Current, Voltage and Resistance

Electricity – involves the movement of electrons from one atom to the next in a conducting wire

Charge

• Is how the flow of electrons is measured i.e. One coulomb of charge is equal to the charge carried by 6 X 1018 electrons.

Current• Electric current is the measurement of the rate of flow of charge

through a circuit.

• It is the amount of electric charge which passes a point in one second.e.g. If a current of 5 A flows through a globe, this means 5 coulombs of electrons pass through one globe each second.

• Current direction is from positive to negative though electrons flow from negative to positive.

• Current is measured in ampere (A)

• In different circuits charge flows at different rate.

Role Play demonstrating current

Measuring Current in a series circuit

• We can place an ammeter at any point in a series circuit and the current will be the same.

e

I

Measuring Current in a parallel circuit

• In a parallel circuit current splits at each path.• The total of the currents from each path

equals the total current in the circuit

• If the resistance in each path is the same then current splits up evenly.

• Current flows more easily in the path with less resistance.

IT

e

I1

I2

I3

IT = I1 + I2 + I3

Measuring Voltage

1. Energy supplied to a circuit i.e. via a battery– This voltage of a battery refers to how much

energy the battery supplies to each coulomb of charge that passes through it.

– A 12 V battery gives each coulomb of charge 12 joules.

2. Energy used by load components (e.g. Light globe) in the circuit.– Energy supplied to a circuit must be entirely used

up by that circuit.

Role play demonstrating energy used in a series circuit

Voltage in load components in series• If a circuit has 3 globes placed in series with a 12V

battery each globe will have voltage of 4 Volts across it

• VT = V1 + V2 + V3

VT

V2

V3

V1

Role play demonstrating energy used in a parallel circuit

Voltage in parallel circuits

• If a circuit has 3 identical globes placed in parallel with a 12V battery each globe will have voltage of 12 Volts across it.

VT = V1 = V2 =V3VT

More on voltage

• When the load components(e.g. Globe) in a circuit have different resistance than the energy used in each component will differ.

• Load components with less resistance use less energy than components with more resistance i.e. You plasma TV uses more energy than your LCD screen or a light globe uses less energy than your heater.

• BUT the overall energy in any series circuit will equal the total energy from the battery (or energy source)

Role play demonstrating energy used in a parallel circuit when

components use different amounts of energy

More on voltage

• If all load components were the same in which type of circuit series or parallel do you think the battery would go flat first?

Practical work

• Resistors in series – the voltage divider p 90 Text

• Resistors in parallel– the voltage divider p 91 Text

• Refer to book for AIM, M&M• Write out result table• Do discussion and evaluation

Resistance

Acticity 1 • Devise an experiment to test 10 objects and

see if they are good or poor conductors of electricity

Activity 2• Devise an experiment to test 10 objects and

see if they are good or poor conductors of electricity

Resistance

• Resistance of an electronic component refers to how difficult it is for electrons to flow through it.

• The unit of resistance is the: ohm.

•

Resistance

• Components can have widely differing resistance values– Eg. metal wires – no resistance– Light globe – resistance of 40 ohms– Volt meter – resistance of 5 million ohms

• There are two main types of resistors:• Fixed resistors• Variable resistors

Resistance

• Made from a solidified carbon mixture• Very useful in electronic circuits because they maintain

their set resistance value• Deliberately placed to ‘use up’ a given amount of voltage

in a circuit, reducing the voltage available to other components in the circuit

• Coloured bands are used to indicate the value of its resistance

• Because the resistor is so small the 1st three bands represent the resistors value, the 4th band is used to indicate the accuracy of the stated resistance value

Resistor Bands

Resistance in series

• In a series circuit the total resistance can be calculated by adding the resistance from each component

RT = R1 + R2 + R3 .....

Resistance in parallel

• In a parallel circuit the total resistance can be calculated by using the following formula

......