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National 5 Physics

ElectricitySummary Notes

&Homework

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National 5 Physics Summary Notes Electricity & Energy

Electrical Charge Carriers and Electric Fields

Electrical Charges

There are 2 types of charges, positive and negative. Electric fields exist around charges. These fields cause other charges to experience a force, either attraction or repulsion. Opposite charges attract. Like charges repel.

In electricity we follow the flow of the negative charges. These are known as electrons.

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CurrentAn electrical current is the quantity of charge that flows past a fixed point every second.

current=chargetime

I=Qt

Current is measured in amperes (A), charge is measured in coulombs (C) and time is measured in seconds (s).

Direct and Alternating Current

A direct current flows in one direction. A good example of this is current from a battery or cell. The flow of electrons is always from the negative terminal to the positive terminal.

An alternating current is when the negative and positive terminals reverse polarity at regular intervals. This causes the current to flow in one direction and then the opposite direction. The mains electrical supply is an alternating current. It has a frequency of 50 Hz.Potential Difference

(Voltage)When a charge is moved by an electric field, the charge is given energy. This energy is then used elsewhere in the circuit. The energy per unit charge is known as the voltage.

voltage=energycharge

V= EQ

Voltage is measured in volts (V), energy is measured in Joules (J) and charge is measured in coulombs (C).

CircuitsA current will only flow in a circuit which is complete. There must be at least one continuous connection between the two terminals of the power supply. Energy is supplied by the power supply and is converted into other forms of energy by the components in the circuit.

In this circuit the electrons flow in a clockwise direction.


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ComponentsCircuits may consist of many components. In a component, an energy change normally takes place. For example, in a lamp electrical energy is converted into light (and heat).

Component Function and useMost common energy source in an electric circuit. Cells have chemically stored energy. Energy is given to the electrons which pass through the cell. This energy is then passed on to componentsA combination of two or more cells connected in series. The voltage supplied by a battery is the sum of the voltages of each of the cells.A lamp converts electrical energy into light energy (and some heat too). It is an example of an output device.A switch is used to “break” the circuit. This creates an open circuit and stops electrons from flowing. A motor converts electrical energy into rotational kinetic energy. It does this using magnets and coils of current carrying wire. A motor is used as an output device.A microphone converts sound energy into electrical energy. It is an example of an input device.

A loudspeaker converts electrical energy into sound energy. It is an example of an output device.

A photovoltaic cell converts light energy into electrical energy. It may be used as an input device. It is the basis for the solar cells used in solar panels.A fuse is a safety device. It contains a piece of wire which melts when the current reaches a certain level. This isolates an appliance from the mains and protects the appliance and the user. Appliances usually have a 3A (under 700W) or 13A (over 700W) fuse.A diode is an electrical one-way-street. It only allows the current to flow in one direction. In the case of the one shown, electrons would only be able to flow from right to left.A capacitor is able to “store” charge. It is initially neutral, however when connected to a DC power supply, like a cell, one plate becomes positively charged while the other becomes negatively charged. If it is then connected across a lamp, the capacitor will discharge and the lamp will come on for a short period. It is often used in electronics for timing circuits, since its charge/discharge time can be


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The Four Circuit Rules

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Series CircuitsA series circuit is one in which the component are connected in a single loop.

In a series circuit:

the current is the same at all points

the voltages across the separate components add up to the supply voltage

Parallel Circuits In a parallel circuit, current can take more than one path.

In a parallel circuit: the currents in the

branches add up to the current from the supply

the voltage is the same across each branch and is equal to the supply voltage

Ammeters and Voltmeters

Each meter measures a different quantity, and each meter is connected in a different way. Ammeters are connected in series (in line with) with components, while voltmeters are connected in parallel (across) components.

ResistanceResistors reduce the current (flow of charge) in a circuit.

Resistance is measured in ohms (Ω).

An ohm meter is used to measure the resistance of a component directly. The ohm meter does not require a separate power supply and so is connected as shown in the diagram.

