physics module 2. what you need to learn circuit symbols measuring current and voltage in series and...

Physics Module 2

What you need to learn

Circuit symbols Measuring current and voltage in

series and parallel circuits Current/Voltage graphs for certain

components Resistance/Current/Voltage

relationship Mains electricity Rate of energy transfer

Can you name these circuit symbols?

…and these?

…what about these?

Exam point

Learn these symbols – you will definitely have to identify some symbols on the exam and you don’t want to lose simple marks.

Current What is current? What is the unit of measurement

for Current? What do we use to measure

current? Is current the same in series and

parallel circuits?

What is current? Current is a flow

of electrons (negative charges) around a circuit.

The electrons carry the electrical energy needed to make things work

Units of measurement Current is

measured in Amperes or Amps.

The unit of measurement is ‘A’

Don’t forget this in the exam.

How do we measure current?

We use an ammeter to measure current. This must be connected in series with the component you wish to measure.

Don’t forget the symbol.

© Boardworks Ltd 2003

Measuring current I

Set up the circuit shown below………and then record the current using an ammeter in the places shown below.What do you notice about the readings?The current is the same f or each ammeter.

In a series circuit the current is the same wherever you measure it.

When measuring current ammeters are always placed in series.

A

AA

Summary for Series Circuits.

1. In a series circuit the current is the same at any point in the circuit.

2. The supply voltage is shared between the components in a series circuit.

3. The current depends on the voltage in ANY circuit.


Measuring current II

Set up the circuit shown below………then record the current at the locations shown.What do you notice about the readings?

For a parallel circuit, the current that leaves the cell or battery is the same as the current that returns to the cell or battery. The current does not get used up by a circuit, just the energy the electrons are carrying.

A1 = A4

The sum of the currents in the branches of a parallel circuit is the equal to the current that leaves the cell or battery.

A1 = A2 + A3 =A4

A4

A3

A2

A1

Current

(A)

Ammeter

0.8

0.8

0.40.4

A1

A2

A3

A4

Current/Voltage graphs This is identified

by a straight line graph through the origin.

The graph obeys Ohms law – current is directly proportional to voltage

Current/Voltage graphs This is identified

by a curved graph It obeys ohms law

at the start The curve shows

that current does not increase even though the voltage does

Current/Voltage graphs A diode does not

let current flow in reverse.

It has a very high resistance when the current is reversed and so will not let it flow.

The relationship between current, voltage and resistance


V=IR

We can express Ohm’s Law mathematically using the equation:

Voltage = Current x Resistance

V=IR

Voltage measured in Volts (V)

Current measured in Amps (A)

Resistance measured in Ohms ()

V = IR The letter ‘I’ is

used to represent current. Do not get it confused with the unit of measurement for current which is ‘A’


Formula triangles

Can you do the calculations?

You need to be able to use the equation triangle to calculate current or voltage or resistance. You must find a way of remembering the equation.

Using the triangle


Formula triangles

V

I R

x

Formula triangles help you to rearrange formula, the triangle for the Ohm’s Law is shown below:

Whatever quantity you are trying to find cover it up and it will leave you with the calculation required.

So if you were trying to find current, I…..

…you would cover I up…

…and you are left with the sum…

I = VR


Resistance for a bulb

If you have a filament bulb and it has a current of 20A running through it, with a potential difference of 100V across it, what is the resistance of the bulb?

V = IR

R = V/I

R = 100V/20A

R = 5


Current for a diode

A diode has a current of 5A running through it, and a resistance of 5. What is the potential difference across the diode?

V = IR

V = 5A x 5

V = 25V


Voltage/current graphs

1. A wire or resistor.

2. A filament lamp.

3. Wires of different materials.

4. A diode.

Which of the components obeys Ohms Law?

I

V

I

V

I

V

I

V

1 2 3 4

..partly

x


What are the units of resistance?

A. Amps

B. Ohms

C. Volts

D. Watts


What does the circuit symbol shown represent?

A. Voltmeter

B. Variable resistor

C. Light dependent resistor

D. Thermistor


If a resistor that obeys Ohm’s Law has a potential difference of 10V across it and a current of 5A running through it. What is its resistance?

A. 50

B. 2

C. 0.5

D. 15

The v_____ is a measure of how big a push the e_______ are given as they leave the cell or b_____. The bigger the voltage, the bigger the p___.

The cell, battery or p____ s_____ produces the voltage.

Voltage

lamps

oltagelectrons

atteryush

ower upply


Measuring voltage I

V2 V3

V1

Set up the apparatus as shown………then record the voltages at the locations shown.What do you notice about the readings?

For a series circuit, the sum of the voltages for each componentis equal to the voltage across the cell or battery.

