4. Electricity and Magnetism Flashcards
Magnets
Can exert a force (attraction or repelling) on another magnet or on magnetic materials
They have a permanent magnetic force.
Attract (magnets)
A force that pulls objects together.
Repel (magnets)
A force that pushes objects apart.
Magnetic materials (Examples)
- Iron
- Nickel
- Cobalt
- Steel (component of steel is Iron)
Magnetic materials
Not magnets, but they are attracted to magnets.
(If connected to a permanent magnet a temporary magnetic field will be induced making it magnetic. –> However, when removed from the magnet the magnetic field is removed and the magnetic material can no longer attract another magnetic material)
Non-magnetic
Do not experience a force when in a magnetic field. Plastic is an example of a non-magnetic material.
Magnetic forces (attraction/ repelling)
Opposites attracts
Similars repel
Magnetic poles
- Every magnet has a north and a south pole, positioned at opposite ends of the magnet.
- If you cut a magnet in half, you get two smaller magnets, each with a north (N) and a south (S) pole. (North pole will still be on the same side as the previous north side, just smaller)
Magnetic field
A region of space where another magnet or magnetic material experiences a force.
Represented by magnetic field lines, ALWAYS going from north to south.
- Field lines never cross
- The strength of the magnetic field is indicated by the density of field lines
The arrow on the magnetic field always shows the direction of force on the N-pole of compass, meaning the direction a compass will point.
Magnetic fields (attraction/ repulsion)
Attraction (North pole next to south pole) : field lines point in the same direction (N to S) and flow between the two magnets.
Repulsion (North pole next to North pole) : field lines point in opposite directions and bend away from each other.
Induced magnetism
- Magnetic materials can attract each other, but only when a permanent magnet is present.
- A permanent magnet always has a magnetic field.
- When a permanent magnet attracts a magnetic material, it induces a magnetic field in the material.
Induced magnetism - Eg. nickel coin on a horseshoe magnet
- When the magnetic material (the nickel coin) is placed in the magnetic field of the permanent magnet (the horseshoe magnet), magnetism is induced in the coin and it becomes attached to the permanent magnet.
- The nickel coin is attracted to either the north or the south pole of the permanent magnet.
- When a coin is attracted to the south pole, a north pole is induced at the top of the coin and a south pole at the bottom.
- The opposite is true when a coin is attracted to the north pole.
- When a third nickel coin is placed in the magnetic field (below the nickel coin), a south pole is induced at the top of this coin and a north pole at the bottom.
Demagnetised
The process of removing magnetism. Also known as unmagnetised.
Eg. When a magnetic material is removed from the magnetic field of the permanent magnet.
Demagnitism - Difference in materials
- A hard magnetic material , such as steel, is hard to magnetise but also hard to demagnetise/unmagnetise. (Used to make permanent magnets in devices that require a constant magnetic field)
- A soft magnetic material (eg. iron) is easy to magnetise but also easy to demagnetise/unmagnetise. (Used for temporary magnets. Iron is used in electronic door locks as it gains and loses its magnetism quickly.
Electromagnet
A magnet caused by the flow of current in a coil. It only creates a magnetic field when current passes through it.
Any time a coil of wire carries current, a magnetic field is induced
Properties of permanent magnets
Constant magnetic field
Cannot be switched on or off
North and south poles cannot be swapped
Properties of electromagnets
Variable strength magnetic field
Can be switched on and off quickly
North and south poles can be changed by changing the direction of current flow
Uses of permanent magents
Guitar pickups
Speakers
Cupboard latches
Uses of electromagnets
Electric door locks
Relays
MRI machines
Conductor
A material through which charges can flow freely.
Insulator
A material that does not allow the free movement of charges.
Charges - Net overall charge /removal/ addition
Most everyday objects have no net overall charge (neutral)
- It is possible to add or remove charges from the surface of an object to make it either positive or negative overall.
- This is due to the transfer of electrons, which are negatively charged particles.
An object that loses electrons will become positively charged, while an object that gains electrons will become negatively charged.
What is electrical charge measured in?
Electrical charge (Q) is measured in coulombs (C).
- A single electron carries a very small charge of 1.6 × 10−19 C.
Static electricity
- Static electricity occurs when friction between two insulators causes electrons to be transferred from one surface to another
- One insulator gains electrons (and becomes negatively charged) while the other loses electrons (and becomes positively charged).
