Physics P4: Electric Circuits (copy)

Charges of protons, electrons and neutrons

• Proton: Positive

• Electron: Negative

• Neutron: Uncharged

What makes an atom neutral?

Same number of protons and electrons

• Equal (but opposite) charges cancel out

Ion

A charged atom

How can you make uncharged atoms have a positive/negative charge?

• Adding electrons to an uncharged atom (negative)

• Removing electrons from an uncharged atom (positive)

Charging by friction- what happens when insulating materials are rubbed against eachother?

They become electrically charged (one positive the other negative)

• The charges remain on the insulators and cannot immediately flow away

Why does friction cause charge?

Electrons are transferred from one material to the other

What will happen when you rub a polythene rod with a dry cloth?

• The rod becomes negatively charged

  • The dry cloth transfers electrons to the surface atoms of the rod

What will happen when you rub a perspex rod with a dry cloth?

• The rod becomes positively charged

  • Electrons are transferred from the surface atoms of the rod to the cloth

What happens when charged objects are bought close together?

They exert a non-contact force on each other

• Direction of this force depends on if the charges are the same or opposite

What does a charged object create around itself?

Electric field

Electric field

A region where charges experience a force

How direction do field lines point around positive and negative charges?

Arrows on field lines show the direction of force when a positive object is bought nearby so they point:

• Away from positive charges (repulsion)

• Towards negative charges (attraction)

What does the strength of an electric field depend on?

The distance from the object creating the field

• Field is strongest as the distance between the 2 charged objects decreases

What force do objects with the same type of charge (like charges) experience?

• Repulsion

  • The 2nd charged object will move away from the charge creating the field

What force do objects with different types of charge (opposite charges) experience?

• Attraction

  • The 2nd charged object will move toward the charge creating the field

What happens to repulsive forces as like charges move further apart?

They decrease

Why are metals good conductors of electricity?

Electrons can easily flow through them

Static electricity

The stationary electric charge produced by friction between insulators caused by the imbalance between negative and positive charges in two objects

• Applies to solids, liquids and gases

How is static electricity produced?

By rubbing surfaces which causes insulators to become charged by friction

How does static electricity work?

• All objects are initially electrically neutral

• When electrons are transferred, one object becomes - charged and the other + charged

• The difference in charges leads to a force of attraction between itself and other neutral objects

Why does static electricity only work for insulators and not conductors?

In conductors, the charge will move through them instead of remaining stationary

Explain static electricity in sticking a balloon to the wall

• Rubbing a balloon on a woolen jumper transfers electrons onto the balloon by friction

• The balloon is now - charged but the jumper is left + charged

• The wall is still neutral, but when the balloon is placed near the wall, the + charges in the wall are brought to the surface because they are attracted to the - charge of the balloon

• Since opposite charges attract, the balloon sticks to the wall from the electrostatic attraction

Why is static electricity more common in dry air conditions?

It is much harder for the charges to dissipate so they build up instead

What causes sparkling

The build-up of electrostatic charge

When does sparkling/electric shocks occur?

• 2 objects which are charged by friction are now oppositely charged

• Electrons in the air molecules between the objects, experience a force towards the + object

• Some electrons are pulled out of air molecules by the force of a field

• The electrons hit other air molecules and knock electrons out of them

• This causes a small current to flow between the objects (spark)

How is lightning an example of sparkling

• In a storm, clouds move over each other causing them to become charged when electrons are transferred between them

• Since the ground is neutral, the negative charge jumps to meet the positive charges on the ground creating a big spark (lightning)

What does a circuit diagram show?

How components in a circuit are connected together

Standard circuit symbols

What and why does an electric circuit need to work?

1. An energy source – Source of PD so a current can flow

2. A closed path / complete circuit – Electrons need to flow in a complete loop for a current to flow

3. Electrical components – Perform a function

Electric charge

A group of electrons

• Measured in Coulombs (C)

How many electrons are there in one Coulomb (C)?

6.24 × 10^18 electrons

Coulomb

The quantity of charge that passes a fixed point per second when a current of 1 A is flowing

What happens when a conductor is connected to a power source?

