Voltage, Current, Resistance
and Ohms Law 

An electrical circuit can be made up from components such
as copper tracks/wires, switches, lamps, buzzers.

For a simple circuit to work, there is one very important rule - all the components used should be part of a continuous loop that connects to the two sides of the battery or power supply.

The complete loop should contain:

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

  • Conductors, such as wire, to let the energy flow from the battery or power source, to the other components. This energy flow is known as current.

  • Components in the circuit which will offer a resistance to
    the current. For example, an old fashioned bulb .

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 manually take control. It is very useful to have switches on our walls to control our room lighting. Control switches on our TV remotes allow us to change the channels we are watching, or apps we are using.

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). High power car batteries are12 volts.

The switch, when closed, will conduct and let the battery
connect to the rest of the circuit. In this position we say 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 we say the switch has made an "open circuit"

In our torch circuit it is important to match the battery voltage
with the correct lamp. This prevents it overloading and getting damaged. This is important for many of the other components we use in electronic circuits.

Current 

For the lamp to light up, current needs to flow through it. In a closed circuit: power from the battery flows through the connecting wires, into the lamp and back to the battery.

Current is measured in Amperes (amps). The smaller currents in electronic circuits are usually measured in milliamps (mA). A milliamp is the same as 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, a wire connects the two ends of the battery together. With very little resistance provided by the wire, the current flowing from the battery is very high and the thin wire soon glows redhot and melts. In this situation the battery has been “short- circuited”.Although these larger batteries are designed to produce the higher currents demanded by a car, if its terminals are connected together (or shorted), this can make them catch fire or even explode. To prevent damage to a car battery and stop it overheating there are many fuses in a car - each designed to "blow" when the current flowing through it becomes too high.

 

When the lamp was included (in the first circuit above), the current was much lower - as the lamp resisted its flow.

 

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 a lot of current will 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 all the gaps using the best-fit words
from the list below before checking your last answer.

{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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