V1 = V2 + V3

NB Voltmeters are connected in parallel

V3

V2

V1

Voltage

(V)

Voltmeter

1.50.75

0.75


Measuring voltage II

V1

V2

V3

V3

V2

V1

Voltage

(V)

Voltmeter

1.5

1.5

1.5

Set up the apparatus as shown….

..measure the voltage at the locations shown.

What do you notice about your readings?

For a parallel circuit, the voltage across the

cell/battery is the same as the voltage across

each branch.

V1 = V2 = V3


Resistors in series

4 2

Total resistance = R1 + R2

What is the total resistance for the circuit shown?


Total resistance = 4 + 2

Total resistance = 6


6 34

What is the total resistance for the circuit shown?


Total resistance = 6 + 34

Total resistance = 40

Resistors in series

Mains electricity We take mains

electricity for granted – what exactly is it?

How do we use it? Where does it

come from?

Facts and figures Mains electricity has an

alternating current a.c. This means it changes

direction It does this 50 times a

second – it has a frequency of 50 Hz

It has voltage in the UK of 230 V

Where does mains electricity come from? Mains electricity is

generated in power stations all around the country.

It travels to our houses along a system of wires and pylons called the National Grid

How do we use it? We use 3 pin

plugs to connect appliances to the mains.


Electrical cable

Electrical cable connects the plug to a device.

What does the cable look like inside?

How is it designed for maximum safety?

Can you label the plug?

Live terminal

Neutral terminal

Earth terminal

Cable grip

Fuse


Electrical cable cross- section

The cable contains three wires.

What material is the wire? Copper

Each wire is encased in plastic.

Why is plastic used?

Plastic is an electrical insulator.

The wires are then encased in

another insulating layer of plastic.


Three copper wires

The three copper wires have different functions.

The colour of the plastic casing tells you what the wire is.

NameColour

Green and yellow

Blue

Brown

Earth

Neutral

Live


The Live wire

The Live wire carries a current that alternates between negative and positive values.

What is this type of current called?

Alternating Current (A.C.)

The Live wire is connected to the Live pin.

Which is the live pin?


The Neutral wire

The Neutral wire stays at a voltage close to zero with respect to Earth.

What colour is the Neutral wire?

Blue

The Neutral wire is connected to the Neutral pin.

Which is the neutral pin?


The Earth wire

If an appliance has a metal case it needs to be earthed. The Earth wire connects the appliance case to the Earth pin. If there is a fault that connects the live wire to the appliance case a very large current flows to earth and overloads the fuse.

What colour is the Earth wire?

Green and yellow

Which is the Earth pin?


Fuses I

What is a fuse?

A fuse is a device that is used to protect devices from a current which is too high.

How does a fuse work?

As current flows along a conductor there is friction between the electrons and the atoms of the conductor. The more current, the more friction. The friction causes heat, and the more current the hotter the fuse gets.Fuses contain pieces of conductor that melt if the current is too high. When the fuse melts, it breaks the circuit.


Fuses II

What is the circuit symbol for a fuse?

Fuses are labelled with the maximum current they are designed to allow.

When choosing the fuse for a device you should always choose one with the ampage above the operating current of the device.

Example:

So if a kettle operates with an electrical current of 4.3A what fuse should you use? You can choose from fuses of 3A, 5A, 8A and 13A.

use a 5A fuse

For the following devices choose fuses from 3A, 5A and 13A.

Operating Current

Fuse

2.1A

11.2A

4.7A

6.0A

4.5A

8.2A

3.0A

13.0A

5.0A

13.0A

5.0A13.0A

Fuses II

How do you work out which fuse to use? You need to know another

equation to calculate the current flowing through the appliance. This will tell you the correct rating of fuse to use.

Power = Current x Voltage (P=IV)


Power calculations and fuses

If you have a lamp and it has a potential difference of 200V across it and it operates at a power of 60W:

A) What current is running through the lamp?

B) What fuse should be fitted to the plug of the lamp?

A) P = IV

I = P/V

I = 60W / 200V

I = 0.3A

B) A 3A fuse should be fitted to the plug.


If you have a cooker and it has a potential difference of 230V across it and it operates at a power of 9.2kW:

A) What current is running through the cooker?

B) Why should a 13A fuse not be fitted to the plug?

A) P = IV

I = P/V

I = 9 200W / 230V

I = 40A

B) The fuse would blow each time the cooker was turned on.

Power calculations and fuses

Power = Current x Voltage Power is a measure of how fast an

appliance transfers electrical energy Power is measured in watts (W) The higher the power – the faster the

appliance works and the more electricity is uses

You can use the power of an appliance to calculate how much electricity has been transferred in a given time

Power = energy transferred /time Power (W) = energy transferred (j)

divided by the time taken (s) You will need to be able to use this

equation. (You learnt it in physics 1a)

Energy transferred ./. ./. power x time

physics module 2. what you need to learn circuit symbols measuring current and voltage in series and...

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