p 2 Electricity

8/13/2019 p 2 Electricity

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GCSE AQA

Additional Science

P2 Physics: Electricity

Steve Bishop

September 2013


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ContentsP2.3 Currents in electrical circuits ......................................................................................... 2

P2.3.1 Static electricity ...................................................................................................... 3P2.3.2 Electrical circuits .................................................................................................... 4

P2.4 Using mains electricity safely and the power of electrical appliances .......................... 11P2.4.1 Household electricity ............................................................................................ 12P2.4.2 Current, charge and power ................................................................................... 15

Useful websites:

Dedicated GCSE-ESOL Additional science site:www.padlet.com/wall/GCSEAddSci

http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/electricity/

http://www.my-gcsescience.com/revision/additional

http://www.docbrown.info/page20/AQAscience2AS.htm#Additional

Electric circuit builder:http://www.teachitscience.co.uk/shared/free/CircuitBuilder_iframe.html

P2.3 Currents in electrical circuits

The current in an electric circuit depends on the resistance of the components and the supply.

You should be able to:

apply the principles of basic electrical circuits to practical situations

evaluate the use of different forms of lighting, in terms of cost and energy efficiency.

Key words

Current

Resistance

Components

Energy efficiency

Static electricity

Electrons

Repel

Attract

Electric charge

Potential difference

Voltage

Work done

Energy transferred

Coulomb

Switch

Lamp

Fuse

Cell

Battery

Ammeter

Resistor

Thermistor

Variable resistor

LDR

LED

Resistance

Filament bulb

Diode
http://www.padlet.com/wall/GCSEAddScihttp://www.padlet.com/wall/GCSEAddScihttp://www.padlet.com/wall/GCSEAddScihttp://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/electricity/http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/electricity/http://www.my-gcsescience.com/revision/additionalhttp://www.my-gcsescience.com/revision/additionalhttp://www.docbrown.info/page20/AQAscience2AS.htm#Additionalhttp://www.docbrown.info/page20/AQAscience2AS.htm#Additionalhttp://www.teachitscience.co.uk/shared/free/CircuitBuilder_iframe.htmlhttp://www.teachitscience.co.uk/shared/free/CircuitBuilder_iframe.htmlhttp://www.teachitscience.co.uk/shared/free/CircuitBuilder_iframe.htmlhttp://www.teachitscience.co.uk/shared/free/CircuitBuilder_iframe.htmlhttp://www.docbrown.info/page20/AQAscience2AS.htm#Additionalhttp://www.my-gcsescience.com/revision/additionalhttp://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa/electricity/http://www.padlet.com/wall/GCSEAddSci


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P2.3.1 Static electricity

a) When certain insulating materials are rubbed against each other they become electrically charged.

Negatively charged electrons are rubbed off one material and onto the other.

b) The material that gains electrons becomes negatively charged. The material that loses electrons is

left with an equal positive charge.

c) When two electrically charged objects are brought together they exert a force on each other.

d) Two objects that carry the same type of charge repel. Two objects that carry different types of

charge attract.

Static electricity has many uses, for example, ink jet printers,

photocopiers, spray painting and removing particles from chimney

smoke.

It can also be dangerous as it creates sparks. Refuelling an aircraft can

create static electricity. A spark for the static could cause an explosion.

So the aircraft needs to be earthed.

Lightening is a form of static electricity. It can do a lot of damage, which is why on some buildings

lightening conductors are used.

e) Electrical charges can move easily through some substances, eg metals. The flow of electric

charges is called current electricity.


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P2.3.2 Electrical circuits

CURRENT ISSUES

A battery is connected to a bulb. The bulb is glowing.

Which of the four diagrams A - D in your opinion best describes the electric current in the wires?

A. There will be no electric current The electric current will be in a

in the wire attached to the base direction towards the bulb in

of the battery. both wires.

The direction of the current will The direction of the electric current

be as shown. The current will will be as shown. The current will

be lessin the return wire be the samein both wires.

Give a reason for your answer:

Devise an experiment to test your answer.


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RESISTANCE MATTERS

A battery is connected to a bulb and two variable resistors, as above. The bulb is glowing

What will happen if

1. Variable resistor R1 is increased?

Why?

2. Variable resistor R1 is decreased?

Why?

3. Variable resistor R2 is increased?

Why?

