4.2 Electrical Quantities (Cambridge (CIE) IGCSE Physics)

The purpose of rubbing insulating solids with a cloth in electrostatic experiments is to transfer electrons between the cloth and the insulator leaving them with opposing net charges.

You can determine if two charged materials have the same charge by observing their interaction.

• If they repel each other, they have the same charge

• If they attract, they have opposite charges

An electric field is the region surrounding a charged object where other charges will experience a force. Field lines always point away from positive charges and towards negative charges.

The direction of electric field lines shows the direction a positive charge would move if placed at that point.

The strength of an electric field decreases with distance from a charged object.

A uniform electric field is when the field strength is constant and the field lines are parallel and straight.

The properties of field lines around a point charge are:

• They extend radially outward (away) from a positive charge and radially inward (toward) a negative charge.

• They indicate the direction of the force on a positively charged particle at any given point in the field.

The electric field between two parallel plates is uniform, with field lines directed from the positive to the negative plate. The lines are parallel and straight.

The symmetry of field lines around a charged conducting sphere is due to the even distribution of charges on the sphere's surface, resulting in symmetrical repulsion.

False.

Electric fields always point away from positive charges and towards negative charges.

True.

Conductors allow charge, usually electrons, to move freely through them.

Conductors are materials that permit the flow of charge, typically electrons, with ease.

False.

Metals are generally excellent conductors due to their ability to facilitate the flow of charge.

True.

Copper is an example of conductor. Most electrical wiring is made of copper.

The key difference between conductors and insulators lies in their ability to allow charge to move.

Conductors allow the movement of charge, while insulators prevent it.

The properties of metal conductors on an atomic scale are:

• have a lattice structure (of metal ions)

• free (delocalised) electrons.

Insulators are materials that prevent the flow of charge and instead accumulate a build up of static charge.

You can distinguish between conductors and insulators using a gold-leaf electroscope by:

• Charging the plate of the gold-leaf electroscope and then touching it with the material being tested.

• If the gold leaf falls, the material is a good conductor.

• If it remains in place, the material is a poor conductor or insulator.

When the gold leaf electroscope is charged the plate, rod and gold leaf all have the same charge so they repel each other and the gold leaf sticks out to the side.

Current is the amount of charge passing a fixed point per unit time.

The equation for charge, current, and time is:

Where:

• = current, measured in amperes (A)

• = charge, measured in coulombs (C)

• = time, measured in seconds (s)

You can distinguish between direct and alternating current because:

• direct current flows continuously in one direction, typically from negative to positive terminals

• alternating current periodically changes direction

Direct current flows in one direction only.

The purpose of an ammeter is to measure the current in a circuit.

False.

Ammeters should be connected in series.

Conventional current is the direction that positive charge would flow around a circuit, from the positive terminal to the negative terminal.

Electrons flow around a circuit from the negative terminal to the positive terminal. This is the opposite direction to conventional current.

False. Conventional current is defined as flowing from positive to negative, even though electrons (negatively charged) actually move in the opposite direction.

A digital ammeter is more accurate than an analogue ammeter because it has a greater sensitivity, a larger range and gives a definite numerical value.

True.

Electromotive force (e.m.f.) refers to the potential difference of the power source in a circuit.

Electromotive force (e.m.f.) is defined as the electrical work done by the power source in moving a unit charge around a complete circuit. It is measured in volts (V).

The equation for electromotive force (e.m.f.) is:

Where:

• = electromotive force, measured in volts (V)

• = energy supplied from the power source, measured in joules (J)

• = charge, measured in coulombs (C)

The equation for potential difference (p.d.), charge and work done is:

Where:

• = potential difference, measured in volts (V)

• = energy transferred to the components, measured in joules (J)

• = charge on each charge carrier, measured in coulombs (C)

Potential difference is the the work done by a unit charge passing through a component.

False.

The potential difference is measured in volts (V).

A voltmeter is a device used to measure potential difference between two points in an electrical circuit. It can be either digital or analogue and is connected in parallel with the component being tested.

False.

Voltmeters are connected in parallel with the component being tested.

True.

Resistance is a measure of how difficult it is for current to pass through a conductor or component.

