P4: electricity and magnetism

  • Ends of magnetics are poles

    • north and south pole

    • magnetic forces are strongest at poles

    • like poles repel

    • opposite poles attract

      → non-contact force

  • Uses:

    • Permanent:

      • compasses

      • toys

      • fridge magnets

    • Electromagnets:

      • MRI scanners

      • speakers and earphones

  • Magnetic materials: iron, cobalt, nickel, steel

    • will always be attracted to the magnet (regardless of poles)

    • magnet → repelled by known magnet

    • magnetic material → attracted to known magnet

    • non-magnetic material → not attracted/repelled

  • Permanent magnets: made of permanent magnetic materials like steel. Produces its own magnetic field.

  • Temporary (induced) magnetism: material with a soft iron core → becomes a temporary magnet when placed in a magnetic field

    • force of attraction between permanent magnet → magnetised

    • end close to the magnet will have opposite pole of the magnet

    • removed → lose its magnetism

  • Magnetic fields: a region in which a magnetic pole experiences a force

    • becomes weaker as the distance from the magnet increases

      → magnetic field lines get further apart

  • Magnetic field lines: an arrow indicating the direction of field line

    • shows the direction that the magnetic force would act

    • always north. → south

  • Strength of magnetic field is shown by spacing of the magnetic filed lines

    • close: strong

    • far: weak

Electric charge

  • Charging by friction: insulating solids that are rubbed together → electrically charged

    • One gains a positive charge

      • electrons move away from the object

    • one gains a negative charge

      • gains electrons

      → both objects are attracted to each other

  • Electric fields: a region of space in which an electric charge experiences a force

  • Direction of an electric field: direction of a for e on a positive charge at that point

    • used to represent the direction and magnitude of an electric field

    • positive → negative

  • Conductors: a material that allows charge to flow

    • silver

    • copper

    • aluminum

    • steel

  • Insulators: a material with no free charges → does not allow the flow of charge

    • rubber

    • plastic

    • glass

    • some can only conduct charge in the form of static electricity

Current

  • The flow of positive charge (conventional current): positive terminal → negative terminal

    • opposite direction to electron flow

  • current flows: when a circuit forms, charge flows from positive terminal → negative terminal

  • measure using an ammeter (A) and in amps

    • connected in series

    • digital or analogue

  • wires in an electric circuit are made of metal (good conductor of electric current

    • current is a flow of negatively charged electrons in wires

  • I = Q/t

    • Q = charge in coulombs (C)

    • I = current in amps (A)

    • t = time in seconds

Electromotive force (e.m.f)

  • electrical work done by a source in a moving unit charge around a complete circuit

  • measured in volts (V)

  • E = W/Q

    • E = e.m.f in volts

    • W = energy transferred to the charges from the power source in joules (J)

    • Q = charge moved in coulombs

Potential difference

  • Work done by a unit charge passing between two points in a circuit

  • measured in volts (V) using voltmeter in parallel with component

  • electrons flow through a cell → gain energy

  • electrons flow through a circuit → lose energy

  • V = W/Q

    • v - potential difference (V)

    • W - energy transferred to the components in Joules (J)

    • Q - charge moved in coulombs (C)

  • 1V = 1 J/C

Ohm’s law

  • Resistance: the opposition to current

    • Occurs when free electrons in the circuit collide with metal ions in the wire → slow down the flow

    • Higher resistance → lower current

    • Can be increased by adding resistors

    • directly proportional to length

      • longer wire → greater resistance (more ions)

    • inversely proportional to cross-sectional area

      • thicker wire → smaller resistance (more space to flow)

  • Current and resistance are inversely proportional

  • R = V/I

    • R - resistance in ohms

    • V - potential difference (V)

    • I - current (A)

  • Resistors are used to control:

    • current in branches of the circuit

    • potential difference across certain components

  • Current decreases as resistance increases

  • p.d increases as resistance increases

  • Current-voltage graphs

    • current - p.d → linear

    • current - voltage → non-linear

Electrical energy

  • amount of energy transferred by an electrical appliance depends on:

    • how long it runs for

    • power rating of the appliance

  • E = VIt

    • E - energy (J)

    • V - p.d (V)

    • I - current (A)

    • t - time (s)

Electrical power

  • The rate at which energy is transferred by an appliance

    • P = E/t

      • P - power (W) or (J/s)

      • E - energy transferred (J)

      • t = time (s)

  • Power dissipated by a component:

    • P = IV

      • P - dissipated power (W)

      • I - current (A)

      • V - p.d (V)

Circuit components

  • Fuses: protect expensive components from current surges and act as a safety measure against fire

  • Diodes: only allows a current in one direction

  • Current in series circuits: same value at any point

    • all components in a closed-loop have the same current

    • increasing voltage drives more current around the circuit

    • increasing components increases resistance→ less current flows through the circuit

  • Current in parallel circuits: current splits along each branch → current from the source is larger than current in each branch

    • amount of current flowing into junctions is equal to the amount of current flowing out (charge is conserved)

    • doesnt always split equally

P.d in series circuits

  • sum of p.d across components is equal to the total e.m.f

P.d in parallel circuits

  • The potential difference across each branch of a parallel circuit is the same as the e.m.f. of the power source

  • if one branch has multiple components → current splits within the branch

Combined resistance

  • series: combined resistance of the component = sum kf individual resistances

  • Parallel: combined resistance is less than resistance of individual components

    • 2 resistors of equal resistance → resistance halves

  • resistor with the largest resistance → greater p.d

  • resistance is increased→ gets a greater share of p.d

Electrical safety

  • Common electrical hazards include:

    • damaged insulation

    • overheating cables

    • damp conditions

    • excess current from overloading of plugs, extension leads, single and multiple sockets when using a mains supply

  • All electrical appliances are connected to the mains supply

  • A mains circuit consists of:

    • a live wire: carries the altering current from main supply to circuit

    • a neutral wire: form the opposite end of the circuit to the live wire to complete the circuit

    • an earth wire: acts as a safety wire to stop the appliance from becoming live

  • safety features built into domestic appliances:

    • double insulation: 2 layers of insulation

    • earthing: provides a low resistance lath to the earth

    • fuses: located in the plug and cut off flow of electricity if the current becomes too large. made of a thin metal wire

    • trip switches: control the amount of current supplied to each circuit within the house

      • When the current is too high the switch 'trips' (automatically flicks to the off position)

      • This stops the current flowing in that circuit


electromagnetic induction

  • Induced e.m.f: whenever there is relative movement between conductor and a magnetic field

    • could be when:

      • conductor moves in a stationary magnetic field

      • conductor is stationary in a changing magnetic field

  • Induced e.m.f. due to a moving conductor

    • For an electrical conductor moving in a fixed magnetic field:

      • the conductor (e.g. a wire) cuts the field lines

      • an e.m.f. is induced in the wire

- Induced e.m.f. due to a moving conductor

  • For an electrical conductor moving in a fixed magnetic field:

    • the conductor (e.g. a wire) cuts the field lines

    • an e.m.f. is induced in the wire

  • Lenz’s law: direction of an induced emf always opposes the change causing it

    • any magnetic field created by an induced emf will act so that it tries to stop the wire or magnet from moving