4. Variable resistor R2 is decreased?

Why?


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a) Electric current is a flow of electric charge. The size of the electric current is the rate of flow of

electric charge. The size of the current is given by the equation:

Q

= I t

Iis the current in amperes (A)Qis the charge in coulombs (C)

tis the time in seconds (s)

A coulomb is 6.2 x 1018

electrons. As the electrons flow around the circuit they do not get used up.

An amp is a 1 Coulomb per second.

Current is measured by an ammeterwhich is placed in series in a circuit.

b) The potential difference (voltage) between two points in an electric circuit is the work done (energy

transferred) per coulomb of charge that passes between the points.

W

= V Q

Vis the potential difference in volts (V)

Wis the work done (energy transferred) in joules (J)

Qis the charge in coulombs (C)

A battery is a number of cells joined together. They supply energy to each coulomb as they flow

around the circuit. The energy is transferred from the battery (chemical potential energy) to the

components in the circuit eg a lamp (light and heat energy).

Batteries are measured in volts. Voltage is the push given to the electrons as they flow, it is often

called potential difference (pd). A volt is a joule per Coulomb.

Voltage is measured by a voltmeter, which is placed across the component to measure its pd.

c) Circuit diagrams are drawn using

standard symbols. The following standard

symbols should be known:


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PUMP UP THE VOLTAGE

Current and voltage are different, but the two are related.

This experiment will investigate how they are related.

Equipment: power pack, voltmeter, ammeter, long piece of wire, variable resistor, lamp, crocodileclips and leads.

Set up the equipment as below:

Slowly increase the voltage on the power pack 1 volt at a time, record the voltage across the wire

using the voltmeter and the current in the circuit using the ammeter.

Switch off the power pack in between taking the readings. Take at least two sets of readingsone

increasing the voltage and one decreasing the voltage.

Record your results in the table:

Independent variable

Pd across the wire (V)

Dependent variable

Current (A)Reading 1 Reading 2 Mean Reading 1 Reading 2 Mean

Plot your results on graph. Plot the current on the y-axis and the pd on thex-axis. Label the graph.

What do you notice?

What is the major risk or hazard in this experiment? How can it be reduced?

What is the major source of error? How could it be overcome?

Take care

the wire

may get

hot.

Replace the wire with

a lamp and repeat the

experiment. Do the

same with a diode.


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d) Currentpotential difference graphs are used to show how the current through a component varies

with the potential difference across it.

e) The currentpotential difference graphs for a resistor at constant temperature is a straight line

(linear)

f) The resistance of a component can be found by measuring the current through, and potential

difference across, the component.

g) The current through a resistor (at a constant temperature) is directly proportional to the potential

difference across the resistor.

h) Calculate current, potential difference or resistance using the equation:

V= I x R V

I R

Vis the potential difference in volts (V)

Iis the current in amperes (A)

Ris the resistance in ohms ()

i) The current through a component depends on its resistance.

The greater the resistance the smaller the current for a given potential difference across the

component.

j) The potential difference provided by cells connected in series is the sum of the potential difference

of each cell (depending on the direction in which they are connected).

The first cell is provides a potential of 1.5 V. What will the total potential difference be when they are

connected as shown above?


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the total resistance is the sum of the resistance of each component

there is the same current through each component

the total potential difference of the supply is shared between the components.

I) For components connected in parallel:

the potential difference across each component is the same the total current through the whole circuit is the sum of the currents through the separate

components.

m) The resistance of a filament bulb increases as the temperature of thefilament increases.

n) The current through a diode flows in one direction only. The diode has a

very high resistance in the reverse direction

o)An LED emits light when a current flows through it in the forward

direction.

p) The resistance of a light-dependent resistor (LDR) decreases as light

intensity increases.

q) The resistance of a thermistor decreases as the temperature increases.


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P2.4 Using mains electricity safely and the power of electrical

appliances

Mains electricity is useful but can be very dangerous. It is important to know how to use it safely.

Electrical appliances transfer energy. The power of an electrical appliance is the rate at which it

transforms energy.

Most appliances have their power and the potential difference of the supply they need printed on

them. From this we can calculate their current and the fuse they need.

You should be able to:

understand the principles of safe practice and recognise dangerous practice in the use of mains

electricity

compare the uses of fuses and circuit breakers

evaluate and explain the need to use different cables for different appliances

consider the factors involved when making a choice of electrical appliances.