Ohm's LawOhm's Law describes the relationship between voltage, current and resistance. For a fixed value resistor there is a direct proportion relationship between the current through the resistor and the voltage across it.

This relationship is known as Ohm's Law.

V=IR

0

From the graph:

R=VI


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Practical electrical and electronic circuitsElectronic Systems

All electronic systems consist of 3 main parts.

Input DevicesInput devices convert one form of energy into electrical energy. This allows an external signal to be converted into an electrical one, for use in the circuit.

Examples include:

solar cell light energy → electrical energy

thermocouple heat energy → electrical energy

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Resistors is Series

In a series circuit, the total resistance is the sum of all the individual resistances.

RT=R1+R2+R3+. . .

Resistors is parallel

The total resistance in parallel circuits is more complicated. The following equation can be used to calculate the total resistance.

1RT

= 1R1

+ 1R2

+ 1R3

+. ..


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Dependant resistors as Input Devices

Light Dependant Resistors

Light Dependant Resistors, often abbreviated to LDR. These resistors have a varying resistance which depends on light level. For an LDR as light level increases, resistance decreases. This is often abbreviated to L.U.R.D. to help you remember.

Thermistors

These resistors have a varying resistance which depends on temperature. For a thermistor as temperature increases, resistance decreases. This is often abbreviated to T.U.R.D. to help you remember.

Variable resistors

Variable resistors which can be controlled by the user.

The Light Emitting Diode

The LED is an example of an output device. It converts electrical energy into light energy. Unlike a lamp it produces very little heat energy. The LED only allows a current to flow through it in one direction and so it must be connected the right way round.

The LED should always be connected in series with a resistor. The purpose of the resistor is to reduce the current passing through the LED since too large a current will damage the LED.

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LED Protective Resistor Calculation

Consider an LED rated at 2V and 10mA connected to a 6V supply.

This circuit is like a voltage divider. If there needs to be 2V across the LED then there must be 4V across the resistor.

In a series circuit the current is the same through each of the component. We can now do a calculation using Ohm's Law.

Voltage DividersIn a circuit with more than one resistor the voltage

of the supply is split across the resistors in that circuit. There are 2 equations that can be helpful in finding missing values in these types of circuits.

The ratio of the resistances is the same as the ratio of the voltages across them:

V 1V 2

=R1R2

The voltage across the second resistor can be calculated using:

V 2=( R2R1+R2 )×V s

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V=IR4=0. 01×R

R=40 .01

R=400Ω


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Alternative Diagrams

In these circuits the reading on the voltmeter is determined by the ratio of the resistors. The ratio can be controlled by the use of variable resistors including thermistors and LDRs. This makes them excellent as input devices.

Voltage Dividers as Input DevicesLight Sensing Temperature Sensing

As light/temperature increases the resistance of the LDR/thermistor decreases. As a result the voltage across the LDR/thermistor decreases. The reading on the voltmeter decreases.

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Connected in reverse they carry out the opposite function:

As light/temperature increases the resistance of the LDR/thermistor decreases. As a result the voltage across the LDR/thermistor decreases. The share of the voltage across the fixed resistor increases and the reading on the voltmeter increases.

The npn transistors and the MOSFETTransistors are electronic switches. When connected in a circuit with a voltage divider they are controlled by the voltage across the lower resistor. There are two main types:

npn transistor MOSFET

Control CircuitsControl circuits use all of the pieces we have looked at so far. They include a sensing section, dependant on light or temperature, an electronic switch and an LED (or other output device).

The control circuit can be reversed by switching the locations of the resistors. The circuit can be calibrated by varying the resistance of the variable resistor.

