KS3 Physics
7I Electric Circuit
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Contents
7I Energy Resources
Charging up
Electric Circuits
Electric Current
Modeling Electric Current
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What are Electric Charge?
There are two types of charges:
 Positive Charge (proton)
 Negative Charge (electron)
Electrostatic attraction or repulsion occurs whenever you
bring charges together:
 A positive charge will repel another positive charge
 A negative charge will repel another negative charge
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What are Electric Charge?
 A negative charge will attract a positive charge
What does “charging” mean?
 You charge an object when you move electrons from
or to it.
 Negatively charged object has excess electrons.
i.e. it has more electrons than protons
 Positively charged object is excess positive
charge. i.e. it has more protons than electrons.
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Charging an object
What does it mean for an object to be neutral?
 Any object that has equal number of positive
and negative charges is said to be neutral.
i. An atom: has equal number of electrons and
protons.
An object can be charged by rubbing it against another
object.
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Charging an object
 When you rub a polythene rod with a cloth, electrons are
transferred from the cloth to the rod.
cloth
i. The polythene rod now has a net negative charge.
ii. The cloth has a net positive charge.
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Charging an object
Not all rods behave like polythene rods. eg.
 When you rub a Perspex rod with a cloth, electrons are
transferred from the rod to the cloth.
cloth
Perspex
i. The Perspex rod now has a net positive charge.
ii. The cloth has a net negative charge.
Not all electrons are transferred during charging
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Charging an object
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Insulators and Conductors
 There’s a region around a charge where another charge
experiences a force. This is called an electric field.
 Static means stationary or standing still
 Electrostatic phenomena such as rods repelling and
attracting each other because electric charges are
stationary in them (polythene rods).
 Charges are static in these rods because they are insulators
 Excess charges on an insulator stays on it and does not move.
 Excess charges on a conductor does not stay on it but moves
through it and then through your hand and into the ground.
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Circuit Components
 Mention machines that runs on electricity.
 All electrical appliances contain electrical circuits that
use circuit components.
 We can make a simple circuit from:
 Lamp
 A battery
 Pieces of wires
 A switch
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Circuit Symbols
 It would take a long time to draw pictures of circuits such
as this one so we use circuit symbols instead.
 You can join circuit symbols in a circuit diagram to show
how the components are connected
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Circuit Symbols
 In Physics, a battery contains two or more cells.
 A cell has a positive and negative terminal.
 The long line represents the positive terminal, and the
short line represents the negative terminal.
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Circuit Symbols
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When does a Circuit work?
 The lamp in the circuit below is not lit, can you explain why?
 The lamp in the circuit is not lit because the switch is open.
 For the lamp to work, we must close the switch to complete
the circuit.
 All electrical circuits work only when the circuit is complete.
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Conductors and Insulators
 We can use a simple circuit like the torch circuit to test if a
material conducts electricity or not.
 Conductor - An object that conducts electricity easily
 Insulator - An object that does not conducts electricity easily
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Conductors and Insulators
 Carbon is the only non-metal that conducts electricity.
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Electric Current
 Have you ever wondered what is going on in an electric wire?
 An electric current is the flow of charges through a
conductor.
A Copper Wire
 Inside a copper wire are copper ions.
 Ions are atoms that have lost or gained electrons.
 Some of the electrons on the outside of the copper atoms are
not strongly bounded to the atom and can move around freely.
 This free electrons are referred to as
sea of electrons.
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Electric Current
 When you connect a battery to a circuit, it provides a push
that makes the “sea of electrons” to move.
 The moving electrons makes an electric current
 Current is the amount of charges flowing per second
Measuring Electric Current
 Electric current is measured with a
meter called the ammeter.
 Current is measured in ampere (A)
or amps for short.
 Small currents are measured in
milliamps (mA).
1 mA = 0.001 A
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