- Insulators with net overall charge can attract to neutral objects by repelling like charges and being relatively more charged.
Static electricity - Eg. Balloon
- Rubbing the balloon causes the transfer of electrons from the jumper, leading to an imbalance of charges.
- As one surface is negative and the other is positive, they attract.
- Rubbing two balloons on the jumper causes the balloons to repel each other because they both have like (negative) charges.
- Balloon is attracted to neutral wall bc relative to wall the ballon is more positive/ negative making the charges opposite.
- Also the postive charged balloon will repel positive charges making the wall more negative + vice versa with negative balloon.
Electric field
When two charged particles approach each other, they experience a force.
The space in which an electric charge experiences a force is called an electric field.
An electric field always points in the direction that a positive charge experiences a force.
Uniform field
Parallel electric field lines betewen two flat surfaces.
Current
Measure of amount/rate of the flow of charges (eg. electrons).
When charges are stationary, there is no current; when they move, there is a current.
How to increase the current
Making each charged particle move faster
Increasing the number of charged particles
Increasing the amount of charge each particle carries.
Current (formula)
I = Q/t
I - Current (A, Amps)
Q - Charge (C, Coulombs)
t - Time (s, seconds)
I = V/R
V - Potential difference (V)
R - Resistance (ohms)
How to measure current
Placing an ammeter IN the circuiot BETWEEN componenets/ the battery etc.
Ammeters can be analogue (needle pointer) or digital (numbers).
Conventional current
- Conventional current is imagined flowing out of the positive terminal of a battery, around the circuit, to the negative terminal.
- The charge carriers, however, are electrons, which have a negative charge. Electrons are repelled from the negative terminal of the battery and attracted to the positive terminal.
- Electron flow is always in the opposite direction to conventional current.
AC/DC - General
- Alternating current (a.c.): electrons continuously change direction.
- The voltage must reduce to zero and then become negative, causing the electrons to move in the opposite direction –> moving back and forth.
- Direct current (d.c.): electrons flow in one direction only.
- One level of voltage –> electrons flowing in one direction only
Voltage
Measure of how much E.M.F. a battery has.
Electromotive Force
The work done or energy per unit charge around the whole circuit by an energy source, such as a battery. Measured in volts.
- Phenomena that enables a charge to flow –> the electrical work done by a source in moving a charge around a complete circuit.
Potential difference
The energy needed per charge to flow between 2 points in a cirvuit.
Work done per unit charge passing through a component.
EMF vs PD
(If circuit stays the same)
EMF = Constant
PD = Variable depending on points chosen within circuit
Calculating EMF in a circuit/ Voltage of a battery (Formula)
EMF (V, Volts) = Work done (J) / Charge (C)
E = W/Q or V= W/Q
Measuring EMF
Voltmeter placed parallel aroudn a component
- Can be analogue (needle-pointer) or digital
PD - Voltmeter = parallel around points.
EMF - Voltmeter = parallel around battery/ energy source.
Resistance
A measure of the opposition to current flow within a circuit.
Measured in ohms (Ω)
Resistance - Formula
Resistance (Ω) = Potential difference (V)/ Current (A)
R = V/I
Factors affecting resistance
Length / Cross-sectional area
If length doubles –> resistance doubles (directly proportional)
If cross-sectional area doubles –> resistance halves (Inversely proportional)
Ohms law
V = IR
V = Voltage (V)
I = Current (A)
R = Resistance (Ω)
Energy in circuits –> power
Power - Measure pf transfer rate of energy
Original energy source in a circuit = chemical energy in battery.
Electricity used to transfer chemical energy in battery to heat/ light/ kinetic energy.
Power (electricity) formula
P = IV
P = Power (W or J/s)
I = Current (A)
V = Potential Difference (V)
Other power electricity formulas
E = IVt
(bc P = E/t –> sub into P = IV formula)
P = I²R
(substituting V = IR into P = IV)
P = V²/R
(substituting I = V/R into P = IV)
Kilowatts
1 kW = 1000 W
E (J) = P (W) * T (s)
E (kWh) = P (kW) * T (h)
(used when units are too high)
How to draw circuits
- Use straight lines to represent wires
- Place voltmeters in parallel with the components
- Place ammeters in series with components
- Use conventional electrical symbols to represent the components, not real-life images.
Switch
Function
- Opens/ closes circuit
Application
- Turns devices on/ off
Cell
Function
- Provides direct current (d.c.)
Application
- Store of energy