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This is a temperature dependant circuit.As temperature increases the resistance of the thermistor decreases. As the resistance of the thermistor decreases the voltage across it also decreases.As the voltage across the thermistor decreases the voltage across resistor 2 increases.When the voltage across resistor 2 rises above 0.7V the transistor starts conducting and the LED goes on.

switches "on" at 0.7V

switches "on" at 2V


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Capacitors as time delays

In this type of circuit, the capacitor will take a set time to “charge”. As it charges, the circuit current decreases, the voltage across the resistor decreases and the voltage across the capacitor increases (eventually reaching the 6V of the power supply). When the voltage across the capacitor reaches 0.7V the transistor will conduct, allowing the LED to light. Swapping the locations of the resistor and capacitor will cause the LED to go off after a delay.

Electrical Power

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Power is the quantity of energy transformed from one form into another form (or other forms) every second.

Power=Energytime

P= Et

Power is measured in watts (W), energy measured in joules (J) and time measured in seconds (s).

1 watt = 1 joule per second

Electrical Power

In electric circuits, the power transferred by a component can be calculated from the voltage across the component and the current passing through it.

Power=Current×Voltage

P=IV

If resistance is known, then the following two equations can also be used to determine power. They are produced by combining Ohm's Law with P=IV.

P=I 2RP=V

2

R


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1 Charge, Current and Time

1. What is an electrical current?

2. Calculate the missing values from this table (DO NOT COPY THE TABLE):

Charge / C Current / A Time / s

(a) 0.14 25

(b) 4.2 15

(c) 270 0.3

(d) 1.6 x 10-3 3.2 x 10-4

3. A current of 6.5A flows through a hairdryer for 5 minutes. What is the charge that flows through the hairdryer during this time?

4. When playing a game, an Xbox 360 has 1368 coulombs of charge flowing through it an hour. What is the current flowing through the console?

5. Calculate the number of electrons passing a point in a circuit when there is a current of 4.0 A for 60 s. (The charge of an electron is -1.6x10-19 C)

2 Circuit Rules

1. What is an electrical current?

2. Find the current at the given points in each series circuit.

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I2

I1

0.4 A(a)

M

I2

I1

6.5 A

(b)


3. Find the voltage across the resistor in each of these series circuits.

(2)

4. In an experiment, two identical resistors are connected to a 9.0 V power supply. Find the voltage across each resistor.

5. Find the missing currents and voltages in these series circuits.

6. Find the current at the given points in each parallel circuit.

(1)

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8.5 VV

M

12 V(b)

V0.6 V

1.5 V

(a)

V2

9.0 V

V1

0.10 A

I

0.15 A(a)

I2

0.7 A

I1

1.6 A(b)

0.45 V

I1

I2

I3

1.2 V

0.5 A

V1

(b)

I1

I2

I3

2 V

1 V

1.6 mAV1

(a)


7. Find the voltage across the lamp in each of these parallel circuits.

8. In an experiment, two identical resistors are connected in parallel to a power supply which has 0.58 A drawn from it. Find the current through each resistor.

9. Find the missing currents and voltages in these parallel circuits.

3 Ohm’s Law & Variable Resistors

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2.5 V(a) 15 V

(b)

I2

I1

0.58 A

I5

I4

I2

I3

0.5 A

1.5 A

I1

V2

V1

12 V(a)

V3

V2

V1

(b)

I4

I5

I3

1.5 V

10 Ω

10 Ω

10 Ω

I2

I1

0.45 A


1. Calculate the missing values in this table. Do not copy the table.

Voltage / V Current / A Resistance / Ω

(a) 0.4 150

(b) 12 60

(c) 230 5

2. What is the resistance of a lamp that allows 600 mA of current to flow through it when there is a potential difference of 12 V across it?

3. What is the current flowing through a piece of 10 kΩ resistance wire when there is a voltage of 15 V is across it.

4. What is the voltage across a 125 Ω lamp that has a current of 1.84 A flowing through it?

5. What is the purpose of a resistor in a circuit?

6. What is the difference between a fixed-value resistor and a variable resistor?

7. A variable resistor can be used to control the speed of the electric motor in a portable fan. The variable resistor has a range of values from 300 Ω to 1.5 kΩ. Assume that the motor has no resistance.

(a)What is the maximum current that can flow through the motor?

(b)What is the minimum current that can flow through the motor?

