Download Electricity

Higher Physics
Electricity
Homework sheets
Current, Voltage, Power and Resistance
1
2
3
4
5
6
Carry out calculations involving potential differences, currents and resistances in circuits
containing resistors.
State that the e.m.f. of a source is the electrical potential energy supplied to each coulomb of
charge which passes through the source.
State that an electrical source is equivalent to a source of e.m.f. with a resistor in series, the
internal resistance.
Describe the principles of a method for measuring the e.m.f. and internal resistance of a source.
Explain why the e.m.f. of a source is equal to the open circuit p.d. across the terminals of the
source.
Use the following terms correctly: charge, current, p.d., resistance, terminal p.d., load resistor,
e.m.f., lost volts, short circuit current.
Alternating Current and Voltage
1
Describe how to measure frequency and voltage using an oscilloscope.
2
Carry out calculations involving peak and r.m.s. values of voltage and current.
Capacitors
1
2
3
4
5
6
7
8
Describe the principles of a method to show that the charge on a capacitor is directly proportional
to the p.d. across the capacitor.
State that capacitance is the ratio of charge to p.d.
State that the unit of capacitance is the farad and that one farad is one coulomb per volt.
Carry out calculations involving the relationship between capacitance, charge and potential
difference.
State that the energy stored in a capacitor is given by the area under the graph of charge against
p.d.
Carry out calculations involving the relationships between capacitance, charge, potential
difference and energy stored in a capacitor.
Draw qualitative graphs of current against time and of p.d. against time for the charge and
discharge of a capacitor in a d.c. circuit .
Carry out calculations involving p.d. and current in CR circuits.
Conductors, Semiconductors and Insulators
1
2
3
4
State that materials can be divided into three categories (conductors, semiconductors or insulators)
by their ability to conduct electricity.
Describe the arrangement of electrons when atoms come together to form solids.
Describe the operation of conduction and valence bands in conductors, semiconductors and
insulators.
Describe how p-n junctions can be used in a number of devices.
Homework 1
Alternating Current
1.
A light bulb has resistance of 960Ω. It is connected up to the 230 Vrms mains. Calculate:
(a)
the peak voltage of the mains
(b)
the rms current in the bulb
(c)
the peak current in the bulb
(d)
the mean power dissipated in the light bulb
(e)
the maximum power dissipated in the light bulb.
2.
In designing an electric fan, an engineer arranges two switches in such a way that the heater
element cannot be on alone without the fan.
(a)
(b)
Suggest a reason why she does this.
[
Draw a circuit which will achieve the manufacturer’s specifications, that is it must be possible
to have :(i) the heater element and fan both switched off
(ii) only the fan on
(iii) both the fan and heater element on
(c)
If the heater element has a power rating of 1.5 kW when it is operated from a 230 V mains
supply, what is it’s resistance?
The 230 V in (c) refers to the root mean square value of the ac mains voltage.
(i) Explain the term “root mean square voltage”
(ii) The mains frequency is 50 Hz. Draw a graph of the instantaneous mains voltage against
time covering a time period of 0.04 s. (Label both graph axes)
(d)
3.
A signal generator has an ac supply of frequency 50Hz and rms voltage equal to 5V.
The frequency is increased
(a) Calculate the peak voltage at 50Hz
(b) Calculate the peak current at 50Hz
(a) Draw a graph to show how the current varies with frequency (no values required)
(b) Explain what happens to the voltage across the resistor when the frequency is increased
4. The sinusoidal output voltage from a signal generator is applied to the input terminals of an
oscilloscope. The trace shown below is obtained on the oscilloscope screen
Y gain setting = 2V/division
Time base = 20ms/division
(a) Calculate the rms voltage
(b) Calculate the frequency of the supply
Homework 2
Resistor circuits
1. In the circuit opposite, resistors R1 , R2 and R3 are connected
in series across a 24 V supply
R1 25 k
+
(a)
(b)
(c)
24 Volts
What is the total resistance?
What is the current in resistor R1?
What is the voltage across R2 ?
-
R2 32 k
R3 7 k
2.
Three resistors are connected to a 12 V supply as shown. An ammeter and a voltmeter are
placed as indicated. What are the readings on the two meters?
R1
+
12 Volts 4 k
(a)
(b)
(c)
(d)
3.
R2
32 k
R3
64 k
V
A
Calculate the total resistance
What is the expected voltmeter reading?
What is the expected ammeter reading?
Why might the reading not be what you have calculated?
In the following arrangement of resistors,
what is the resistance between
points P and Q?
4. A small aquarium heater is designed to operate at 100 V and 0.1 A. It can be correctly operated
from a 230 V mains supply by inserting a suitable resistor R in series.
(a) Draw a labelled circuit diagram .
(b) Calculate the value of the resistor R
(c) Calculate the power dissipated by the resistor R
Homework 3
Potential Dividers
+Vs
1.
Consider the circuit.
15k
20V
5k
4V
(a)
(b)
Calculate the value of the unknown voltage
Calculate the supply voltage
V= ?
10k
Rx
12V
2. Consider the following circuit.
+Vs
(a) Calculate the value of the unknown resistor
(b) Calculate the supply voltage
5k
6k
Rx
8V
4V
10V
12V
(c) Another resistor of value 20k is now put in parallel with the resistor Rx
+Vs
Calculate the new value of voltage across Rx
6k5k
20k
20k
Rx
3. Each of the following circuits have no current flowing between AB
A
A
X
5k 10k
20k 2k
Y
10k
5k
5k20k
+ 5V
+ 5V
0V
0V
R
2k
20k
R
2.2k
35k
B
B
(a) Calculate the unknown resistance in the above circuits
(b) What effect does increasing the supply voltage have on the meter reading. Explain your
answer.