Key words

Cells

Batteries

Direct current

Alternating current

Frequency

Cycles

Hertz

Three pin plug

Fuse

RCBsEarth

Live wire

Neutral wire

Electrical charge

Resistor


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P2.4.1 Household electricitya) Cells and batteries supply current that always passes in the same direction. This is called direct

current (d.c.).

b)An alternating current (a.c.) is one that is constantly changing direction.

c) Mains electricity is an a.c. supply. In the UK it has a frequency of 50 cycles per second (50 hertz)

and is about 230 V.

Alternating current is used in the UK because it can easily be converted from higher to lower voltages

this means that it can be transferred from a power station with a low current and so avoid heating up

the electricity lines.

Electricity comes to the home via the live wire (brown)

d) Most electrical appliances are connected to the mains using cable and a three-pin plug.

e) The structure of electrical cable.

f) The structure and wiring of a three-pin plug.


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g) If an electrical fault causes too great a current, the circuit is disconnected by a fuse or a circuit

breaker in the live wire.

h) When the current in a fuse wire exceeds the rating of the fuse it will melt, breaking the circuit.

Fuses are placed in series with the live wire. The fuse rating needs to be lower than the normal

working current of the appliance.

i) Some circuits are protected by Residual Current Circuit Breakers (RCCBs).

This type of circuit breaker works by comparing the current going in to an appliance with the

current coming out.

When an appliance is working correctly all of the current entering the appliance through the live

wire is returned to the power supply through the neutral wire.

In the picture below the strength of the magnetic field is the same in both coils because they both

have the same current.

If something goes wrong with the appliance some of the electric current will flow through theearth wire.

The amount of current flowing through the neutral wire decreases and now there is a difference

between the current entering the appliance through the live wire and the current returned to the

power supply through the neutral wire. This difference is called the residual current.

The coil connected to the neutral wire now has a weaker magnetic field than the coil connected

to the live wire.

The iron rocker turns about the pivot and the contacts are disconnected which switches off the

appliance and makes it safe. See the picture below.

Residual Current Circuit Breaker - Appliance Switched Off

The RCCB acts to switch off the electricity much faster than a fuse.

From:http://www.gcsescience.com/pme9.htm
http://www.gcsescience.com/pme9.htmhttp://www.gcsescience.com/pme9.htmhttp://www.gcsescience.com/pme9.htmhttp://www.gcsescience.com/pme9.htm


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j)Appliances with metal cases are usually earthed.

Electrical appliances with metal cases usually contain an earth wire. If there is a fault in the

appliance and the live wire makes contact with the metal casing, a large current flows from the

live wire to the earth wire. This larger current melts the fuse in the live wire breaking the circuit

and isolating the appliance from the live wire.

Some appliances are double-insulated this means they have a plastic insulating case andtherefore do not require an earth wire connect to them. Double-insulated appliance such as hair

dryers are marked with the symbol:

k) The earth wire and fuse together protect the wiring of the circuit.

FITTING THE RIGHT FUSES

Example

What is the best fuse to fit in the plug of a hair dryer rated at 1200 W?

Find the normal current using

Power = Vx I

V= 240 V

Current I= P/V = 1200/ 240 = 5A

The correct fuse to fit must be higher than the normal current. So the best fuse would be a 13A fuse.

Questions

1. Calculate the best fuse for a 40 W desk lamp.

2. Calculate the best fuse for a 1500 W electric fan heater.

3. What would happen if a fuse that was rated too low was used?

4. What would happen if a fuse rated too high was used?


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P2.4.2 Current, charge and power

a) When an electrical charge flows through a resistor, the resistor gets hot.

b) The rate at which energy is transferred by an appliance is called the power.

=

Power is measured in Watts (W)

c) Power, potential difference and current are related by the equation:

P = I x V

And because V= Ix Rthen P= I2R

Example

A lamp has a voltage of 12 V and a current of 2 A passing through it. Find its power.

Using P = I x V

then Power (W) =2 x 12 = 24 W

Now try these

A lamp has a voltage of 230 V and a current of 3 A passing through it. Find its power.

d) Energy transferred, potential difference and charge are related by the equation:

E = V x Q

E = V I t


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Questions

1. If the current through a floodlamp is 5A, what charge passes in (a) 1 second (b) 10 seconds (c) 5

mins?

