Unit G482: Electrons, Waves and Photons

• Module 1: Electric current• 2.1.1 Electric current

• Module 2: Resistance• 2.2.1 Circuit symbols• 2.2.2 E.m.f. and p.d.• 2.2.3 Resistance• 2.2.4 Resistivity• 2.2.5 Power

• Module 3: DC circuits• 2.3.1 Series and parallel circuits• 2.3.2 Practical circuits

Learning outcomesYou should all be able to:• Recall and use appropriate circuit symbols

as set out in SI Units, Signs, Symbols and Abbreviations (ASE, 1981) and Signs, Symbols and Systematics (ASE, 1995)

• Interpret and draw circuit diagrams using these symbols.

EMF

E.m.f and p.d - Learning Outcomes

You should all be able to: • define potential difference (p.d.); • select and use the equation W = VQ; • define the volt; • describe how a voltmeter may be used to determine the

p.d. across a component; • define electromotive force (e.m.f.) of a source such as a

cell or a power supply; • describe the difference between e.m.f. and p.d. in terms

of energy transfer

Electric Potential (a bit like Gravitational Potential)

To give the ball more gravitational ‘potential’ energy

work must be done on it.Now it is

‘higher’ it has more ‘potential’.

This is a bit like electrical ‘potential’. The position of a charge (rather than in a ball) in an electric field (rather than a

gravitational field) determines its potential.

Electric Potential (a bit like Gravitational Potential)

The direction of current flow is conventionally taken to be from

points of higher potential (i.e. the top of a hill) to a point of lower potential.

Note: Conventional current flows from +ve to –ve, but the electrons (that are –ve) flow to the positive terminal.

If the work done in causing one coulomb of electric charge to flow between two points is one joule, then the PD between the points is one volt (i.e. 1V = 1 JC-1)

It follows that W = QV

Where W = the work done (J)Q = the charge (C)V = the potential difference (V)

e.m.f is the energy transferred per unit charge when one type of energy is converted into electrical energy.

Potential difference is the energy transferred per unit charge when electrical energy is transferred into another form of energy.

Key Definitions

Potential difference 12V

What is the voltage across each resistor? 6V

What happens to the charge as it flows through the resistor?

Electrical energy is transformed into another energy (heat in this case)

Electrical Energy and Potential Difference Questions

From ‘Practice in PHYSICS’ you are to complete the following questions:

5.415.425.435.445.465.485.495.505.535.56

From ‘Advanced Physics for you’ you are to complete the following questions:

Page 2059101112

Resistance

Resistance - Learning Outcomes

You should all be able to: • define resistance; • select and use the equation for resistance (V = I.R)• define the ohm; • state and use Ohm’s law; • describe the I–V characteristics of a resistor at constant

temperature, filament lamp and light-emitting diode (LED); • describe an experiment to obtain the I–V characteristics of

a resistor at constant temperature, filament lamp and light-emitting diode (LED);

• describe the uses and benefits of using light emitting diodes (LEDs).

A

V

A

V

A

Vbit of wire

http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

How to measure resistance

Ohm’s LawResistance = Potential Difference (volts)

(ohms) Current (amperes)

V is Potential Difference measured in voltage, V

I is current measured in Amps, A

R is resistance measured in Ohms, Ω

R = V I V = I R

Ohm’s Law

The current through a resistor at a constant temperature is directly

proportional to the potential difference across the resistor.

This means if you double the current you double the voltage over a component.

It also means that the resistance of the component does not change when you put more current through it.

• define resistivity of a material;• select and use the resistivity equation ;• describe how the resistivities of metals

and semiconductors are affected by temperature;

• describe how the resistance of a pure metal wire and of a negative temperature coefficient (NTC) thermistor is affected by temperature.

Resistivity - Learning Outcomes

Resistivity

The resistivity ρ of a wire of length l, resistance R and cross sectional area A is given by:

ρ = RAl

Power

Candidates should be able to:• describe power as the rate of energy transfer;• select and use power equations P = VI, P = I2R and

P = V2 / R• explain how a fuse works as a safety device• determine the correct fuse for an electrical device;• select and use the equation W = IVt;• define the kilowatt-hour (kW h) as a unit of energy;• calculate energy in kW h and the cost of this energy

when solving problems.

Power Power (watts) = Energy Transformed (joules) Time (seconds)

This means the more powerful something is, the more energy is transferred every second.

Power Power (watts) = Energy Transformed (joules) Time (seconds)

For example:

If a bulb transforms 300 J of electrical energy into light in 3 second, the power is:

P = Energy Transformed ÷TimeP = 300 (J) ÷ 3 (s)P = 100 W

Power in a circuit Power = Current x Potential Difference(watts, W) = (ampere, A) x (volts, V) P = I x V

For example:

If a bulb has a p.d. across it of 3.0V, and a current flowing through it of 2.0A then the power is:

P = I x VP = 2.0 (A) x 3.0 (V)P = 6.0 W