Ohm's Law states that the current flowing through a conductor is directly proportional to the potential difference across the conductor.

The equation for Ohm's law is:

Where:

• = resistance, measured in ohms (Ω)

• = potential difference, measured in volts (V)

• = current, measured in amperes (A)

False.

The unit of resistance is the ohm, represented by the Greek symbol omega (Ω).

An I-V graph, or current-voltage graph, is a graphical representation of the relationship between the current flowing through a component and the potential difference across it. It shows how the current changes with potential difference.

The shape of the I-V graph for a fixed resistor is a straight line passing through the origin, indicating that the current is directly proportional to the potential difference.

True.

A diode is a non-ohmic conductor. This means current and potential difference are not directly proportional.

The unique characteristic of an I-V graph for a diode is a sharp increase in current when it is in forward bias, and a flat line indicating zero current when it is in reverse bias.

The resistance of a wire is the opposition encountered by electrons as they pass through a wire due to collisions with metal ions. It is a measure of how difficult it is for current to flow through the wire.

The resistance of a wire is directly proportional to its length. This means that as the length of the wire increases, its resistance also increases by the same factor.

The resistance of a wire is inversely proportional to its cross-sectional area. As the wire's cross-sectional area (thickness) increases, its resistance decreases, and vice versa.

False.

If a wire is thicker (greater diameter), its resistance is less.

True.

If a wire is longer, its resistance is higher because each electron will collide with more ions along the wire's length, resulting in a greater opposition to the flow of electrons.

The resistance of a wire is directly proportional to its length, so if the length is increased by a factor of 3, the resistance will increase by a factor of 3.

The resistance of a wire is inversely proportional to its cross-sectional area, therefore if its cross-sectional area is halved, its resistance will double.

Electrical energy is the energy transferred by an electric current.

This is typically from a power source to the components connected in an electrical circuit, where it is transformed into other stores of energy, such as thermal or kinetic energy.

True.

Potential difference, current, and time are related in electrical energy transfer, as described by the equation .

The electrical energy equation is:

Where:

• = electrical energy transferred, measured in joules (J)

• = voltage supplied, measured in volts (V)

• = current, measured in amperes (A)

• = time, measured in seconds (s)

In a circuit, energy is transferred from the power source to the components connected in the circuit.

As charge passes through the power supply, it gains energy in its kinetic store, which is then transferred to the energy stores of the components as it passes through them.

The amount of energy transferred in an appliance depends on:

• How long the appliance is switched on for

• The power rating of the appliance

The unit kW stands for kilowatt, equal to 1000 watts.

True.

Energy in a circuit is transferred from the power source to the components and the surroundings.

In a toaster:

Electrical energy is transferred from the mains supply to the thermal store of the heating element and then to the thermal store of the bread.

Electrical energy is transferred from the mains supply to the kinetic store of the motor, to the kinetic store of the drum as it spins.

Electrical power is defined as the rate at which electrical energy is transferred or converted per unit of time, typically measured in watts (W).

True.

Power is defined as the rate of doing work, or, the rate of energy transfer.

It is typically measured in watts (W).

The electrical power equation is:

Where:

• = power, measured in watts (W)

• = voltage, measured in volts (V)

• = current, measured in amperes (A)

Power is defined as the rate of doing work.

Therefore, electrical power describes the rate at which electrical energy is transferred or converted.

The equation for electrical power in terms of resistance is:

Where:

• = power, measured in watts (W)

• = current, measured in amperes (A)

• = resistance, measured in ohms (Ω)

False.

The unit of power is watts (W), where 1 watt is equivalent to 1 joule per second.

The kilowatt-hour (kWh) is a unit of energy. It is equivalent to one kilowatt of power expended for one hour.

It is commonly used to measure and compare energy usage in homes and businesses.

False.

Energy usage in homes and businesses is commonly measured in kilowatt-hours (kWh), where 1 kWh is the energy consumed using 1 kilowatt of power for 1 hour.

The equation that defines the kilowatt hour (kWh) is:

Where:

• = energy, measured in kilowatt-hours (kWh)

• = power, measured in kilowatts (kW)

• = time, measured in hours (h)