# uint 2

## Resultant forces

a) Whenever two objects interact, the forces they exert on each other are equal and opposite.

b) A number of forces acting at a point may be replaced by a single force that has the same effect on the motion as the original forces all acting together. This single force is called the resultant force.

c) A resultant force acting on an object may cause a change in its state of rest or motion.

d) If the resultant force acting on a stationary object is:

- zero, the object will remain stationary
- not zero, the object will accelerate in the direction of the resultant force.

e) If the resultant force acting on a moving object is:

- zero, the object will continue to move at the same speed and in the same direction
- not zero, the object will accelerate in the direction of the resultant force.

## forces and motions

a) The acceleration of an object is determined by the resultant force acting on the object and the mass of the object. a=Fm or F=m×a

*F*is the resultant force in newtons, N*m*is the mass in kilograms, kg*a*is the acceleration in metres per second squared, m/s^{2}

b) The gradient of a distance–time graph represents speed.

d) The velocity of an object is its speed in a given direction.

## forces and motions part 2

e) The acceleration of an object is given by the equation: The gradient of a velocity–time graph represents acceleration. a=v−ut

*a*is the acceleration in metres per second squared, m/s^{2}*v*is the final velocity in metres per second, m/s*u*is the initial velocity in metres per second, m/s*t*is the time taken in seconds, s

f) The gradient of a velocity–time graph represents acceleration.

## forces and braking

a) When a vehicle travels at a steady speed the resistive forces balance the driving force.

b) The greater the speed of a vehicle the greater the braking force needed to stop it in a certain distance.

c) The stopping distance of a vehicle is the sum of the distance the vehicle travels during the driver's reaction time (thinking distance) and the distance it travels under the braking force (braking distance).

d) A driver's reaction time can be affected by tiredness, drugs and alcohol.

e) When the brakes of a vehicle are applied, work done by the friction force between the brakes and the wheel reduces the kinetic energy of the vehicle and the temperature of the brakes increase.

f) A vehicle's braking distance can be affected by adverse road and weather conditions and poor condition of the vehicle.

## Forces and terminal velocity

a) The faster an object moves through a fluid the greater the frictional force that acts on it.

b) An object falling through a fluid will initially accelerate due to the force of gravity. Eventually the resultant force will be zero and the object will move at its terminal velocity (steady speed).

c) Draw and interpret velocity-time graphs for objects that reach terminal velocity, including a consideration of the forces acting on the object.

d) Calculate the weight of an object using the force exerted on it by a gravitational force: W=m×g

- W is the weight in newtons, N
- m is the mass in kilograms, kg
- g is the gravitational field strength in newtons per kilogram, N/kg

## Forces and elasticity

a) A force acting on an object may cause a change in shape of the object.

b) A force applied to an elastic object such as a spring will result in the object stretching and storing elastic potential energy.

c) For an object that is able to recover its original shape, elastic potential energy is stored in the object when work is done on the object to change its shape.

d) The extension of an elastic object is directly proportional to the force applied, provided that the limit of proportionality is not exceeded: F=k×e

- F is the force in newtons, N
- k is the spring constant in newtons per metre, N/m
- e is the extension in metres, m

## Forces and energy

a) When a force causes an object to move through a distance work is done.

b) Work done, force and distance are related by the equation: W=F×d

*W*is the work done in joules, J*F*is the force applied in newtons, N*d*is the distance moved in the direction of the force in metres, m

c) Energy is transferred when work is done.

d) Work done against frictional forces.

## Forces and energy part 2

e) Power is the work done or energy transferred in a given time. PEt

*P*is the power in watts, W*E*is the energy transferred in joules, J*t*is the time taken in seconds, s

f) Gravitational potential energy is the energy that an object has by virtue of its position in a gravitational field. Ep=m×g×h

*E*_{p}is the change in gravitational potential energy in joules, J*m*is the mass in kilograms, kg*g*is the gravitational field strength in newtons per kilogram, N/kg*h*is the change in height in metres, m

## Forces and energy part 3

g) The kinetic energy of an object depends on its mass and its speed. Ek=12×m×v2

- E
_{k}is the kinetic energy in joules, J - m is the mass in kilograms, kg
- v is the speed in metres per second, m/s

## Momentum

a) Momentum is a property of moving objects. p=m×v

*p*is momentum in kilograms metres per second, kg m/s*m*is the mass in kilograms, kg*v*is the velocity in metres per second, m/s

b) In a closed system the total momentum before an event is equal to the total momentum after the event. This is called conservation of momentum.

## Static electricity

a) When certain insulating materials are rubbed against each other they become electrically charged. Negatively charged electrons are rubbed off one material and onto the other.

b) The material that gains electrons becomes negatively charged. The material that loses electrons is left with an equal positive charge.

c) When two electrically charged objects are brought together they exert a force on each other.

d) Two objects that carry the same type of charge repel. Two objects that carry different types of charge attract.

e) Electrical charges can move easily through some substances, eg metals.

## Electrical circuits

a) Electric current is a flow of electric charge. The size of the electric current is the rate of flow of electric charge. The size of the current is given by the equation: I=Qt

*I*is the current in amperes (amps), A*Q*is the charge in coulombs, C*t*is the time in seconds, s

b) The potential difference (voltage) between two points in an electric circuit is the work done (energy transferred) per coulomb of charge that passes between the points. V=WQ

*V*is the potential difference in volts, V*W*is the work done in joules, J*Q*is the charge in coulombs, C

## Electrical circuits part 2

c) Circuit diagrams using standard symbols. The following standard symbols should be known:

d) Current–potential difference graphs are used to show how the current through a component varies with the potential difference across it.

e) The current–potential difference graphs for a resistor at constant temperature.

