Physics 2

• Have a nucleus containing protons and neutrons

• Electrons surround the nucleus

• Atom has no overall charge

Electrostatic charges:

Insulating materials can be given an electrostatic charge by rubbing two materials together.

Electrons are transferred from one material to another.

The material that has gained electrons has a negative charge and the other a positive charge.

A charged object can attract uncharged objects, it induces a charge on the object.

Uses and Dangers

Electrons can move to cancel out a charge on you, maybe cause a ‘shock’. This is called earthing.

Lightning happens when a charge builds up in clouds.

Using static electricity:

Used in paint sprayers. Droplets all get the same kind of charge so they repel and spray evenly and attracts to the object of opposite charge.

A bonding line has to be used to earth any static charge on an aeroplane or truck before refuelling starts.

Electric Currents

 The size of a current is a measure of how much charge flows past a point each second, the rate of flow of charged particles.

Measured in coulombs(C).

One amp is one C of charge per second.

 Charge = current × time

 Q = I × T

 Key points:

• Conductors are difficult to charge because the electrons can easily move through them

• A flow of electrons is an electric current

Current and Voltage

The current in a circuit is measured using an ammeter.

The voltage is measured using a voltmeter.

1 volt = 1 joule per coulomn

If the voltage is increased, the current increases, if it is decreased, the current decreases.

Resistance, Current and voltage

Resistance is a way of measuring how hard it is for electricity to flow through it.

Measured in ohms(Ω).

 Higher the resistance, smaller the current.

 A resistors or variable resistors can change the resistance.

 Calculations:

 Potential difference/voltage(volts) = current(amps) × resistance(ohms)

V = I × R

 Shorter wire = less resistance = more current = brighter bulb

Transferring Energy

Power:

Power is the energy per second

Measured in Watts

1 Watt = 1 Joule per second

• Electrical power(watt) = current(amp) × potential difference/voltage(volt)

  P = I × V 

  P = V²/R

  P= I²R

•  

Energy:

Total energy transferred by an appliance depends on its power and how longs its switched on

• Energy transferred(joule) = current(amp) × potential difference/voltage(volts) × time(seconds)

  E = I × V × t

   

  Must be converted to seconds.

Vectors and Velocity

Vectors have both size and direction.

Distance – path/route taken

• Displacement  – distance in a straight line from start to finish

  Distance-time graphs:

• Gradient = speed

• Speed is always positive

• Velocity can be positive or negative

  Velocity(m/s) =

Velocity and Acceleration

Acceleration:

• Is a change in velocity

• Is a vector quantity

  Acceleration(m/s²) =

  a =

a = acceleration

v = final velocity

u = initial velocity

t = time taken

Velocity-time graphs:

• The area under the graph represents the distance travelled by the object

   

Resultant Forces

If there is more than one force on something, all the forces can be combined into a resultant force.

Free-body diagrams:

• Shows all the forces on something

• Shows the direction

• Larger forces shown using longer arrows

• Used for forces on the same body

  Action and reaction forces:

• Two touching objects exert force onto each other

Forces and Acceleration

Acceleration depends on 2 things:

• Mass of the object

• Size of the force

  Force(N) = mass(kg) × acceleration(m/s²)

Terminal Velocity

Calculating weight:

• The gravitational field strength on Earth is 10N

• Mass must always be in kilograms

  Weight(N) = mass(kg) × gravitational field strength(N/kg)

  W = m × g

• Mass – quantity of matter (doesn’t change)

• Weight – measure of the pull of gravity on an object (can change)

Terminal Velocity:

• The force of gravity on a large mass is more than on a smaller mass

• But the large mass also needs a greater force to accelerate it

• The two affects cancel out

• All masses fall at the same rate in a vacuum

• The acceleration due to gravity on Earth is 10 m/s²

  Gravity has units of acceleration too. It is not a force, but acts on objects to cause downwards acceleration.