Static and charge — OCR GCSE Physics
Test yourself on Static and charge with OCR GCSE practice questions.
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Static and charge explained
Charge flow is the total charge that passes a point in a circuit, measured in coulombs (C).
Read the full explanation
Current is the rate of flow of charge, measured in amperes (A), where one ampere is one coulomb per second. Time is measured in seconds (s). The relationship is Q = I × t, so a current of 2 A flowing for 30 s transfers 2 × 30 = 60 C. Rearranged, I = Q ÷ t and t = Q ÷ I. Always convert time to seconds first: 3 minutes is 180 s, and 0.5 minutes is 30 s. If a question gives charge and time and asks for current, divide charge by time. Check units: coulombs divided by seconds gives amperes. This equation links the definition of current to measurable quantities and is recalled and applied in both foundation and higher tiers.
describe that charge is a property of all matter and that there are positive and negative charges (to include: the understanding that in most bodies there are an equal number of positive and negative charges resulting in the body having zero net charge)
Charge is a property that every form of matter possesses, because matter is built from particles that carry charge. Two kinds of charge exist: positive and negative. In a neutral body the total amount of positive charge equals the total amount of negative charge, so the charges cancel and the net charge is zero. For example, an atom has positively charged protons in its nucleus and negatively charged electrons around it; when the numbers balance, the atom is neutral overall. A body becomes charged only when these quantities become unequal, for instance when electrons are transferred to or from it. When answering a multiple-choice question, decide whether the option correctly states that charge belongs to all matter, that there are two types, and that equal numbers give zero net charge.
describe the production of static electricity, and sparking, by rubbing surfaces, and evidence that charged objects exert forces of attraction or repulsion on one another when not in contact (to include: the understanding that static charge only builds up on insulators)
Static electricity is produced when two different insulating surfaces are rubbed together. Friction transfers electrons from one surface to the other, so one surface gains electrons and becomes negatively charged while the other loses electrons and becomes positively charged. Static charge builds up only on insulators, because in a conductor the charge would flow away. A spark can occur when the built-up charge is large enough to jump across a gap, for example from a charged object to a conductor. Charged objects also exert forces on each other without touching: like charges repel and opposite charges attract. Evidence for this is seen when a charged rod attracts small pieces of paper or repels another rod of the same charge at a distance. In a multiple-choice question, check that the option names rubbing, insulator build-up, sparking and non-contact forces correctly.
explain how transfer of electrons between objects can explain the phenomena of static electricity
Static electricity arises because electrons, which carry negative charge, can be moved from one object to another when two different insulating materials are rubbed together. The object that gains electrons becomes negatively charged; the object that loses electrons is left with a deficit of negative charge and so becomes positively charged. The total charge is conserved, so the two objects always carry equal and opposite charges. A charged object can then attract small neutral objects such as scraps of paper, because the charged object repels or attracts electrons within the neutral object and induces a charge separation. Two objects carrying like charges repel; two carrying opposite charges attract. The electrons themselves do not travel through the air in these everyday effects; the charge stays on the surface of the insulator.
explain the concept of an electric field and how it helps to explain the phenomena of static electricity (to include: how electric fields relate to the forces of attraction and repulsion) (content found only in the physics separate science qualification)
An electric field is a region in which a charged object experiences a force. The field is represented by field lines whose direction shows the force on a positive charge, and the spacing of the lines shows the strength of the field: closer lines mean a stronger field. Around an isolated positive charge the lines point radially outwards; around an isolated negative charge they point radially inwards. Between two like charges the field lines push apart, giving repulsion; between two opposite charges the lines run from the positive to the negative charge, giving attraction. Static phenomena such as a charged rod attracting small neutral scraps of paper are explained by the field: the field of the charged object polarises the neutral object, and the resulting force acts without the objects touching. This content is assessed only in the physics separate science qualification.
recall that current is a rate of flow of charge (electrons) and the conditions needed for charge to flow (to include: conditions for charge to flow: source of potential difference and a closed circuit)
Electric current measures how much charge passes a point each second, so a current of 2 A means 2 coulombs of charge flow past that point every second. In a metal wire the moving charges are electrons, which drift through the wire while the current direction is taken as opposite to their motion. Charge flows only when two conditions are met together: there must be a source of potential difference, such as a cell or battery, to push the charge, and there must be a closed circuit so charge can travel all the way round. Break the loop with a switch or remove the cell and the current stops. A complete path with no source of potential difference also gives no current.
recall that current has the same value at any point in a single closed loop
In a single closed loop, charge is not used up as it travels round, so the current is the same at every point. If 0.5 A leaves a cell, then 0.5 A passes through each lamp, each wire and back into the cell. This follows because charge is conserved: the rate at which charge enters any section equals the rate at which it leaves. A common trap is to think current is smaller after a component because energy has been transferred, but it is energy, not charge, that is transferred to the surroundings. The rule applies to one loop only; where a circuit branches, the current splits between the branches.
