Voltage, Current, Resistance
and Ohms Law

An electrical circuit can be made up from components such
as copper wire, batteries, switches, lamps etc.

There is one very important rule for any circuit to work - All
the components used should produce a continuous loop
that connects to the power supply.

The loop should contain:

  • A battery or power source (with a known voltage or electromotive force (e.m.f.).

  • Connecting wires to let the energy flow from the battery to the other components. This
    flow is known as current.

  • Components in the circuit which offer a resistance to
    the current.

Torch Circuit

A switch can be used to make the following circuit
open or closed.

Click on the switches to close the circuits.

This is a very simple control system.

Without the switch we couldn't take control. Imagine how
inconvenient it would be at home to have no switches on
your wall to control the room lighting, or on your TV to
switch on the power.

How the circuit works

Voltage

The power for the circuits above comes from the battery.
Batteries can be low voltage types (1.5 volts) to higher
voltage types (9 volts) and cars even have 12 volt batteries.

The switch, when closed, will conduct and let the battery
connect to the circuit. In this position the switch “makes the
circuit”.

The switch, when opened, will insulate and disconnect the
power source from the rest of the circuit. In this position the
switch has made an "open circuit"

In our circuit it is also important to match the battery voltage
with the correct lamp to prevent it overloading and getting
damaged. This is the same for many other components in
electronic circuits.

Current

The battery gets its power to the lamp through the wires. This
is because the battery in a closed circuit creates “current” in
the wires which flows around the circuit. When current flows

through the lamp it lights up.

Current is measured in Amperes (Amps) - smaller currents in
electronic circuits are usually measured in milliamps (mA). A
milliamp is equal to one thousandth of an Amp.

1mA = 1/1000 Amp

Close the switch
to see the current flow from the positive (+) side of the battery to the negative (-) side.

If you had no lamp and just connected the battery ends
together with wire you would allow a very high current to flow.

Click on the switch to see what would happen to the wire if there was no lamp and a car battery was being used.

 

Resistance

In the second circuit by connecting the two ends of the
battery together with wire the battery has been “short-
circuited”. Because of this, there is very little or no
resistance
to the current that flows from the battery. The
wires connected to the battery are glowing redhot and would
soon melt. The heat produced in this way can
seriously burn you or create a fire. This can happen because
a car battery is very powerful and can produce an extremely
high current when its terminals are connected together
(shorted).

In the first example, when the lamp was in the circuit, the
current was much less as the lamp resisted its flow. To
prevent damage to the battery and stop overheating, you
should avoid “short-circuits” in your practical work.

 

Q9, Is this statement true or false ? (you may need to
look back at the “Resistance” section above).

“A "short circuit" across the contacts of a battery has a very
low resistance - So lots of current can flow through it”.


True
False

In a circuit

Ohm's Law

says:

Voltage

= Current

x Resistance

(volts)

(amps)

(ohms)

 

Power

Different electrical circuits use different amounts of power. A
torch can use quite a lot of battery power and you have
probably found that its batteries need to be replaced more
often than the batteries in a wrist watch. Power is dependent
on the voltage and the current in the circuit. It is measured in
Watts.

Power (Watts) = Voltage (V) x Current (I)

 

Q10/11/12 Fill in the gaps using the best-fit words
from the list below.

{high-low-zero-colourful-tall-switch-bulb-current}

“A switch controls and lets if flow or not flow. If
the switch contacts are open then the current flowing
from the battery is . When the switch is in this
position
the circuit has a very resistance.

 



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