• Electrons flow out of the cell and move around the circuit

• The electrons are carrying energy from the cell

• They pass energy to the components in the circuit like a lamp

  • In the lamp, electrical energy is transferred to light and thermal energy

• When electrons return to the + end of the cell, they are carrying less energy than when they left the - end

Electric current

The flow of electrical charge per second

• Measured in Amperes, amps (A)

• Symbol for current is I

What direction does electric current flow in?

From the negative terminal of the cell to the positive terminal

What affects the size of current

The number of electrons passing through a component per second

Conventional current

The flow of positive charge from the positive terminal of a cell to the negative terminal

Why is the direction of conventional current opposite to the direction of electron flow?

Conventional current was described before electric current was understood

Equation for charge flow

Current, I x Time taken, t = Charge flow, Q

(amperes, A) (seconds, s) (coulombs, C)

How is current measured in a circuit

Using an ammeter

How should an ammeter be connected in a circuit?

In series with the part of the circuit you wish to measure the current through

How and why does current behave in a series circuit?

• Current is the same value at any point

  • The same number of electrons passes through each component every second

Series circuit

A circuit where current can only flow in one path

• There are no branches

• Current is never used up in a circuit

Parallel circuit

Contains branches so current can flow in more than 1 path

How does current behave in parallel circuits?

The current in the branches adds up to the total current leaving the cell

Potential difference (voltage)

The energy transferred per unit charge flowing from one point to another

• Measured in volts (V)

• Volts = Joule per coulomb (J C-^1)

What does a potential difference of 1 volt tell us?

1 joule of energy is transferred for each coulomb of charge moving through the circuit

Sources of potential difference

• A cell

• Batteries (multiple cells)

• Electrical generator

How do you measure voltage

Voltmeter

How is a voltmeter connected in a circuit

In parallel to the component you are measuring the voltage for

How does voltage behave in series circuits

The supply voltage is shared between components

Why are two lamps connected in a series circuit dimmer than if there were just only one?

• The total energy carried by the current has been shared between the 2 lamps

• If there was only one lamp, all of the energy carried by the current is transferred to that one lamp

How does voltage behave in parallel circuits

Voltage is the same as the supply voltage across components

Relationship between voltage and energy transferred to a component

The bigger the voltage, the more energy is transferred

Equation for energy transferred

Charge (Q) x Voltage (V) = Energy

(coulombs, C) (volts, V) (joules, J)

Battery

2 or more cells connected together

• The cells in a battery must be connected in the same direction (eg positive ends both pointing to the left)

Resistance

The potential difference needed to drive a current through a component

• Measured in ohms, Ω

• Ohm= one volt per ampere

What factor about components causes them to transfer energy and why?

Resistance

• As current moves, electrons collide with atoms in the metal

• Electrical energy is transferred into other forms (like thermal)

Equation for the resistance of a component

Potential difference, V (volts, V)

--------------------------------- = Resistance, R (ohms, Ω)

Current, I (amperes, A)

How does resistance affect current?

The higher the resistance of a circuit/resistor, the lower the current

Do conductors and insulators have a high or low resistance?

• Conductors: Low resistance

• Insulators: High resistance

Why does adding resistors increase resistance in a series circuit?

• The total voltage is shared between more resistors, which makes the voltage across each one of them less than before

• The current through the resistors is hence less than before

• As the total voltage is unchanged, the total resistance is greater

Resistor

A device that limits the flow of electrical current in a circuit

2 types of resistors

• Fixed resistors

• Variable resistors

Fixed resistor

They have a resistance that remains constant

Variable resistor

They can change the resistance through the circuit

• Hence they can vary the amount of current through the circuit

Ohm’s Law

The current through a resistor at a constant temperature will be directly proportional to the potential difference across the resistor

• If V and I are directly proportional, this means that the resistance R remains constant

• The smaller the voltage the higher the resistance!!

What happens if you reverse the potential difference, and hence the direction of current, across a resistor?

• It makes no difference to the shape of the line (the values are just negative)

• The resistance is the same in whichever direction the current is in

What does the gradient of the line depend on?

