Edexcel physics topic 10

What three subatomic components make up an atom?

electrons, protons, neutrons

Describe the structure of an atom

A nucleus containing protons and neutrons, surrounded by electrons in shells.

An atom contains 2 types of particles, name both of them and state their charges

Proton (+1), electron (-1)

In a closed circuit, if there is a potential difference in the circuit there will also be a....

Current

How is current defined?

As the rate of flow of charge (or electrons) around a circuit.

What are the units of charge?

Coulombs, C

What are the two ways that a component can be connected in a circuit?

1. Series (same loop)

2. Parallel (adjacent loop)

Voltage is also known as...

Potential difference

How does the potential difference across two components vary when connected in series and parallel?

● In a series circuit the total P.D is shared between each component

● In a parallel circuit the P.D across each component is the same

If two resistors are connected in parallel, what can be said about their combined total resistance?

Their total resistance is less than the smallest of the two individual resistances.

If two resistors are connected in series, what can be said about their total resistance?

Their total combined resistance is equal to the sum of the two individual resistances.

Where must a voltmeter be placed in a circuit?

in parallel with the component that is being measured

Give an equation relating potential difference with energy transferred and charge

Potential difference (V) = energy transferred (J) / charge (C)

V=E/Q

A volt can also be described as...

Joule per coulomb

What is an electric current?

The rate of flow of charge

State the equation linking charge, current and time. Give the units for the quantities involved

Q = I t

Charge (Coulombs), Current (Amperes), Time (Seconds)

What can be said about the value of current at any point in a single closed loop?

Current is the same at all points in a closed loop.

What two factors does the current in a circuit depend on?

1. Potential Difference (V)

2. Resistance (R)

What equation should be used to calculate potential difference if current and resistance are known? State the units for all 3 quantities.

V = I R

Potential Difference (V), Current (A), Resistance (Ω)

What is an ammeter and where must it be connected in a circuit?

An ammeter measures current. It is placed in series with the component it is required to measured.

What happens when current reaches a junction in a circuit?

Current is conserved; the total current remains the same and is split between the two branches.

How does resistance affect current in a circuit?

As the total resistance of a circuit increases, the current flowing through the circuit decreases.

How can the current in a circuit be varied?

Using a variable resistor

Give an equation linking current with resistance

P.d. (V) = current (A) x resistance (ohm)

V = IR

How is total resistance affected by two resistors in series?

The total resistance increases; it is equal to the sum of the two resistors.

How is total resistance affected by two resistors in parallel?

The total resistance decreases; it is less than the resistance of the resistor with the lowest resistance.

What is an 'Ohmic Conductor'? State the condition required

- conductor for which current and potential difference are directly proportional

- resistance remains constant as current changes

- temperature must be constant

List four components for which resistance is not constant as current changes.

1. Lamps

2. Diodes

3. Thermistors

4. Light Dependant Resistors (LDRs)

What happens to the resistance of a filament lamp as the temperature increases? Why?

● Resistance increases

● Ions in metal have more energy, so vibrate

more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow

What is different about current flow through a diode?

- The current only flows in one direction

- Resistance is very high in the other direction, preventing current flow

State what happens to the resistance of a thermistor as temperature increases.

The thermistor's resistance decreases.

Give two examples of when a thermistor may be used

1. In a thermostat to turn a heater on below a certain temperature

2. In a freezer to turn on a cooler when the temperature becomes too high

State what happens to the resistance of an LDR as light intensity decreases

The LDR's resistance increases.

Give an application for a LDR

● Street lights often use LDRs

● When light levels become too low, the

light gains sufficient current to turn on

How do diodes work?

A diode only allows current to flow in one direction. If current is flowing the right way, the resistance is large for small voltages (up until about 0.6V), but then at higher voltages becomes very small.

What factors affect the energy transferred when charge flows through a component?

● Amount of charge

● The potential difference across the component

Give an equation linking energy, current and potential difference giving all SI units

Energy (J) = potential difference (V) x current (A) x time (s)

Define potential difference in terms of charge

Work done per unit charge

Give an equation relating potential difference to charge

energy transferred (J) = charge (C) x p.d. (V)

E = QV

When an electrical current flows through a resistor why does it heat up?

There are collisions between the electrons and the ions in the resistor's lattice. This causes a transfer of kinetic energy into thermal energy, which is released into the surroundings.

How do low resistance wires reduce unwanted energy transfers?

A smaller resistance will mean there are fewer collisions, therefore less energy will be wasted through heating.

What are some advantages of the heating effect?

It is useful for appliances such as toasters or electrical fires, where the heat is the desired product.

What are some disadvantages of the heating effect?

- the loss of energy as heat energy can make an appliance inefficient

- if an appliance overheats it can catch fire or overheat, which could ruin the device or injure the user

The energy transferred per second is also known as

power (watts)

Give an equation linking power and potential difference, giving all SI units

power (W) = current (A) x p.d. (V)

Give an equation to work out power without potential difference

Power = current^2 x resistance

Give an equation linking power and energy

Power = energy/time

What does AC mean?

Alternating Current. This is current which is constantly changing between a negative and a positive maximum.

What does DC mean?

Direct Current. This is current which takes a constant value.

Is mains electricity an AC supply or a DC supply?

Mains electricity is an AC supply.

What is meant by the frequency of a supply?

This is the rate at which an AC current changes.

What is the frequency and voltage of the UK mains electricity supply?

● Frequency: 50 Hz

● Voltage: 230V

What type of current do batteries and cells supply?

direct current

What is the power rating of an appliance?

It shows how much energy a device converts per second.

What wires does a typical domestic appliance have?

1. Live wire

2. Neutral wire

3. Earth wire

What colour is the live wire?

brown

What colour is the neutral wire?

blue

What colour is the earth wire?

Green and yellow stripes

Explain when the Earth wire does and doesn't carry a current.

● Under normal circumstances, no current flows through the Earth wire

● If a fault occurs in the appliance (such as a surge or the casing becoming live), current will flow to the ground

What potential is the neutral wire at?

0 volts

State the potential difference between the live and earth wires.

230 volts

What is the purpose of the neutral wire?

To complete the circuit by connecting the appliance back to the mains supply.

For metal appliances, where is the Earth wire connected to? Why?

● Earth wire is connected to the metal casing of the appliance

● If live wire becomes loose and touches the casing, the current will flow through the Earth wire, preventing electrocution

How does a fuse stop a device from getting damaged?

When the current becomes too high the fuse wire will melt and break, therefore stopping current from flowing to the appliance and preventing damage.

Why is it important to connect a fuse or switch to the live wire?

When the switch is turned off, or when the fuse breaks, it will break the circuit and stop current running through the wire. This provides a method to stop current flowing into the appliance.

Why is it dangerous to have a connection between the earth wire and the live wire?

It can result in a current surge, causing electrical shocks.