8. A variable resistor is used as a dimmer switch in a lighting circuit.

(a) What is the resistance of the variable resistor if the current flowing through the variable resistor is 0.20 A and the voltage across it is 4 V?

(b) What is the resistance of the variable resistor when the current flowing through the lamp is 5 mA and the voltage across the lamp is 2 V?

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300 Ω - 1.5 kΩ

M

3.0 V

12 V

R


4 Resistors in Series & Parallel

1. What happens to the total resistance of a circuit as more resistors are connected in series?

2. Calculate the total resistance of these combinations of resistors.

4. What happens to the total resistance of a circuit as more resistors are added in parallel?

5. Calculate the total resistance of these combinations of resistors.

5 Combination Circuits

1. Calculate the total resistance between X and Y in these circuits:

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190 Ω240 Ω120 Ω(a)

1.5 kΩ390 Ω4.5 kΩ(b)

120 Ω

120 Ω

120 Ω

(a)

1.5 kΩ

1.5 kΩ

500 Ω

(b)

YX 20 Ω

40 Ω

60 Ω

(a)

YX

40 Ω

40 Ω

80 Ω

(b)


2. In a science lesson, a student is given three 1.2 kΩ resistors.

What is the lowest possible resistance that the student could achieve by combining these resistors in to a circuit?

3. In another science lesson, a student is given five 40 Ω resistors.

Show how the student could combine all five resistors so that the total resistance of the circuit is:(a)200 Ω (b) 8 Ω (c) 50 Ω (d) 80 Ω

6 LEDs & Components

1. An LED is connected in series with a cell and a resistor, as shown. The LED operates when the voltage across it is 1.8 V and the current flowing through it is 1.5 mA.

(a)Why is the LED connected in series with a resistor?

(b)What value of resistor is required for the LED to operate properly?

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1.2 kΩ1.2 kΩ1.2 kΩ

50 Ω

50 ΩYX

100 Ω

100 Ω

(c)

40 Ω40 Ω

40 Ω 40 Ω

40 Ω

6 V1.5 mA

R

1.8 V


2. Which of these LEDs will operate?

3. What happens to the resistance of a thermistor as temperature increases?

4. What happens to the current flowing through a thermistor as temperature increases?

5. At a temperature of 23 °C, the resistance of a thermistor is 5700 Ω. Suggest a possible resistance of the thermistor when the temperature is 20 °C.

6. A thermistor is connected to an ohmmeter, as shown.

The temperature of the thermistor is measured and the resistance is observed.

The temperature of the thermistor is increased in 10 °C increments and the resistance is measured at each temperature. Draw a line graph of these results.

7. What happens to the resistance of an LDR as the brightness of light incident on it increases?

8. What happens to the current flowing through an LDR as the brightness of incident light increases?

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1.8 V

D

C

BA

t

Ω

Temperature / °C

Resistance / kΩ

0 10010 2020 1030 540 250 1


9. An LDR is used in an outdoor light sensor system. At noon, the resistance of the LDR is 3.3 kΩ. Suggest a possible resistance of the LDR at midnight.

10. A discharged capacitor is placed in to a series circuit as shown.

(a) What is the reading on the voltmeter, V2, when switch S is open?

(b) The switch, S, is closed. What is the reading on voltmeter, V1, immediately after the switch is closed?

(c) The switch, S, is closed. What is the reading on voltmeter, V2, immediately after the switch is closed?

(d) The switch, S, remains closed. Draw a voltage-time graph that shows how the reading on the voltmeter, V1, changes until the capacitor is fully charged.

(e) Describe two changes that could be made to the circuit to increase the time taken for the capacitor, C, to charge.

11. The capacitor is now discharged using the circuit below.

(a) Draw a voltage-time graph that shows how the reading on voltmeter, V2, changes with time until it is fully discharged.

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V1

S6 V

V2

CR

V1

S

V2

CR


(b) State two changes that could be made to the circuit to make the capacitor discharge faster.

12. A thermistor is used in a switching circuit, as shown.

(a)What is the name of component X?

