Electrical Circuits

Define Charge

Charge is a physical quantity of matters that refers to how it reacts within an electrical or magnetic field

Define current

A measure of the rate of flow of electrical charge (measured in amps)

What is the main formula for current ?

Current = total charge/time taken

What is potential difference?

It is the total electrical energy used per coulomb of charge within the circut

How can we measure potential difference and current?

• Current is measured in amps by an AMMETER

• Voltage/potential difference is measured in volts by a VOLTMETER

How does current behave between parallel and series circuits?

→ Series circuits: the current is the same everywhere in the circuit

→ Parallel circuits: the current branches out depending on the resistance of each branch (higher resistance = lower current) or equally if there is no resistance

How does voltage behave between parallel and series circuits?

→ Series circuits: The total voltage splits between the circuit components based on their individual resistance (more resistance = higher voltage to push electric charge through)

→ Parallel circuits: The same throughout the circuit

What is the formula for voltage?

Voltage = total energy supplied or work done/ total charge

What is Kirchoff’s Law for current?

→ The total current flowing into any junction of a circuit = the total current flowing out of it

What did Kirchoff’s Law for voltage?

The sum of voltage in any closed loop = 0

Define resistance

a measurement of how much opposition is placed against the flow of electrons (measured in Ohms)

What is Ohms Law?

→ Ohms law states that resistance, current and voltage are connected in the formula

R = V/I

How does the total resistance act in series and parallel circuits?

→ Series: Rt = R1+R2+R3…

→ Parallel: 1/Rt = 1/R1 + 1/R2 + 1/R3…

What factors can change the resistance of a circuit?

1) Length of the wires: The longer the length of the wires = the higher the resistance (electrons have to travel a greater distance and collide more frequently with metal ions within the lattice)

2) Cross-sectional area: The smaller the cross-sectional area = the higher the resistance ( electrons have less room to pass through and will collide more with the metal ions in the lattice)

3) Temperature: In conductors the higher the temp = the higher the resistance because the metal ions gain more KE to move around and bump into electrons

In semi-conductors the higher the temp = the lower the resistance because the electrons have more energy to pass through

What is emf?

Emf is the measure of the total energy supplied per coulomb of charge passing through the cell of a circuit (also measured in Volts)

How do different components provide resistance in a circuit?

1) Fixed resistors = supply a specific resistance and reduce the flow of current and control it within the circuit

2) Variable resistors = change their resistance depending on the status of current throughout the circuit (lower it if the current is low and raises it if the current is high)

3) Thermistors = based around temperature of surrounding. If the temperature is high the resistance is low but if the surroundings are cold the resistance is raised

4) LDR (Light dependant resistors) = based around light intensity. If the surrounding light is high the resistance is low but in darker conditions the resistance is higher

5) Diodes =. maintain the flow of current in one direction as resistance is very low in the forwards direction but extremely high in the opposing direction

Define power in terms of electricity

The rate at which energy is transferred in an electrical circuit

How do we calculate power?

P = V x I

How can we calculate total energy transferred in a circuit?

Total energy transferred = V x I x Time taken

What is a potential divider?

A potential divider is a specific circuit set up that works on a series circuit where two different components need different levels of voltage with both being supplied with the same current. My attaching a resistor with a higher resistance in parallel with the component in need of more voltage, it divides more voltage into that component (more resistance = more voltage) and away from the component requiring less voltage.

What is the ohms law for a potential divider circuit?

V1/VT = R1/RT and V out = V in x R2/RT (which is the main one)

What is a diode?

A diode is a two terminal semi-conductor that regulates the flow of current in one direction only by creating very little resistance in the forward direction and very high resistance in the opposing direction

Determine the difference between the motion of AC and DC current

→ D.C current = electrons flow in a one way direction in a straight line

→ A.C current = electrons flow back and forth creating a wave-like motion (they can push forwards and back creating a positive and negative cycle)

How can diodes convert A.C to D.C current?

→ Two diodes = the one way direction of current in diodes removes the negative flow of electrons (back flow) leaving only the positive flow and reroutes the reverse flow into a single line -forward flow = a half-wave rectification-

→ More than two diodes = while two diodes pairs rectify the reverse flow the other two recycle them back into the main circuit flipping them foeward to create a full wave rectifcation.

Why do diodes need to translate ACcurrent into DC current?

More sensitive components cannot process the eratic electron movement in AC current and prefer the smooth single line flow of DC current, Diodes convert the faster moving AC current into DC current for them to process.

How does voltage change if resistance increases but current remains the same?

The voltage increases

Is the resistance stronger in a parallel of series circuit?

→ Resistance is stronger in series than parallel and the resistance of a single resistor is stronger than that of resistors in parallel

What is a relay?

An electromagnetic switch that uses a small input current to turn on/off a stronger output current using electromagnetism

State the core components of a relay

1) The coil = contain an internal electromagnet that activates when a small electric current flows through it

2) The Armature: A small moveable leaver that is activated by the coil (has contacts on both ends)

3) The Contacts: The heavy-duty switch terminals that open or close the high-power circuit.

• Normally Open (NO): Disconnected by default; turns ON when the relay is energized.

• Normally Closed (NC): Connected by default; turns OFF when the relay is energized.

4) Spring: Pulls the armeture back into its original position when the coil is no longer energised

Walk through the process of a relay

1) A small current is sent through the first circuit towards the coil

2) The coil is energised which generates a magnetic field pulling the armature down

3) As the armature clamps down it snaps down the heavy-duty switches

4) when the contacts snap together theyr generate a higher current the flows into the second circuit

5) When the small current is switched off, the magnetic field shuts down and the spring retracts the armature back into place

When might we use relays?

→ Protect sensitive sensors circuits from a sudden high voltage supply

→ The output voltage of a sensor is too small on its own to trigger a high voltage device

How does a variable resistor work?

→ Basic variable resistors have dial that is physically operated to increase or decrease resistance

→ Thermistors change their resistance based on the surrounding temperature: High temp = lower resistance

→ LDR (Light-dependant resistors) change their resistance based on light intensity : Bright light = lower resistance

Why can variable resistors be better than fixed resistors?

→ Easier circuit calibration as fixed resistors need to be plugged in or out to change resistance, variable resistors can be easily manipulated

→ Acts as both a controller for current and a sensor

→ Changes dynamically to environment and user input (not rigid in resistance)