Download 2.9 Understanding electricity

23 May, 2017
2.9 Understanding electricity
Key terms
• Series: in a series circuit the components
are connected in a line, end to end, so that
the current flows through all of them one
after another
• Parallel: in a parallel circuit the
components are in separate paths and the
current is split between the two paths
What is electricity?
• Electricity is the flow of electric charge
• This charge could be positive and negative ions
in your phone or negative electrons in your PS3
• When charge flows there is a current
• Electrical energy allows a current to flow in a
circuit
• A measure of the energy carried between two
points in a circuit is called voltage (or potential
difference)
Measuring electricity
• We use a voltmeter to measure voltage
and an ammeter to measure current
• The way we connect them is important
• A voltmeter is always connected in
parallel
• An ammeter is always connected in series
Case study: fault finder
• Sophia is a technician at an electronics
company. Today she is repairing a DVD
player that seems to have no power. She
wants to measure the voltage and find out
if there is a break in the circuit.
• How could she do this?
Ohms Law
V  IR
Voltage equals the product of the current and the resistance
Resistors
• You can use an ammeter and a voltmeter
to check the values you calculate for a
circuit using Ohm’s law
• High levels of current can be dangerous,
in the lab we only use low levels such as
mA (milliamps) or even μA (microamps)
• All electrical devices have resistors which
limit the current that flows through a
component so they are not damaged
Worked example
• What is the voltage across a 300 Ω resistor if
the current flowing is 0.01 A?
V  IR
V  0.01A  300
V  3V
Worked example
• If the voltage across the resistor was 9 V what
current would be flowing?
V  IR
V
I
R
9V
I
300
I  0.03 A
Units and symbols
Electrical
property
Voltage
Current
Resistance
Unit
Symbol
Units and symbols
Electrical
property
Unit
Symbol
Voltage
Volt
V
Current
Ampere
(Amp)
A
Resistance
Ohm
Ω
Activity 1
• What meter is used to measure current?
• How should it be connected in order to
measure current?
• Draw a diagram to show this
Activity 2
• You are an electrician. Part of your work is to
make sure electrical circuits are working
correctly. To do this you must understand Ohm’s
law and how to use measuring instruments
– Draw the symbols for a voltmeter and an ammeter
– If a resistor in a circuit is 1500 Ω, what is the current if
it is connected to a 1.5 V supply?
– Using a circuit diagram, show how you could confirm
the current and voltage readings in question 2 by
using the correct measuring instruments