2. What is the current in a circuit if the charge passing each point is:

(a) 10C in 2 s

(b) 20 C in 40 s

(c) 240 C in 2 mins?

I =Q/t V =W/Q R =V/I

E =VIt P =VI C =Q/V P = IR

Worked examples

The current through a car headlamp is 3A. What is the charge that flows through the lamp in 10 s?

Step 1 Write down what you know and what you want to know

I = 3A t = 10s Q= ?

Step 2 Find the equation(s) which contains the unknownI =Q/t C =Q/V

Step 3 Identify the equation which is most suitable and use the corrrect form of the equation

I =Q/tthus Q =Ix t

Step 4 Substitute in the values and work out the answer

Q = 3A x 10 s = 30 C

Step 5 Check that the units are correct

Q [C] = [A] [s]

A current of 0.25 A flows through a 240 V light bulb. What is the power rating of the bulb?

Step 1 V= 240 V I= 0.25 A P= ?

Step 2 P =VI P = IR

Step3 P =VI

Step 4 P = 240 V x 0.25 A = 60 [V] [A]

Step 5 P= 60 W

A 100 heating coil is connected to a 5 V power supply. What power does it provide?

Step 1 R = 100 V= 5 V P = ?

Step 2 P =VI P = IR

Step 3 We will have to find I first from V =IR

I =V/R so I = 5 V / 100 = 0.25 A

Step 4 Substituting in P =VIwe have

P= 5 x 0.05 = 0.25 W

Step 5 Power = [W]


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Now try these

Complete this table

Quantity Symbol Units Symbol for unit

Current

Voltage

Resistance

Power

Charge

Energy

Transferred

A. Quest ions on charge

1. What current flows when 36 C of charge flows past a point in a circuit in one minute?

2. A flashbulb requires a current of 5 A. The flash lasts 0.02 s. How much charge flows through the

bulb in this time?

3. A rechargeable battery stores 7200 C of charge. How long can it make a current of 4 A flowthrough a resistor?

B. Quest ions on electr ical power

1. A current of 5 A flows through a heater when it is connected to the 240 V mains. What is its

power?

2. A car headlamp is rated at 36 W. It runs off the car's 12 V battery. What current flows through thebulb when it is working?

3. A loudspeaker is labelled '30 W - maximum safe current 5A'. What is the greatest potential

difference which could safely be connected across it?

4. An electric cooker has an oven rated at 2 kW, and four rings each rated at 500W. The cooker

operates from the 250V mains. Would a 30 A fuse be suitable for the cooker?


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C. Miscellaneous qu est ions

1. A 2V cell is connected in series with a 25 resistor. Calculate the current flowing.

2. If a current of 0.5 A flows through a 1 kresistor what is the potential difference?

3. What is the charge flowing when a current of 2A flows for 30 seconds?

4. A current of 0.5 A flows when a pd of 5 V is applied across a wire. Calculate the resistance of the

wire.

5. In a circuit with a combined resistance of 500 W a current of 2 A flows. Calculate the circuit's

power.

6. A current of 5 A flows through a resistance of 2 W. What is the potential difference across

7. In terms of charge transfer describe what happens when you rub a glass rod with a cloth.

8. An electric kettle is rated at 1.5 kW.

(a) Assuming that the domestic voltage supply is 250 V, what is the correct-sized fuse for the plug?

(b) A unit of electricity costs 6p. How much does it cost to operate the kettle each day for a week, 20

minutes a day?

9. (a) What is AC electricity?

(b) Why is mains electricity AC?


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10. Study the circuits below.

a) In which circuit is the current from the cell greatest?

b) Why is the current in B less than in A?

c) What is the current from the cell in D likely to be?

d) If the bulb in circuit A has a brightness of 10 write a number next to every other bulb to

indicate its brightness.

11. Why is the cover of a three-pin plug made of plastic?

12. How does a fuse work?

13. How does a RCCD work?

14. Draw the circuit symbol for

(a) An open switch (b) a thermistor (c) a Light-dependent resistor

15. What is an ohmic conductor?

16. Sketch the current-potential difference graph for

(a) a filament bulb (b) a diode

p 2 Electricity

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