## Electrical circuits part 3

f) The resistance of a component can be found by measuring the current through, and potential difference across, the component.

g) The current through a resistor (at a constant temperature) is directly proportional to the potential difference across the resistor.

h) Calculate current, potential difference or resistance using the equation: V=I×R

*V*is the potential difference in volts, V*I*is the current in amperes (amps), A*R*is the resistance in ohms, Ω

## Electrical circuits part 4

i) The current through a component depends on its resistance. The greater the resistance the smaller the current for a given potential difference across the component.

j) The potential difference provided by cells connected in series is the sum of the potential difference of each cell (depending on the direction in which they are connected).

k) For components connected in series:

- the total resistance is the sum of the resistance of each component
- there is the same current through each component
- the total potential difference of the supply is shared between the components.

I) For components connected in parallel:

- the potential difference across each component is the same
- the total current through the whole circuit is the sum of the currents through the separate components.

## Electrical circuits part 5

m) The resistance of a filament bulb increases as the temperature of the filament increases.

n) The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.

## Electrical circuits part 6

o) An LED emits light when a current flows through it in the forward direction.

p) The resistance of a light-dependent resistor (LDR) decreases as light intensity increases.

q) The resistance of a thermistor decreases as the temperature increases.

## Household electricity

a) Cells and batteries supply current that always passes in the same direction. This is called direct current (d.c.).

b) An alternating current (a.c.) is one that is constantly changing direction.

c) Mains electricity is an a.c. supply. In the UK it has a frequency of 50 cycles per second (50 hertz) and is about 230 V.

d) Most electrical appliances are connected to the mains using cable and a three-pin plug.

e) The structure of electrical cable.

f) The structure and wiring of a three-pin plug.

g) If an electrical fault causes too great a current, the circuit is disconnected by a fuse or a circuit breaker in the live wire.

## Household electricity part2

h) When the current in a fuse wire exceeds the rating of the fuse it will melt, breaking the circuit.

i) Some circuits are protected by Residual Current Circuit Breakers (RCCBs).

Candidates should be aware of the fact that this device operates much faster than a fuse.

j) Appliances with metal cases are usually earthed.

k) The earth wire and fuse together protect the wiring of the circuit.

## Current, charge and power

a) When an electrical charge flows through a resistor, the resistor gets hot.

Candidates should understand that there is a choice when buying new appliances in how efficiently they transfer energy.

b) The rate at which energy is transferred by an appliance is called the power.

P=E / t

*P*is power in watts, W*E*is energy in joules, J*t*is time in seconds, s

## Current, charge and power part 2

c) Power, potential difference and current are related by the equation:

P=I×V

*P* is power in watts, W

*I*is current in amperes (amps), A*V*is potential difference in volts, V

**Additional guidance:**

*E*is energy in joules, J*V*is potential difference in volts, V*Q*is charge in coulombs, C

## Atomic structure

a) The basic structure of an atom is a small central nucleus composed of protons and neutrons surrounded by electrons.

b) The relative masses and relative electric charges of protons, neutrons and electrons.

c) In an atom the number of electrons is equal to the number of protons in the nucleus. The atom has no overall electrical charge.

d) Atoms may lose or gain electrons to form charged particles called ions.

e) The atoms of an element always have the same number of protons, but have a different number of neutrons for each isotope. The total number of protons in an atom is called its atomic number. The total number of protons and neutrons in an atom is called its mass number.

## Atoms and radiation

a) Some substances give out radiation from the nuclei of their atoms all the time, whatever is done to them. These substances are said to be radioactive.

b) The origins of background radiation.

c) Identification of an alpha particle as two neutrons and two protons, the same as a helium nucleus, a beta particle as an electron from the nucleus and gamma radiation as electromagnetic radiation.

e) Properties of the alpha, beta and gamma radiations limited to their relative ionising power, their penetration through materials and their range in air.

f) Alpha and beta radiations are deflected by both electric and magnetic fields but gamma radiation is not.

g) The uses of and the dangers associated with each type of nuclear radiation.

h) The half-life of a radioactive isotope is the average time it takes for the number of nuclei of the isotope in a sample to halve, or the time it takes for the count rate from a sample containing the isotope to fall to half its initial level.

## Nuclear fission

a) There are two fissionable substances in common use in nuclear reactors: uranium-235 and plutonium-239.

The majority of nuclear reactors use uranium-235.

b) Nuclear fission is the splitting of an atomic nucleus.

c) For fission to occur the uranium-235 or plutonium-239 nucleus must first absorb a neutron.

d) The nucleus undergoing fission splits into two smaller nuclei and two or three neutrons and energy is released.

e) The neutrons may go on to start a chain reaction.

## Nuclear fusion

a) Nuclear fusion is the joining of two atomic nuclei to form a larger one.

b) Nuclear fusion is the process by which energy is released in stars.

c) Stars form when enough dust and gas from space is pulled together by gravitational attraction. Smaller masses may also form and be attracted by a larger mass to become planets.

d) During the 'main sequence' period of its life cycle a star is stable because the forces within it are balanced.

e) A star goes through a life cycle. This life cycle is determined by the size of the star.

## Nuclear fusion

f) Fusion processes in stars produce all of the naturally occurring elements. These elements may be distributed throughout the Universe by the explosion of a massive star (supernova) at the end of its life.

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