Homework 4
Emf and Internal Resistance
1.
A battery is marked as having an Emf of 6 V.
(a) What does Emf stand for?
(b) Describe Emf in terms of energy and charge.
2.
Explain how you would find the internal resistance of a battery using an ammeter, voltmeter,
variable resistor and a switch. Your answer should include:
(a) A diagram of how the apparatus is connected
(b) How you would obtain suitable measurements
(c) How you would use your measurements to obtain a value for the internal resistance of the
battery.
3.
A car mechanic measures the Emf of a car battery drawing no current as 12.6 V. When the rear
screen heater (resistance 30.0 Ω) is switched on the p.d. produced by the battery drops to 12.0 V
(a) What characteristic of the battery causes the voltage drop?
(b) What is the term used to describe the drop in p.d. observed?
(c) Draw a labelled circuit diagram to illustrate the situation when the rear screen heater is
operating.
(d) Calculate the current flowing in the circuit
(e) Calculate the internal resistance of the battery
The mechanic accidentally drops a spanner of resistance zero ohms across the terminals of the
battery, effectively short circuiting it.
(f) Draw a circuit diagram to represent this situation
(g) What is the output p.d. from the battery under these conditions?
(h) Calculate the current flowing in the battery as a result of the short circuit
4.
The following circuit is set up
The following measurements are taken
The current flowing = 2A. The resistance R = 4ohms and the emf E =12V
(a) What is meant by lost volts
(b) Calculate the lost volts
(c) Calculate the internal resistance of the supply
(d) A thick wire is dropped across the terminals of the power supply. Calculate the reading on the
ammeter
5.
A student sets up the following circuit to determine the internal resistance of a battery
The student measures values of potential difference for different values of current and the results
plotted on graph
(c) Calculate the internal resistance
(d) Calculate the emf
(e) Explain why the potential difference decreases as the current increases
(d) Calculate the current if a thick wire is dropped across the terminals of the supply
6.
A pupil sets up the circuit as shown, the resistance of the external resistor being 4ohms
R
With S open the voltmeter reading = 8.0V
With S closed the voltmeter reading = 7.2V
(a)
(b)
(c)
(d)
What is the emf of the power supply
Calculate the internal resistance
Calculate the power dissipated in the power supply
Calculate the power dissipated in the external resistor
Homework 5
Capacitors
1. A capacitor has a capacitance of 4μF. What does this mean
2. The following circuit is used to find the capacitance of a capacitor and a graph drawn of the
charge against voltage as shown.
Charge/nC
Coulombmeter
V
C
20
10
2
4
Voltage/V
(a) Calculate the capacitance of the capacitor
(b) Calculate the energy store in the capacitor when the voltage across C is 4V
3. A 100μF capacitor is fully charged using a 9V battery.
(a) Calculate the total charge stored in the capacitor
(b) Calculate the maximum energy stored in the capacitor
4. A capacitor is fully charged using the following circuit
A
5V
2.2 F
10k
(a)
(b)
(c)
(d)
(e)
Calculate the initial current
Calculate the maximum charge stored by the capacitor
Calculate the maximum energy stored by the capacitor
How would you know when the capacitor is fully charged
Draw a graph to show how the current varies with time from the moment the circuit is
switched on (values on current axis only)
(f) On the same sketch draw the graph if a larger resistor is used
5. The circuit below is used to charge up a capacitor
1k
9V
10 F
V
A
(a) Calculate the voltage across the capacitor when the current is 0.6mA
(b) Calculate the charge stored in the capacitor when the current is 0.6mA
(c) Calculate the maximum energy stored in the capacitor
(d) Sketch a graph to show how the voltage across the capacitor varies with time
6. The circuit below is used to investigate the charging of a capacitor
Vc
8
V
6
10k
4
2
5V
V
1000 F
2
A
4
6
8
time/s
(a) The graph shows how the voltage across the capacitor varies with time. Sketch a graph with
values showing how the voltage across the resistor varies with time. Values required
(b) Sketch a graph with values showing how the current varies with time as the capacitor is
charged up
(c) Calculate the maximum charge stored in the capacitor
(d) Calculate the maximum energy stored in the capacitor
(e) After what time will the voltage across the resistor be 3V
(f) How much charge is stored in the capacitor at this time.
7. The diagram shows a flash lamp for a camera.
S1
1k
9V
S2
100 ohms
(a)
(b)
(c)
(d)
(e)
S1 is closed and the capacitor charges up. Calculate the initial current
What is the voltage across the capacitor when it is fully charged
S1 is open and S2 is closed. Explain what happens to the lamp
Calculate the initial current through the lamp
How does the time taken to charge compare with that taken to discharge? Explain your answer
Homework 6
1.
Semiconductors
A physicist has discovered six new materials:
Name
Drakiesum
Drummossium
Raigmite
Crownium
Culcaboke
Kingsmillium
Appearance
silvery liquid
dull green crystal
bright yellow solid
clear solid
blue crystalline solid
grey powder
Resistance of sample
0.01 Ω
depends on voltage
0.3 Ω
> 10 MΩ
10 k Ω in dark, 20 Ω in light
> 10 MΩ
Classify the six materials into three groups based on their properties and make a table summarising
this classification.
2.
Describe the chemical and electrical differences between intrinsic and extrinsic (doped)
semiconductors.
3.
Describe the movement of charge in a p type semiconductor in terms of electrons, holes and
electron outer shells.
Question 4 on next page
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