The resistance of the resistor

• The greater the resistance of the resistor, the less steep the line is

Ohmic conductor

Components (resistors) that follow Ohm’s law

• Fixed resistors, wires, heating elements

How do resistors in series behave and why?

The total resistance of 2 (or more) components in series is equal to the sum of the resistance of each component

• Current has to pass through each resistor in turn

• It can’t bypass any resistor

• The more components the charge has to travel through, the higher the number of collisions that occur

How do resistors in parallel behave and why?

The total resistance of two (or more) resistors in parallel:

• Is less than the resistance of the smallest individual resistor

• This is because the charge has more than one pathway to take, so only some charge will flow along each path

• The more pathways there are, the smaller the amount of charge in each path

How can you control the potential difference across a component using a variable resistor?

• It contains a long piece of wire and a coil

• Using the slider, we can change the length of the wire the current runs through

  • Increase the length of the wire → increases resistance → less PD flows through the wire

Factors affecting resistance and why

Thickness, length, temperature

1. Thicker wires = lower resistance because moving electrons have more space to move

2. Shorter wires = lower resistance because moving electrons have a shorter distance to travel

3. Hot wires = higher resistance because metal particles move faster and so electrons collide more with them

How does a filament lamp work?

The tightly coiled wire gets very hot when an electric current passes through it which causes it to glow and give out light

Why is a filament lamp a non-ohmic conductor?

Current and PD aren’t directly proportional

• The filament gets hot so the resistance of the filament lamp increases

What does the IV graph for a filament lamp tell us?

• Where the graph is a straight line, the resistance is constant

• The resistance increases as the graph curves

• Reversing the PD doesn’t make a difference to the shape of the line

Why does the IV graph for a filament lamp show current increasing at a slower rate than the voltage?

• As the current increases, the temperature of the filament in the lamp increases

• Higher temperature → atoms in the metal lattice of the filament vibrate more

• So resistance increases

• Resistance opposes the current, causing the current to increase at a slower rate

Why will an increase in temperature cause an increase in resistance?

• The higher the temperature, the faster atoms in a solid vibrate

• The electrons collide with the vibrating atoms which impedes their flow

• So current decreases, but resistance increases

Diode

A non-ohmic conductor that allows current to flow in one direction only (forward bias)

Why can current through a diode only flow in one direction only?

The diode has a very high resistance in the reverse direction (current is virtually 0)

• Called reverse bias

What does the IV graph for a diode tell us?

• No current can flow in the reverse direction

• In the forward direction, current increases as potential difference increases

What are diodes useful for?

Controlling the flow of current in circuits

Light-emitting diode (LED)

A diode that gives off light when a current flows through it in the forward direction

• Very energy efficient source of light

Examples of non-ohmic conductors

• Filament lamps

• Diodes and LEDs

• LDRs

• Thermistors

Thermistor

A temperature-dependent resistor

• It is non-ohmic

What happens to the resistance of a thermistor as temperature increases?

As the temperature increases the resistance decreases

Why is a thermistor used as a thermostat?

• It is a temperature sensor

• It automatically regulates temperature or activates a device when the temperature reaches a certain point

Where can thermistors be found?

• Ovens

• Refrigerators

• Fire alarms

• Digital thermometers

• Boilers

LDR

Light-dependent resistor

• Non-ohmic conductor

What happens to the resistance of an LDR as light level increases?

As the light intensity increases the resistance decreases

What can an LDR do as a light sensor?

• It automatically regulates the amount of light intensity on it

• It activates a device when the light intensity reaches above or below a certain point

Advantage of using LDRs

The circuits are automatic and don’t need any human time to function correctly everyday

How do cells behave in series circuits?

The total potential difference of cells in series is the sum of the potential difference of each cell

Why does adding resistors in parallel decrease the total resistance?

• This happens because each extra resistor creates an extra path along which the charge can flow

  • This allows more charge to flow overall

  • This leads to a smaller overall resistance

Advantages of parallel circuits

• Components can be individually controlled, using their own switches

• If one component stops working the others will continue to function

Disadvantages of series circuits

• If a components breaks, all of the others will stop working

• The components cannot be controlled (switched on and off) separately