recall and use the relationship between quantity of charge, current and time
Charge, current and time are linked by Q = I × t, where Q is charge in coulombs (C), I is current in amperes (A) and t is time in seconds (s). Current is the rate of flow of charge, so one ampere means one coulomb passing a point each second. To use the relationship, identify the two known quantities, convert time to seconds (for example 3 minutes = 180 s) and multiply current by time to find charge, or divide charge by time to find current. A current of 2 A flowing for 30 s transfers 2 × 30 = 60 C. Rearranged forms are I = Q ÷ t and t = Q ÷ I. In a multiple-choice question, work the calculation before looking at the options, then check the unit matches coulombs, amperes or seconds as asked.
Your focus
- State the relationship charge flow = current × time with correct units.
- Calculate charge flow, current or time using the relationship.
- Convert time to seconds and give answers with the correct unit.
Show all 24 objectives
- State that charge is a property of all matter and that charges are positive or negative.
- Explain that most bodies contain equal numbers of positive and negative charges.
- Conclude that a body with equal positive and negative charge has zero net charge.
- Describe how rubbing insulating surfaces produces static electricity and can cause sparking.
- Explain that static charge builds up only on insulators.
- Give evidence that charged objects exert non-contact forces of attraction or repulsion.
- Describe how rubbing two insulating materials transfers electrons and produces equal and opposite charges.
- Relate the gain or loss of electrons to an object becoming negatively or positively charged.
- Use the ideas of induced charge separation and forces between charges to explain static phenomena.
- Define an electric field and describe how field lines represent its direction and strength.
- Use electric fields to explain the forces of attraction and repulsion between charged objects.
- Apply the concept of an electric field to explain a static phenomenon such as attraction of a neutral object.
- State that current is the rate of flow of charge carried by electrons.
- Identify a source of potential difference as a condition for charge to flow.
- Identify a closed circuit as a condition for charge to flow.
- State that current is the same at any point in a single closed loop.
- Justify equal current using conservation of charge.
- Distinguish a single loop from a branched circuit when applying the rule.
- State the relationship Q = I × t and the units of each quantity.
- Calculate charge from a given current and time, converting time to seconds.
- Rearrange the relationship to find current or time from the other two quantities.
Static and charge exam tips
Marking Points
- Recalls the relationship charge flow = current × time and writes it as Q = I × t.
- Substitutes current in amperes and time in seconds to obtain charge in coulombs.
- Rearranges the relationship to I = Q ÷ t or t = Q ÷ I when current or time is the unknown.
- Converts a time given in minutes to seconds by multiplying by 60 before calculating.
- Checks that the answer carries the unit coulomb and is consistent with the values used.
- States that charge is a property of all matter, not only of certain special materials.
- Identifies that charge exists in two types, positive and negative.
- Explains that in most bodies the number of positive charges equals the number of negative charges.
- Concludes that equal numbers of positive and negative charges give a body zero net charge.
- Describes that rubbing two different insulating surfaces together transfers electrons and produces static charge.
- Explains that static charge builds up only on insulators because charge cannot flow away as it would in a conductor.
- States that a spark occurs when built-up static charge jumps across a gap.
- Gives evidence that charged objects exert attraction or repulsion on each other without touching, such as a charged rod attracting small pieces of paper.
- States that rubbing two different insulating materials transfers electrons from one object to the other.
- Identifies the object that gains electrons as becoming negatively charged and the object that loses electrons as becoming positively charged.
- Explains that the two objects end up with equal and opposite charges because charge is conserved.
- Explains attraction of a neutral object in terms of induced charge separation caused by the charged object.
- States that like charges repel and opposite charges attract, linking this to the observed forces.
- Defines an electric field as a region in which a charged object experiences a force.
- States that electric field lines show the direction of the force on a positive charge and that closer lines indicate a stronger field.
- Explains that like charges repel and opposite charges attract because of the forces described by their electric fields.
- Uses the electric field of a charged object to explain how it attracts a neutral object through induced charge separation without contact.
- States that current is the rate of flow of charge, measured as charge passing a point per unit time.
- Identifies electrons as the charge carriers that flow in a metal wire.
- Names a source of potential difference, such as a cell or battery, as one condition for charge to flow.
- States that the circuit must be closed, forming a complete path, as the other condition for charge to flow.
- Recognises that removing the source of potential difference or breaking the loop stops the current.
- States that the current is the same at every point in a single closed loop.
- Explains that charge is conserved, so the rate of flow entering a section equals the rate leaving it.
- Distinguishes current from energy, noting that components transfer energy without reducing the current.
- Applies the rule to a series loop, giving the same ammeter reading at different positions.