(b)State how this circuit will warn a chef that the temperature of a fridge is too high.

(c)How could this circuit be altered to warn a chef that the temperature of a fridge is too low?

13. An LDR is used in a switching circuit, as shown.

(a) What is the name of component Y?

(b) State how this circuit will automatically turn on a porch light when it gets dark.

(c) What is the advantage of component R1 being a variable resistor?

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X

R3

R2

t

0 V

+ Vs

R1

Y

R2

+ Vs

R1

0 V


7 Electrical Power

1. A student makes a statement:

‘The power of a light bulb is 60 W.’

What does this statement mean, in terms of energy?

2. Calculate the missing values from this table. Do not copy the table.

Power / W Energy / J Time / s

(a) 800 10

(b) 1500 30

(c) 218 54 500

3. What is the power of a radio that uses up 27 kJ of electrical energy in five minutes?

4. How much electrical energy is used up by a 725 W fridge in one day?

5. How long will it take a 1.2 kW vacuum cleaner to use up 720 kJ of electrical energy?

6. A 42 inch LED television has a power rating of 52 W when it is fully operational. When it is on standby, the television has a power rating of 0.8 W.

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(a)How much electrical energy will the television use if it is fully operational for 4 hours?

(b)How much electrical energy will the television use if it is on standby for 10 hours?

(c)Will the television use up more electrical energy being on standby for two days or being fully operational for 45 minutes?

8 Power, Current and Voltage

Note: Mains voltage in the UK is 230 V.


Power / W Current / A Voltage / V

(a) 0.3 4.5

(b) 750 25

(c) 40 0.8

2. What is the power rating of a microwave that has a current of 3.3 A flowing through it when it is plugged in to the mains?

3. What is the current flowing through a 65 W laptop that has a potential difference of 18.5 V across it?

4. What is the voltage across a 6 W light bulb that has a current of 500 mA flowing through it?

5. Three 40 W light bulbs are connected in parallel with the mains power supply, as shown.

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40 W 40 W

230 V

40 W


What is the current drawn from the mains?

9 Power and Resistance

1. Use the equations P = IV and V = IR in order to derive the equation, P = I2 R.


3. What is the power rating of a lamp that has a resistance of 5 Ω and a current of 1.2 A flowing through it?

4. What is the current flowing through a 50 Ω heating element in a toaster if it has a power rating of 800 W?

5. What is the resistance of a 1200 W electric convection heater that has a current of 5.0 A flowing through it.


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Power / W Current / A Resistance / Ω

(a) 1.5 100

(b) 500 125

(c) 36 0.06


7. What is the power

rating of a mains television that has a resistance of 529 Ω?

8. What is the voltage across a portable electric shaver that has a resistance of 2.45 Ω and a power rating of 20 W?

9. What is the resistance of an 800 W mains powered coffee machine?

10. A hairdryer has three heat settings: cold, warm and hot. The hairdryer is made up of two 300 Ω resistors and a 50 Ω motor that are connected in parallel with the mains supply, as shown.

(a) What is the energy change that occurs when current flows through a resistor?

(b) Which switches are closed when the hairdryer is blowing out warm air?

(c) How much current is drawn from the mains when the hairdryer is blowing out cold air?

(d) What is the power rating of the hairdryer when it is blowing out cold air?

(e) What is the total resistance of the hairdryer when it is blowing out hot air?

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S3

S2

S1 230 V

M

300 Ω

300 Ω

50 Ω

Power / W Voltage / V Resistance / Ω

(a) 6 72

(b) 25 9

(c) 400 230


(f) What is the power rating of the hairdryer when it is blowing out hot air?

(g) How much electrical energy will the hairdryer use if it blows cold air for 1 minute and hot air for 8 minutes?

11. Wires used in transmission lines have a resistance of 0.0025 Ω/m. How much power is lost by transmission lines carrying a current of 12 A if the length of the lines are:

(a)1 m?

(b)30 m?

(c)2.5 km?

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