating
around.
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CfE Higher Physics Unit 3
Electricity
In a similar way, a semiconductor can be made into a P-type material by doping
the semiconductor with an element that contains one less electron such as
indium. This creates a hole – a gap where an electron could fit into if the
conductivity is increased by temperature.
The P-N Juntion and Diodes
Diodes are semiconductor materials that can conduct when the correct amount
of voltage is applied to it. Diodes can be used in electronic switchs or small
electronic devices. Unlike other componets, like a lamp or motor, a diode will
only work if it is connected the correct way.
A diode is created using a P-N junction. A P-N junction is formed when a p-type
material and an n-type material are brought together. The p-type material will
contain holes and the n-type material contains electrons. At room temperature,
and when no voltage is applied, there is a small depletion layer between the
materials – this area has no free charge particles and at room temperature,
electrons cannot pass through.
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CfE Higher Physics Unit 3
Electricity
P-N juntions can be connected in either forward or reverse bias – this is
determined by how the voltage is applied across the juntion.
In forward bias, the diode is connected the correct way to the power supply.
This causes the depletion layer between the P-N junction to become negligable
and electrons can flow towards the holes.
In reverse bias, the diode is not connected the right way round. The deplection
layer becomes greater and no conduction can take place.
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CfE Higher Physics Unit 3
Electricity
LEDs
Light Emitting Diodes (LEDs) are semiconductor diodes that, when stimulated
with the right amount of energy, will emit a photon of light. The colour of this
phton depends on the energy change within the semiconductor material. The
light is meitted by electrons meeting with holes. Just like all diodes, the LED
must be conncted in forward bias in order to work.
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