- Recognises that the rule applies to a single loop and that current divides where a circuit branches.
- Selects the option that applies Q = I × t with current in amperes and time in seconds.
- Converts a time given in minutes into seconds before multiplying by the current.
- Rearranges the relationship correctly to find current as charge divided by time.
- Identifies the coulomb as the unit of charge corresponding to one ampere for one second.
Examiner Tips
- 💡Write Q = I × t, then rearrange before substituting numbers.
- 💡Convert every time to seconds and every current to amperes before calculating.
- 💡Carry the unit through the calculation so the final answer is clearly in coulombs, amperes or seconds.
- 💡Read each option for the two ideas needed: charge belongs to all matter, and equal positive and negative charges cancel.
- 💡Reject any option that says a neutral object has no charged particles inside it.
- 💡Use the word net when describing the overall charge of a balanced body.
- 💡Look for the word insulator when an option describes where static charge builds up.
- 💡Check that an option about forces mentions attraction or repulsion without contact.
- 💡Link sparking to a large build-up of charge jumping across a gap.
- 💡Name the particle that moves (electrons) and state the direction of transfer before describing the resulting charges.
- 💡Use the words 'gain' and 'lose' electrons explicitly so the sign of each charge is unambiguous.
- 💡When explaining attraction of a neutral object, mention induced charge separation rather than saying the neutral object is simply 'attracted for no reason'.
- 💡Define the field in terms of the force on a charge before describing the pattern of field lines.
- 💡Link the direction of field lines to the force on a positive charge so attraction and repulsion follow logically.
- 💡Use the field idea to explain a named static effect, such as a charged rod attracting small neutral scraps, rather than leaving the field abstract.
- 💡Learn the two conditions as a pair and check each option against both before choosing.
- 💡Link the word rate to per second so you can reject answers that describe a total amount of charge rather than a flow.
- 💡When an option mentions only one condition, treat it as incomplete rather than fully correct.
- 💡Picture an ammeter moved to different positions in a series loop and expect identical readings.
- 💡Use the phrase charge is conserved to justify equal current rather than relying on intuition.
- 💡Check whether the circuit is a single loop or has branches before applying the rule.
- 💡Write down Q = I × t and substitute values before scanning the answer options.
- 💡Convert all times to seconds first, since the equation assumes seconds.
- 💡Check the unit of your answer against the unit named in the question stem.
Common Mistakes
- Uses time in minutes without converting to seconds; multiply the minutes by 60 to obtain seconds.
- Multiplies charge by time instead of current by time; apply Q = I × t with current and time as the factors.
- Gives the answer in amperes when charge was asked for; state the unit as coulombs for charge flow.
- Divides time by current when finding current; use I = Q ÷ t so charge is divided by time.
- Claims that only some materials contain charge; correct this by stating that charge is a property of all matter.
- Assumes a neutral body contains no charge at all; correct this by explaining that it contains equal positive and negative charge giving zero net charge.
- Treats positive and negative charges as adding to a larger total; correct this by explaining that they cancel when equal in number.
- Claims that static charge builds up on metals; correct this by stating that static charge builds up only on insulators.
- Says charged objects only affect each other on contact; correct this by describing attraction or repulsion observed at a distance.
- Describes rubbing as creating new charge from nothing; correct this by explaining that electrons are transferred between the surfaces.
- Claims that protons are transferred between the objects; correct this by stating that only electrons move between materials in static charging.
- Says the object that gains electrons becomes positive; correct this by linking gain of negative electrons to a negative charge.
- Describes electrons flowing through the air to a nearby object; correct this by explaining that the charge remains on the surface of the insulator and acts at a distance through the electric field.
- Describes the electric field as a substance or as 'electricity in the air'; correct this by defining it as a region where a charge experiences a force.
- Says field lines point towards a positive charge; correct this by stating that field lines point away from a positive charge and towards a negative charge.
- Claims that a charged object must touch a neutral object to attract it; correct this by explaining that the field acts at a distance and polarises the neutral object.
- Confuses current with voltage; keep current as the rate of flow of charge and potential difference as what drives it.
- Claims charge flows in an open circuit; require a closed loop for any steady current.
- Says a closed circuit alone is enough; add a source of potential difference as the second condition.
- Describes current as the flow of positive charge in a metal wire; identify electrons as the moving charges.
- Assumes current decreases after each component; keep the current equal all round a single loop.
- Confuses current with energy and says a lamp uses up current; state that the lamp transfers energy while the current stays the same.
- Applies the single-loop rule to a parallel circuit; note that current splits between branches there.
- Thinks the current is larger near the cell; give the same value at every point in the loop.
- Uses time in minutes without converting to seconds; convert every time to seconds before calculating.
- Multiplies charge by time when finding current; divide charge by time instead.
- Confuses the unit of charge with the unit of current; label charge in coulombs and current in amperes.